2A Linux CD-ROM standard
   5:Author: David van Leeuwen <>
   6:Date: 12 March 1999
   7:Updated by: Erik Andersen (
   8:Updated by: Jens Axboe (
  14Linux is probably the Unix-like operating system that supports
  15the widest variety of hardware devices. The reasons for this are
  18- The large list of hardware devices available for the many platforms
  19  that Linux now supports (i.e., i386-PCs, Sparc Suns, etc.)
  20- The open design of the operating system, such that anybody can write a
  21  driver for Linux.
  22- There is plenty of source code around as examples of how to write a driver.
  24The openness of Linux, and the many different types of available
  25hardware has allowed Linux to support many different hardware devices.
  26Unfortunately, the very openness that has allowed Linux to support
  27all these different devices has also allowed the behavior of each
  28device driver to differ significantly from one device to another.
  29This divergence of behavior has been very significant for CD-ROM
  30devices; the way a particular drive reacts to a `standard` *ioctl()*
  31call varies greatly from one device driver to another. To avoid making
  32their drivers totally inconsistent, the writers of Linux CD-ROM
  33drivers generally created new device drivers by understanding, copying,
  34and then changing an existing one. Unfortunately, this practice did not
  35maintain uniform behavior across all the Linux CD-ROM drivers.
  37This document describes an effort to establish Uniform behavior across
  38all the different CD-ROM device drivers for Linux. This document also
  39defines the various *ioctl()'s*, and how the low-level CD-ROM device
  40drivers should implement them. Currently (as of the Linux 2.1.\ *x*
  41development kernels) several low-level CD-ROM device drivers, including
  42both IDE/ATAPI and SCSI, now use this Uniform interface.
  44When the CD-ROM was developed, the interface between the CD-ROM drive
  45and the computer was not specified in the standards. As a result, many
  46different CD-ROM interfaces were developed. Some of them had their
  47own proprietary design (Sony, Mitsumi, Panasonic, Philips), other
  48manufacturers adopted an existing electrical interface and changed
  49the functionality (CreativeLabs/SoundBlaster, Teac, Funai) or simply
  50adapted their drives to one or more of the already existing electrical
  51interfaces (Aztech, Sanyo, Funai, Vertos, Longshine, Optics Storage and
  52most of the `NoName` manufacturers). In cases where a new drive really
  53brought its own interface or used its own command set and flow control
  54scheme, either a separate driver had to be written, or an existing
  55driver had to be enhanced. History has delivered us CD-ROM support for
  56many of these different interfaces. Nowadays, almost all new CD-ROM
  57drives are either IDE/ATAPI or SCSI, and it is very unlikely that any
  58manufacturer will create a new interface. Even finding drives for the
  59old proprietary interfaces is getting difficult.
  61When (in the 1.3.70's) I looked at the existing software interface,
  62which was expressed through `cdrom.h`, it appeared to be a rather wild
  63set of commands and data formats [#f1]_. It seemed that many
  64features of the software interface had been added to accommodate the
  65capabilities of a particular drive, in an *ad hoc* manner. More
  66importantly, it appeared that the behavior of the `standard` commands
  67was different for most of the different drivers: e. g., some drivers
  68close the tray if an *open()* call occurs when the tray is open, while
  69others do not. Some drivers lock the door upon opening the device, to
  70prevent an incoherent file system, but others don't, to allow software
  71ejection. Undoubtedly, the capabilities of the different drives vary,
  72but even when two drives have the same capability their drivers'
  73behavior was usually different.
  75.. [#f1]
  76   I cannot recollect what kernel version I looked at, then,
  77   presumably 1.2.13 and 1.3.34 --- the latest kernel that I was
  78   indirectly involved in.
  80I decided to start a discussion on how to make all the Linux CD-ROM
  81drivers behave more uniformly. I began by contacting the developers of
  82the many CD-ROM drivers found in the Linux kernel. Their reactions
  83encouraged me to write the Uniform CD-ROM Driver which this document is
  84intended to describe. The implementation of the Uniform CD-ROM Driver is
  85in the file `cdrom.c`. This driver is intended to be an additional software
  86layer that sits on top of the low-level device drivers for each CD-ROM drive.
  87By adding this additional layer, it is possible to have all the different
  88CD-ROM devices behave **exactly** the same (insofar as the underlying
  89hardware will allow).
  91The goal of the Uniform CD-ROM Driver is **not** to alienate driver developers
  92whohave not yet taken steps to support this effort. The goal of Uniform CD-ROM
  93Driver is simply to give people writing application programs for CD-ROM drives
  94**one** Linux CD-ROM interface with consistent behavior for all
  95CD-ROM devices. In addition, this also provides a consistent interface
  96between the low-level device driver code and the Linux kernel. Care
  97is taken that 100% compatibility exists with the data structures and
  98programmer's interface defined in `cdrom.h`. This guide was written to
  99help CD-ROM driver developers adapt their code to use the Uniform CD-ROM
 100Driver code defined in `cdrom.c`.
 102Personally, I think that the most important hardware interfaces are
 103the IDE/ATAPI drives and, of course, the SCSI drives, but as prices
 104of hardware drop continuously, it is also likely that people may have
 105more than one CD-ROM drive, possibly of mixed types. It is important
 106that these drives behave in the same way. In December 1994, one of the
 107cheapest CD-ROM drives was a Philips cm206, a double-speed proprietary
 108drive. In the months that I was busy writing a Linux driver for it,
 109proprietary drives became obsolete and IDE/ATAPI drives became the
 110standard. At the time of the last update to this document (November
 1111997) it is becoming difficult to even **find** anything less than a
 11216 speed CD-ROM drive, and 24 speed drives are common.
 114.. _cdrom_api:
 116Standardizing through another software level
 119At the time this document was conceived, all drivers directly
 120implemented the CD-ROM *ioctl()* calls through their own routines. This
 121led to the danger of different drivers forgetting to do important things
 122like checking that the user was giving the driver valid data. More
 123importantly, this led to the divergence of behavior, which has already
 124been discussed.
 126For this reason, the Uniform CD-ROM Driver was created to enforce consistent
 127CD-ROM drive behavior, and to provide a common set of services to the various
 128low-level CD-ROM device drivers. The Uniform CD-ROM Driver now provides another
 129software-level, that separates the *ioctl()* and *open()* implementation
 130from the actual hardware implementation. Note that this effort has
 131made few changes which will affect a user's application programs. The
 132greatest change involved moving the contents of the various low-level
 133CD-ROM drivers\' header files to the kernel's cdrom directory. This was
 134done to help ensure that the user is only presented with only one cdrom
 135interface, the interface defined in `cdrom.h`.
 137CD-ROM drives are specific enough (i. e., different from other
 138block-devices such as floppy or hard disc drives), to define a set
 139of common **CD-ROM device operations**, *<cdrom-device>_dops*.
 140These operations are different from the classical block-device file
 141operations, *<block-device>_fops*.
 143The routines for the Uniform CD-ROM Driver interface level are implemented
 144in the file `cdrom.c`. In this file, the Uniform CD-ROM Driver interfaces
 145with the kernel as a block device by registering the following general
 146*struct file_operations*::
 148        struct file_operations cdrom_fops = {
 149                NULL,                   /* lseek */
 150                block _read ,           /* read--general block-dev read */
 151                block _write,           /* write--general block-dev write */
 152                NULL,                   /* readdir */
 153                NULL,                   /* select */
 154                cdrom_ioctl,            /* ioctl */
 155                NULL,                   /* mmap */
 156                cdrom_open,             /* open */
 157                cdrom_release,          /* release */
 158                NULL,                   /* fsync */
 159                NULL,                   /* fasync */
 160                NULL                    /* revalidate */
 161        };
 163Every active CD-ROM device shares this *struct*. The routines
 164declared above are all implemented in `cdrom.c`, since this file is the
 165place where the behavior of all CD-ROM-devices is defined and
 166standardized. The actual interface to the various types of CD-ROM
 167hardware is still performed by various low-level CD-ROM-device
 168drivers. These routines simply implement certain **capabilities**
 169that are common to all CD-ROM (and really, all removable-media
 172Registration of a low-level CD-ROM device driver is now done through
 173the general routines in `cdrom.c`, not through the Virtual File System
 174(VFS) any more. The interface implemented in `cdrom.c` is carried out
 175through two general structures that contain information about the
 176capabilities of the driver, and the specific drives on which the
 177driver operates. The structures are:
 180  This structure contains information about the low-level driver for a
 181  CD-ROM device. This structure is conceptually connected to the major
 182  number of the device (although some drivers may have different
 183  major numbers, as is the case for the IDE driver).
 186  This structure contains information about a particular CD-ROM drive,
 187  such as its device name, speed, etc. This structure is conceptually
 188  connected to the minor number of the device.
 190Registering a particular CD-ROM drive with the Uniform CD-ROM Driver
 191is done by the low-level device driver though a call to::
 193        register_cdrom(struct cdrom_device_info * <device>_info)
 195The device information structure, *<device>_info*, contains all the
 196information needed for the kernel to interface with the low-level
 197CD-ROM device driver. One of the most important entries in this
 198structure is a pointer to the *cdrom_device_ops* structure of the
 199low-level driver.
 201The device operations structure, *cdrom_device_ops*, contains a list
 202of pointers to the functions which are implemented in the low-level
 203device driver. When `cdrom.c` accesses a CD-ROM device, it does it
 204through the functions in this structure. It is impossible to know all
 205the capabilities of future CD-ROM drives, so it is expected that this
 206list may need to be expanded from time to time as new technologies are
 207developed. For example, CD-R and CD-R/W drives are beginning to become
 208popular, and support will soon need to be added for them. For now, the
 209current *struct* is::
 211        struct cdrom_device_ops {
 212                int (*open)(struct cdrom_device_info *, int)
 213                void (*release)(struct cdrom_device_info *);
 214                int (*drive_status)(struct cdrom_device_info *, int);
 215                unsigned int (*check_events)(struct cdrom_device_info *,
 216                                             unsigned int, int);
 217                int (*media_changed)(struct cdrom_device_info *, int);
 218                int (*tray_move)(struct cdrom_device_info *, int);
 219                int (*lock_door)(struct cdrom_device_info *, int);
 220                int (*select_speed)(struct cdrom_device_info *, int);
 221                int (*select_disc)(struct cdrom_device_info *, int);
 222                int (*get_last_session) (struct cdrom_device_info *,
 223                                         struct cdrom_multisession *);
 224                int (*get_mcn)(struct cdrom_device_info *, struct cdrom_mcn *);
 225                int (*reset)(struct cdrom_device_info *);
 226                int (*audio_ioctl)(struct cdrom_device_info *,
 227                                   unsigned int, void *);
 228                const int capability;           /* capability flags */
 229                int (*generic_packet)(struct cdrom_device_info *,
 230                                      struct packet_command *);
 231        };
 233When a low-level device driver implements one of these capabilities,
 234it should add a function pointer to this *struct*. When a particular
 235function is not implemented, however, this *struct* should contain a
 236NULL instead. The *capability* flags specify the capabilities of the
 237CD-ROM hardware and/or low-level CD-ROM driver when a CD-ROM drive
 238is registered with the Uniform CD-ROM Driver.
 240Note that most functions have fewer parameters than their
 241*blkdev_fops* counterparts. This is because very little of the
 242information in the structures *inode* and *file* is used. For most
 243drivers, the main parameter is the *struct* *cdrom_device_info*, from
 244which the major and minor number can be extracted. (Most low-level
 245CD-ROM drivers don't even look at the major and minor number though,
 246since many of them only support one device.) This will be available
 247through *dev* in *cdrom_device_info* described below.
 249The drive-specific, minor-like information that is registered with
 250`cdrom.c`, currently contains the following fields::
 252  struct cdrom_device_info {
 253        const struct cdrom_device_ops * ops;    /* device operations for this major */
 254        struct list_head list;                  /* linked list of all device_info */
 255        struct gendisk * disk;                  /* matching block layer disk */
 256        void *  handle;                         /* driver-dependent data */
 258        int mask;                               /* mask of capability: disables them */
 259        int speed;                              /* maximum speed for reading data */
 260        int capacity;                           /* number of discs in a jukebox */
 262        unsigned int options:30;                /* options flags */
 263        unsigned mc_flags:2;                    /*  media-change buffer flags */
 264        unsigned int vfs_events;                /*  cached events for vfs path */
 265        unsigned int ioctl_events;              /*  cached events for ioctl path */
 266        int use_count;                          /*  number of times device is opened */
 267        char name[20];                          /*  name of the device type */
 269        __u8 sanyo_slot : 2;                    /*  Sanyo 3-CD changer support */
 270        __u8 keeplocked : 1;                    /*  CDROM_LOCKDOOR status */
 271        __u8 reserved : 5;                      /*  not used yet */
 272        int cdda_method;                        /*  see CDDA_* flags */
 273        __u8 last_sense;                        /*  saves last sense key */
 274        __u8 media_written;                     /*  dirty flag, DVD+RW bookkeeping */
 275        unsigned short mmc3_profile;            /*  current MMC3 profile */
 276        int for_data;                           /*  unknown:TBD */
 277        int (*exit)(struct cdrom_device_info *);/*  unknown:TBD */
 278        int mrw_mode_page;                      /*  which MRW mode page is in use */
 279  };
 281Using this *struct*, a linked list of the registered minor devices is
 282built, using the *next* field. The device number, the device operations
 283struct and specifications of properties of the drive are stored in this
 286The *mask* flags can be used to mask out some of the capabilities listed
 287in *ops->capability*, if a specific drive doesn't support a feature
 288of the driver. The value *speed* specifies the maximum head-rate of the
 289drive, measured in units of normal audio speed (176kB/sec raw data or
 290150kB/sec file system data). The parameters are declared *const*
 291because they describe properties of the drive, which don't change after
 294A few registers contain variables local to the CD-ROM drive. The
 295flags *options* are used to specify how the general CD-ROM routines
 296should behave. These various flags registers should provide enough
 297flexibility to adapt to the different users' wishes (and **not** the
 298`arbitrary` wishes of the author of the low-level device driver, as is
 299the case in the old scheme). The register *mc_flags* is used to buffer
 300the information from *media_changed()* to two separate queues. Other
 301data that is specific to a minor drive, can be accessed through *handle*,
 302which can point to a data structure specific to the low-level driver.
 303The fields *use_count*, *next*, *options* and *mc_flags* need not be
 306The intermediate software layer that `cdrom.c` forms will perform some
 307additional bookkeeping. The use count of the device (the number of
 308processes that have the device opened) is registered in *use_count*. The
 309function *cdrom_ioctl()* will verify the appropriate user-memory regions
 310for read and write, and in case a location on the CD is transferred,
 311it will `sanitize` the format by making requests to the low-level
 312drivers in a standard format, and translating all formats between the
 313user-software and low level drivers. This relieves much of the drivers'
 314memory checking and format checking and translation. Also, the necessary
 315structures will be declared on the program stack.
 317The implementation of the functions should be as defined in the
 318following sections. Two functions **must** be implemented, namely
 319*open()* and *release()*. Other functions may be omitted, their
 320corresponding capability flags will be cleared upon registration.
 321Generally, a function returns zero on success and negative on error. A
 322function call should return only after the command has completed, but of
 323course waiting for the device should not use processor time.
 327        int open(struct cdrom_device_info *cdi, int purpose)
 329*Open()* should try to open the device for a specific *purpose*, which
 330can be either:
 332- Open for reading data, as done by `mount()` (2), or the
 333  user commands `dd` or `cat`.
 334- Open for *ioctl* commands, as done by audio-CD playing programs.
 336Notice that any strategic code (closing tray upon *open()*, etc.) is
 337done by the calling routine in `cdrom.c`, so the low-level routine
 338should only be concerned with proper initialization, such as spinning
 339up the disc, etc.
 343        void release(struct cdrom_device_info *cdi)
 345Device-specific actions should be taken such as spinning down the device.
 346However, strategic actions such as ejection of the tray, or unlocking
 347the door, should be left over to the general routine *cdrom_release()*.
 348This is the only function returning type *void*.
 350.. _cdrom_drive_status:
 354        int drive_status(struct cdrom_device_info *cdi, int slot_nr)
 356The function *drive_status*, if implemented, should provide
 357information on the status of the drive (not the status of the disc,
 358which may or may not be in the drive). If the drive is not a changer,
 359*slot_nr* should be ignored. In `cdrom.h` the possibilities are listed::
 362        CDS_NO_INFO             /* no information available */
 363        CDS_NO_DISC             /* no disc is inserted, tray is closed */
 364        CDS_TRAY_OPEN           /* tray is opened */
 365        CDS_DRIVE_NOT_READY     /* something is wrong, tray is moving? */
 366        CDS_DISC_OK             /* a disc is loaded and everything is fine */
 370        int tray_move(struct cdrom_device_info *cdi, int position)
 372This function, if implemented, should control the tray movement. (No
 373other function should control this.) The parameter *position* controls
 374the desired direction of movement:
 376- 0 Close tray
 377- 1 Open tray
 379This function returns 0 upon success, and a non-zero value upon
 380error. Note that if the tray is already in the desired position, no
 381action need be taken, and the return value should be 0.
 385        int lock_door(struct cdrom_device_info *cdi, int lock)
 387This function (and no other code) controls locking of the door, if the
 388drive allows this. The value of *lock* controls the desired locking
 391- 0 Unlock door, manual opening is allowed
 392- 1 Lock door, tray cannot be ejected manually
 394This function returns 0 upon success, and a non-zero value upon
 395error. Note that if the door is already in the requested state, no
 396action need be taken, and the return value should be 0.
 400        int select_speed(struct cdrom_device_info *cdi, int speed)
 402Some CD-ROM drives are capable of changing their head-speed. There
 403are several reasons for changing the speed of a CD-ROM drive. Badly
 404pressed CD-ROM s may benefit from less-than-maximum head rate. Modern
 405CD-ROM drives can obtain very high head rates (up to *24x* is
 406common). It has been reported that these drives can make reading
 407errors at these high speeds, reducing the speed can prevent data loss
 408in these circumstances. Finally, some of these drives can
 409make an annoyingly loud noise, which a lower speed may reduce.
 411This function specifies the speed at which data is read or audio is
 412played back. The value of *speed* specifies the head-speed of the
 413drive, measured in units of standard cdrom speed (176kB/sec raw data
 414or 150kB/sec file system data). So to request that a CD-ROM drive
 415operate at 300kB/sec you would call the CDROM_SELECT_SPEED *ioctl*
 416with *speed=2*. The special value `0` means `auto-selection`, i. e.,
 417maximum data-rate or real-time audio rate. If the drive doesn't have
 418this `auto-selection` capability, the decision should be made on the
 419current disc loaded and the return value should be positive. A negative
 420return value indicates an error.
 424        int select_disc(struct cdrom_device_info *cdi, int number)
 426If the drive can store multiple discs (a juke-box) this function
 427will perform disc selection. It should return the number of the
 428selected disc on success, a negative value on error. Currently, only
 429the ide-cd driver supports this functionality.
 433        int get_last_session(struct cdrom_device_info *cdi,
 434                             struct cdrom_multisession *ms_info)
 436This function should implement the old corresponding *ioctl()*. For
 437device *cdi->dev*, the start of the last session of the current disc
 438should be returned in the pointer argument *ms_info*. Note that
 439routines in `cdrom.c` have sanitized this argument: its requested
 440format will **always** be of the type *CDROM_LBA* (linear block
 441addressing mode), whatever the calling software requested. But
 442sanitization goes even further: the low-level implementation may
 443return the requested information in *CDROM_MSF* format if it wishes so
 444(setting the *ms_info->addr_format* field appropriately, of
 445course) and the routines in `cdrom.c` will make the transformation if
 446necessary. The return value is 0 upon success.
 450        int get_mcn(struct cdrom_device_info *cdi,
 451                    struct cdrom_mcn *mcn)
 453Some discs carry a `Media Catalog Number` (MCN), also called
 454`Universal Product Code` (UPC). This number should reflect the number
 455that is generally found in the bar-code on the product. Unfortunately,
 456the few discs that carry such a number on the disc don't even use the
 457same format. The return argument to this function is a pointer to a
 458pre-declared memory region of type *struct cdrom_mcn*. The MCN is
 459expected as a 13-character string, terminated by a null-character.
 463        int reset(struct cdrom_device_info *cdi)
 465This call should perform a hard-reset on the drive (although in
 466circumstances that a hard-reset is necessary, a drive may very well not
 467listen to commands anymore). Preferably, control is returned to the
 468caller only after the drive has finished resetting. If the drive is no
 469longer listening, it may be wise for the underlying low-level cdrom
 470driver to time out.
 474        int audio_ioctl(struct cdrom_device_info *cdi,
 475                        unsigned int cmd, void *arg)
 477Some of the CD-ROM-\ *ioctl()*\ 's defined in `cdrom.h` can be
 478implemented by the routines described above, and hence the function
 479*cdrom_ioctl* will use those. However, most *ioctl()*\ 's deal with
 480audio-control. We have decided to leave these to be accessed through a
 481single function, repeating the arguments *cmd* and *arg*. Note that
 482the latter is of type *void*, rather than *unsigned long int*.
 483The routine *cdrom_ioctl()* does do some useful things,
 484though. It sanitizes the address format type to *CDROM_MSF* (Minutes,
 485Seconds, Frames) for all audio calls. It also verifies the memory
 486location of *arg*, and reserves stack-memory for the argument. This
 487makes implementation of the *audio_ioctl()* much simpler than in the
 488old driver scheme. For example, you may look up the function
 489*cm206_audio_ioctl()* `cm206.c` that should be updated with
 490this documentation.
 492An unimplemented ioctl should return *-ENOSYS*, but a harmless request
 493(e. g., *CDROMSTART*) may be ignored by returning 0 (success). Other
 494errors should be according to the standards, whatever they are. When
 495an error is returned by the low-level driver, the Uniform CD-ROM Driver
 496tries whenever possible to return the error code to the calling program.
 497(We may decide to sanitize the return value in *cdrom_ioctl()* though, in
 498order to guarantee a uniform interface to the audio-player software.)
 502        int dev_ioctl(struct cdrom_device_info *cdi,
 503                      unsigned int cmd, unsigned long arg)
 505Some *ioctl()'s* seem to be specific to certain CD-ROM drives. That is,
 506they are introduced to service some capabilities of certain drives. In
 507fact, there are 6 different *ioctl()'s* for reading data, either in some
 508particular kind of format, or audio data. Not many drives support
 509reading audio tracks as data, I believe this is because of protection
 510of copyrights of artists. Moreover, I think that if audio-tracks are
 511supported, it should be done through the VFS and not via *ioctl()'s*. A
 512problem here could be the fact that audio-frames are 2352 bytes long,
 513so either the audio-file-system should ask for 75264 bytes at once
 514(the least common multiple of 512 and 2352), or the drivers should
 515bend their backs to cope with this incoherence (to which I would be
 516opposed). Furthermore, it is very difficult for the hardware to find
 517the exact frame boundaries, since there are no synchronization headers
 518in audio frames. Once these issues are resolved, this code should be
 519standardized in `cdrom.c`.
 521Because there are so many *ioctl()'s* that seem to be introduced to
 522satisfy certain drivers [#f2]_, any non-standard *ioctl()*\ s
 523are routed through the call *dev_ioctl()*. In principle, `private`
 524*ioctl()*\ 's should be numbered after the device's major number, and not
 525the general CD-ROM *ioctl* number, `0x53`. Currently the
 526non-supported *ioctl()'s* are:
 531.. [#f2]
 533   Is there software around that actually uses these? I'd be interested!
 535.. _cdrom_capabilities:
 537CD-ROM capabilities
 540Instead of just implementing some *ioctl* calls, the interface in
 541`cdrom.c` supplies the possibility to indicate the **capabilities**
 542of a CD-ROM drive. This can be done by ORing any number of
 543capability-constants that are defined in `cdrom.h` at the registration
 544phase. Currently, the capabilities are any of::
 546        CDC_CLOSE_TRAY          /* can close tray by software control */
 547        CDC_OPEN_TRAY           /* can open tray */
 548        CDC_LOCK                /* can lock and unlock the door */
 549        CDC_SELECT_SPEED        /* can select speed, in units of * sim*150 ,kB/s */
 550        CDC_SELECT_DISC         /* drive is juke-box */
 551        CDC_MULTI_SESSION       /* can read sessions *> rm1* */
 552        CDC_MCN                 /* can read Media Catalog Number */
 553        CDC_MEDIA_CHANGED       /* can report if disc has changed */
 554        CDC_PLAY_AUDIO          /* can perform audio-functions (play, pause, etc) */
 555        CDC_RESET               /* hard reset device */
 556        CDC_IOCTLS              /* driver has non-standard ioctls */
 557        CDC_DRIVE_STATUS        /* driver implements drive status */
 559The capability flag is declared *const*, to prevent drivers from
 560accidentally tampering with the contents. The capability flags actually
 561inform `cdrom.c` of what the driver can do. If the drive found
 562by the driver does not have the capability, is can be masked out by
 563the *cdrom_device_info* variable *mask*. For instance, the SCSI CD-ROM
 564driver has implemented the code for loading and ejecting CD-ROM's, and
 565hence its corresponding flags in *capability* will be set. But a SCSI
 566CD-ROM drive might be a caddy system, which can't load the tray, and
 567hence for this drive the *cdrom_device_info* struct will have set
 568the *CDC_CLOSE_TRAY* bit in *mask*.
 570In the file `cdrom.c` you will encounter many constructions of the type::
 572        if (cdo->capability & ~cdi->mask & CDC _<capability>) ...
 574There is no *ioctl* to set the mask... The reason is that
 575I think it is better to control the **behavior** rather than the
 581A final flag register controls the **behavior** of the CD-ROM
 582drives, in order to satisfy different users' wishes, hopefully
 583independently of the ideas of the respective author who happened to
 584have made the drive's support available to the Linux community. The
 585current behavior options are::
 587        CDO_AUTO_CLOSE  /* try to close tray upon device open() */
 588        CDO_AUTO_EJECT  /* try to open tray on last device close() */
 589        CDO_USE_FFLAGS  /* use file_pointer->f_flags to indicate purpose for open() */
 590        CDO_LOCK        /* try to lock door if device is opened */
 591        CDO_CHECK_TYPE  /* ensure disc type is data if opened for data */
 593The initial value of this register is
 594`CDO_AUTO_CLOSE | CDO_USE_FFLAGS | CDO_LOCK`, reflecting my own view on user
 595interface and software standards. Before you protest, there are two
 596new *ioctl()'s* implemented in `cdrom.c`, that allow you to control the
 597behavior by software. These are::
 599        CDROM_SET_OPTIONS       /* set options specified in (int)arg */
 600        CDROM_CLEAR_OPTIONS     /* clear options specified in (int)arg */
 602One option needs some more explanation: *CDO_USE_FFLAGS*. In the next
 603newsection we explain what the need for this option is.
 605A software package `setcd`, available from the Debian distribution
 606and ``, allows user level control of these flags.
 609The need to know the purpose of opening the CD-ROM device
 612Traditionally, Unix devices can be used in two different `modes`,
 613either by reading/writing to the device file, or by issuing
 614controlling commands to the device, by the device's *ioctl()*
 615call. The problem with CD-ROM drives, is that they can be used for
 616two entirely different purposes. One is to mount removable
 617file systems, CD-ROM's, the other is to play audio CD's. Audio commands
 618are implemented entirely through *ioctl()\'s*, presumably because the
 619first implementation (SUN?) has been such. In principle there is
 620nothing wrong with this, but a good control of the `CD player` demands
 621that the device can **always** be opened in order to give the
 622*ioctl* commands, regardless of the state the drive is in.
 624On the other hand, when used as a removable-media disc drive (what the
 625original purpose of CD-ROM s is) we would like to make sure that the
 626disc drive is ready for operation upon opening the device. In the old
 627scheme, some CD-ROM drivers don't do any integrity checking, resulting
 628in a number of i/o errors reported by the VFS to the kernel when an
 629attempt for mounting a CD-ROM on an empty drive occurs. This is not a
 630particularly elegant way to find out that there is no CD-ROM inserted;
 631it more-or-less looks like the old IBM-PC trying to read an empty floppy
 632drive for a couple of seconds, after which the system complains it
 633can't read from it. Nowadays we can **sense** the existence of a
 634removable medium in a drive, and we believe we should exploit that
 635fact. An integrity check on opening of the device, that verifies the
 636availability of a CD-ROM and its correct type (data), would be
 639These two ways of using a CD-ROM drive, principally for data and
 640secondarily for playing audio discs, have different demands for the
 641behavior of the *open()* call. Audio use simply wants to open the
 642device in order to get a file handle which is needed for issuing
 643*ioctl* commands, while data use wants to open for correct and
 644reliable data transfer. The only way user programs can indicate what
 645their *purpose* of opening the device is, is through the *flags*
 646parameter (see `open(2)`). For CD-ROM devices, these flags aren't
 647implemented (some drivers implement checking for write-related flags,
 648but this is not strictly necessary if the device file has correct
 649permission flags). Most option flags simply don't make sense to
 650CD-ROM devices: *O_CREAT*, *O_NOCTTY*, *O_TRUNC*, *O_APPEND*, and
 651*O_SYNC* have no meaning to a CD-ROM.
 653We therefore propose to use the flag *O_NONBLOCK* to indicate
 654that the device is opened just for issuing *ioctl*
 655commands. Strictly, the meaning of *O_NONBLOCK* is that opening and
 656subsequent calls to the device don't cause the calling process to
 657wait. We could interpret this as don't wait until someone has
 658inserted some valid data-CD-ROM. Thus, our proposal of the
 659implementation for the *open()* call for CD-ROM s is:
 661- If no other flags are set than *O_RDONLY*, the device is opened
 662  for data transfer, and the return value will be 0 only upon successful
 663  initialization of the transfer. The call may even induce some actions
 664  on the CD-ROM, such as closing the tray.
 665- If the option flag *O_NONBLOCK* is set, opening will always be
 666  successful, unless the whole device doesn't exist. The drive will take
 667  no actions whatsoever.
 669And what about standards?
 672You might hesitate to accept this proposal as it comes from the
 673Linux community, and not from some standardizing institute. What
 674about SUN, SGI, HP and all those other Unix and hardware vendors?
 675Well, these companies are in the lucky position that they generally
 676control both the hardware and software of their supported products,
 677and are large enough to set their own standard. They do not have to
 678deal with a dozen or more different, competing hardware
 679configurations\ [#f3]_.
 681.. [#f3]
 683   Incidentally, I think that SUN's approach to mounting CD-ROM s is very
 684   good in origin: under Solaris a volume-daemon automatically mounts a
 685   newly inserted CD-ROM under `/cdrom/*<volume-name>*`.
 687   In my opinion they should have pushed this
 688   further and have **every** CD-ROM on the local area network be
 689   mounted at the similar location, i. e., no matter in which particular
 690   machine you insert a CD-ROM, it will always appear at the same
 691   position in the directory tree, on every system. When I wanted to
 692   implement such a user-program for Linux, I came across the
 693   differences in behavior of the various drivers, and the need for an
 694   *ioctl* informing about media changes.
 696We believe that using *O_NONBLOCK* to indicate that a device is being opened
 697for *ioctl* commands only can be easily introduced in the Linux
 698community. All the CD-player authors will have to be informed, we can
 699even send in our own patches to the programs. The use of *O_NONBLOCK*
 700has most likely no influence on the behavior of the CD-players on
 701other operating systems than Linux. Finally, a user can always revert
 702to old behavior by a call to
 703*ioctl(file_descriptor, CDROM_CLEAR_OPTIONS, CDO_USE_FFLAGS)*.
 705The preferred strategy of *open()*
 708The routines in `cdrom.c` are designed in such a way that run-time
 709configuration of the behavior of CD-ROM devices (of **any** type)
 710can be carried out, by the *CDROM_SET/CLEAR_OPTIONS* *ioctls*. Thus, various
 711modes of operation can be set:
 714   This is the default setting. (With *CDO_CHECK_TYPE* it will be better, in
 715   the future.) If the device is not yet opened by any other process, and if
 716   the device is being opened for data (*O_NONBLOCK* is not set) and the
 717   tray is found to be open, an attempt to close the tray is made. Then,
 718   it is verified that a disc is in the drive and, if *CDO_CHECK_TYPE* is
 719   set, that it contains tracks of type `data mode 1`. Only if all tests
 720   are passed is the return value zero. The door is locked to prevent file
 721   system corruption. If the drive is opened for audio (*O_NONBLOCK* is
 722   set), no actions are taken and a value of 0 will be returned.
 725   This mimics the behavior of the current sbpcd-driver. The option flags are
 726   ignored, the tray is closed on the first open, if necessary. Similarly,
 727   the tray is opened on the last release, i. e., if a CD-ROM is unmounted,
 728   it is automatically ejected, such that the user can replace it.
 730We hope that these option can convince everybody (both driver
 731maintainers and user program developers) to adopt the new CD-ROM
 732driver scheme and option flag interpretation.
 734Description of routines in `cdrom.c`
 737Only a few routines in `cdrom.c` are exported to the drivers. In this
 738new section we will discuss these, as well as the functions that `take
 739over` the CD-ROM interface to the kernel. The header file belonging
 740to `cdrom.c` is called `cdrom.h`. Formerly, some of the contents of this
 741file were placed in the file `ucdrom.h`, but this file has now been
 742merged back into `cdrom.h`.
 746        struct file_operations cdrom_fops
 748The contents of this structure were described in cdrom_api_.
 749A pointer to this structure is assigned to the *fops* field
 750of the *struct gendisk*.
 754        int register_cdrom(struct cdrom_device_info *cdi)
 756This function is used in about the same way one registers *cdrom_fops*
 757with the kernel, the device operations and information structures,
 758as described in cdrom_api_, should be registered with the
 759Uniform CD-ROM Driver::
 761        register_cdrom(&<device>_info);
 764This function returns zero upon success, and non-zero upon
 765failure. The structure *<device>_info* should have a pointer to the
 766driver's *<device>_dops*, as in::
 768        struct cdrom_device_info <device>_info = {
 769                <device>_dops;
 770                ...
 771        }
 773Note that a driver must have one static structure, *<device>_dops*, while
 774it may have as many structures *<device>_info* as there are minor devices
 775active. *Register_cdrom()* builds a linked list from these.
 780        void unregister_cdrom(struct cdrom_device_info *cdi)
 782Unregistering device *cdi* with minor number *MINOR(cdi->dev)* removes
 783the minor device from the list. If it was the last registered minor for
 784the low-level driver, this disconnects the registered device-operation
 785routines from the CD-ROM interface. This function returns zero upon
 786success, and non-zero upon failure.
 790        int cdrom_open(struct inode * ip, struct file * fp)
 792This function is not called directly by the low-level drivers, it is
 793listed in the standard *cdrom_fops*. If the VFS opens a file, this
 794function becomes active. A strategy is implemented in this routine,
 795taking care of all capabilities and options that are set in the
 796*cdrom_device_ops* connected to the device. Then, the program flow is
 797transferred to the device_dependent *open()* call.
 801        void cdrom_release(struct inode *ip, struct file *fp)
 803This function implements the reverse-logic of *cdrom_open()*, and then
 804calls the device-dependent *release()* routine. When the use-count has
 805reached 0, the allocated buffers are flushed by calls to *sync_dev(dev)*
 806and *invalidate_buffers(dev)*.
 809.. _cdrom_ioctl:
 813        int cdrom_ioctl(struct inode *ip, struct file *fp,
 814                        unsigned int cmd, unsigned long arg)
 816This function handles all the standard *ioctl* requests for CD-ROM
 817devices in a uniform way. The different calls fall into three
 818categories: *ioctl()'s* that can be directly implemented by device
 819operations, ones that are routed through the call *audio_ioctl()*, and
 820the remaining ones, that are presumable device-dependent. Generally, a
 821negative return value indicates an error.
 823Directly implemented *ioctl()'s*
 826The following `old` CD-ROM *ioctl()*\ 's are implemented by directly
 827calling device-operations in *cdrom_device_ops*, if implemented and
 828not masked:
 831        Requests the last session on a CD-ROM.
 833        Open tray.
 835        Close tray.
 837        If *arg\not=0*, set behavior to auto-close (close
 838        tray on first open) and auto-eject (eject on last release), otherwise
 839        set behavior to non-moving on *open()* and *release()* calls.
 841        Get the Media Catalog Number from a CD.
 843*Ioctl*s routed through *audio_ioctl()*
 846The following set of *ioctl()'s* are all implemented through a call to
 847the *cdrom_fops* function *audio_ioctl()*. Memory checks and
 848allocation are performed in *cdrom_ioctl()*, and also sanitization of
 849address format (*CDROM_LBA*/*CDROM_MSF*) is done.
 852        Get sub-channel data in argument *arg* of type
 853        `struct cdrom_subchnl *`.
 855        Read Table of Contents header, in *arg* of type
 856        `struct cdrom_tochdr *`.
 858        Read a Table of Contents entry in *arg* and specified by *arg*
 859        of type `struct cdrom_tocentry *`.
 861        Play audio fragment specified in Minute, Second, Frame format,
 862        delimited by *arg* of type `struct cdrom_msf *`.
 864        Play audio fragment in track-index format delimited by *arg*
 865        of type `struct cdrom_ti *`.
 867        Set volume specified by *arg* of type `struct cdrom_volctrl *`.
 869        Read volume into by *arg* of type `struct cdrom_volctrl *`.
 871        Spin up disc.
 873        Stop playback of audio fragment.
 875        Pause playback of audio fragment.
 877        Resume playing.
 879New *ioctl()'s* in `cdrom.c`
 882The following *ioctl()'s* have been introduced to allow user programs to
 883control the behavior of individual CD-ROM devices. New *ioctl*
 884commands can be identified by the underscores in their names.
 887        Set options specified by *arg*. Returns the option flag register
 888        after modification. Use *arg = \rm0* for reading the current flags.
 890        Clear options specified by *arg*. Returns the option flag register
 891        after modification.
 893        Select head-rate speed of disc specified as by *arg* in units
 894        of standard cdrom speed (176\,kB/sec raw data or
 895        150kB/sec file system data). The value 0 means `auto-select`,
 896        i. e., play audio discs at real time and data discs at maximum speed.
 897        The value *arg* is checked against the maximum head rate of the
 898        drive found in the *cdrom_dops*.
 900        Select disc numbered *arg* from a juke-box.
 902        First disc is numbered 0. The number *arg* is checked against the
 903        maximum number of discs in the juke-box found in the *cdrom_dops*.
 905        Returns 1 if a disc has been changed since the last call.
 906        For juke-boxes, an extra argument *arg*
 907        specifies the slot for which the information is given. The special
 908        value *CDSL_CURRENT* requests that information about the currently
 909        selected slot be returned.
 911        Returns the status of the drive by a call to
 912        *drive_status()*. Return values are defined in cdrom_drive_status_.
 913        Note that this call doesn't return information on the
 914        current playing activity of the drive; this can be polled through
 915        an *ioctl* call to *CDROMSUBCHNL*. For juke-boxes, an extra argument
 916        *arg* specifies the slot for which (possibly limited) information is
 917        given. The special value *CDSL_CURRENT* requests that information
 918        about the currently selected slot be returned.
 920        Returns the type of the disc currently in the drive.
 921        It should be viewed as a complement to *CDROM_DRIVE_STATUS*.
 922        This *ioctl* can provide *some* information about the current
 923        disc that is inserted in the drive. This functionality used to be
 924        implemented in the low level drivers, but is now carried out
 925        entirely in Uniform CD-ROM Driver.
 927        The history of development of the CD's use as a carrier medium for
 928        various digital information has lead to many different disc types.
 929        This *ioctl* is useful only in the case that CDs have \emph {only
 930        one} type of data on them. While this is often the case, it is
 931        also very common for CDs to have some tracks with data, and some
 932        tracks with audio. Because this is an existing interface, rather
 933        than fixing this interface by changing the assumptions it was made
 934        under, thereby breaking all user applications that use this
 935        function, the Uniform CD-ROM Driver implements this *ioctl* as
 936        follows: If the CD in question has audio tracks on it, and it has
 937        absolutely no CD-I, XA, or data tracks on it, it will be reported
 938        as *CDS_AUDIO*. If it has both audio and data tracks, it will
 939        return *CDS_MIXED*. If there are no audio tracks on the disc, and
 940        if the CD in question has any CD-I tracks on it, it will be
 941        reported as *CDS_XA_2_2*. Failing that, if the CD in question
 942        has any XA tracks on it, it will be reported as *CDS_XA_2_1*.
 943        Finally, if the CD in question has any data tracks on it,
 944        it will be reported as a data CD (*CDS_DATA_1*).
 946        This *ioctl* can return::
 948                CDS_NO_INFO     /* no information available */
 949                CDS_NO_DISC     /* no disc is inserted, or tray is opened */
 950                CDS_AUDIO       /* Audio disc (2352 audio bytes/frame) */
 951                CDS_DATA_1      /* data disc, mode 1 (2048 user bytes/frame) */
 952                CDS_XA_2_1      /* mixed data (XA), mode 2, form 1 (2048 user bytes) */
 953                CDS_XA_2_2      /* mixed data (XA), mode 2, form 1 (2324 user bytes) */
 954                CDS_MIXED       /* mixed audio/data disc */
 956        For some information concerning frame layout of the various disc
 957        types, see a recent version of `cdrom.h`.
 960        Returns the number of slots in a juke-box.
 962        Reset the drive.
 964        Returns the *capability* flags for the drive. Refer to section
 965        cdrom_capabilities_ for more information on these flags.
 967         Locks the door of the drive. `arg == 0` unlocks the door,
 968         any other value locks it.
 970         Turns on debugging info. Only root is allowed to do this.
 971         Same semantics as CDROM_LOCKDOOR.
 974Device dependent *ioctl()'s*
 977Finally, all other *ioctl()'s* are passed to the function *dev_ioctl()*,
 978if implemented. No memory allocation or verification is carried out.
 980How to update your driver
 983- Make a backup of your current driver.
 984- Get hold of the files `cdrom.c` and `cdrom.h`, they should be in
 985  the directory tree that came with this documentation.
 986- Make sure you include `cdrom.h`.
 987- Change the 3rd argument of *register_blkdev* from `&<your-drive>_fops`
 988  to `&cdrom_fops`.
 989- Just after that line, add the following to register with the Uniform
 990  CD-ROM Driver::
 992        register_cdrom(&<your-drive>_info);*
 994  Similarly, add a call to *unregister_cdrom()* at the appropriate place.
 995- Copy an example of the device-operations *struct* to your
 996  source, e. g., from `cm206.c` *cm206_dops*, and change all
 997  entries to names corresponding to your driver, or names you just
 998  happen to like. If your driver doesn't support a certain function,
 999  make the entry *NULL*. At the entry *capability* you should list all
1000  capabilities your driver currently supports. If your driver
1001  has a capability that is not listed, please send me a message.
1002- Copy the *cdrom_device_info* declaration from the same example
1003  driver, and modify the entries according to your needs. If your
1004  driver dynamically determines the capabilities of the hardware, this
1005  structure should also be declared dynamically.
1006- Implement all functions in your `<device>_dops` structure,
1007  according to prototypes listed in  `cdrom.h`, and specifications given
1008  in cdrom_api_. Most likely you have already implemented
1009  the code in a large part, and you will almost certainly need to adapt the
1010  prototype and return values.
1011- Rename your `<device>_ioctl()` function to *audio_ioctl* and
1012  change the prototype a little. Remove entries listed in the first
1013  part in cdrom_ioctl_, if your code was OK, these are
1014  just calls to the routines you adapted in the previous step.
1015- You may remove all remaining memory checking code in the
1016  *audio_ioctl()* function that deals with audio commands (these are
1017  listed in the second part of cdrom_ioctl_. There is no
1018  need for memory allocation either, so most *case*s in the *switch*
1019  statement look similar to::
1021        case CDROMREADTOCENTRY:
1022                get_toc_entry\bigl((struct cdrom_tocentry *) arg);
1024- All remaining *ioctl* cases must be moved to a separate
1025  function, *<device>_ioctl*, the device-dependent *ioctl()'s*. Note that
1026  memory checking and allocation must be kept in this code!
1027- Change the prototypes of *<device>_open()* and
1028  *<device>_release()*, and remove any strategic code (i. e., tray
1029  movement, door locking, etc.).
1030- Try to recompile the drivers. We advise you to use modules, both
1031  for `cdrom.o` and your driver, as debugging is much easier this
1032  way.
1037Thanks to all the people involved. First, Erik Andersen, who has
1038taken over the torch in maintaining `cdrom.c` and integrating much
1039CD-ROM-related code in the 2.1-kernel. Thanks to Scott Snyder and
1040Gerd Knorr, who were the first to implement this interface for SCSI
1041and IDE-CD drivers and added many ideas for extension of the data
1042structures relative to kernel~2.0. Further thanks to Heiko Ei\xC3\x9Ffeldt,
1043Thomas Quinot, Jon Tombs, Ken Pizzini, Eberhard M\xC3\xB6nkeberg and Andrew Kroll,
1044the Linux CD-ROM device driver developers who were kind
1045enough to give suggestions and criticisms during the writing. Finally
1046of course, I want to thank Linus Torvalds for making this possible in
1047the first place.