linux/Documentation/PCI/pci.txt
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   2                        How To Write Linux PCI Drivers
   3
   4                by Martin Mares <mj@ucw.cz> on 07-Feb-2000
   5        updated by Grant Grundler <grundler@parisc-linux.org> on 23-Dec-2006
   6
   7~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   8The world of PCI is vast and full of (mostly unpleasant) surprises.
   9Since each CPU architecture implements different chip-sets and PCI devices
  10have different requirements (erm, "features"), the result is the PCI support
  11in the Linux kernel is not as trivial as one would wish. This short paper
  12tries to introduce all potential driver authors to Linux APIs for
  13PCI device drivers.
  14
  15A more complete resource is the third edition of "Linux Device Drivers"
  16by Jonathan Corbet, Alessandro Rubini, and Greg Kroah-Hartman.
  17LDD3 is available for free (under Creative Commons License) from:
  18
  19        http://lwn.net/Kernel/LDD3/
  20
  21However, keep in mind that all documents are subject to "bit rot".
  22Refer to the source code if things are not working as described here.
  23
  24Please send questions/comments/patches about Linux PCI API to the
  25"Linux PCI" <linux-pci@atrey.karlin.mff.cuni.cz> mailing list.
  26
  27
  28
  290. Structure of PCI drivers
  30~~~~~~~~~~~~~~~~~~~~~~~~~~~
  31PCI drivers "discover" PCI devices in a system via pci_register_driver().
  32Actually, it's the other way around. When the PCI generic code discovers
  33a new device, the driver with a matching "description" will be notified.
  34Details on this below.
  35
  36pci_register_driver() leaves most of the probing for devices to
  37the PCI layer and supports online insertion/removal of devices [thus
  38supporting hot-pluggable PCI, CardBus, and Express-Card in a single driver].
  39pci_register_driver() call requires passing in a table of function
  40pointers and thus dictates the high level structure of a driver.
  41
  42Once the driver knows about a PCI device and takes ownership, the
  43driver generally needs to perform the following initialization:
  44
  45        Enable the device
  46        Request MMIO/IOP resources
  47        Set the DMA mask size (for both coherent and streaming DMA)
  48        Allocate and initialize shared control data (pci_allocate_coherent())
  49        Access device configuration space (if needed)
  50        Register IRQ handler (request_irq())
  51        Initialize non-PCI (i.e. LAN/SCSI/etc parts of the chip)
  52        Enable DMA/processing engines
  53
  54When done using the device, and perhaps the module needs to be unloaded,
  55the driver needs to take the follow steps:
  56        Disable the device from generating IRQs
  57        Release the IRQ (free_irq())
  58        Stop all DMA activity
  59        Release DMA buffers (both streaming and coherent)
  60        Unregister from other subsystems (e.g. scsi or netdev)
  61        Release MMIO/IOP resources
  62        Disable the device
  63
  64Most of these topics are covered in the following sections.
  65For the rest look at LDD3 or <linux/pci.h> .
  66
  67If the PCI subsystem is not configured (CONFIG_PCI is not set), most of
  68the PCI functions described below are defined as inline functions either
  69completely empty or just returning an appropriate error codes to avoid
  70lots of ifdefs in the drivers.
  71
  72
  73
  741. pci_register_driver() call
  75~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  76
  77PCI device drivers call pci_register_driver() during their
  78initialization with a pointer to a structure describing the driver
  79(struct pci_driver):
  80
  81        field name      Description
  82        ----------      ------------------------------------------------------
  83        id_table        Pointer to table of device ID's the driver is
  84                        interested in.  Most drivers should export this
  85                        table using MODULE_DEVICE_TABLE(pci,...).
  86
  87        probe           This probing function gets called (during execution
  88                        of pci_register_driver() for already existing
  89                        devices or later if a new device gets inserted) for
  90                        all PCI devices which match the ID table and are not
  91                        "owned" by the other drivers yet. This function gets
  92                        passed a "struct pci_dev *" for each device whose
  93                        entry in the ID table matches the device. The probe
  94                        function returns zero when the driver chooses to
  95                        take "ownership" of the device or an error code
  96                        (negative number) otherwise.
  97                        The probe function always gets called from process
  98                        context, so it can sleep.
  99
 100        remove          The remove() function gets called whenever a device
 101                        being handled by this driver is removed (either during
 102                        deregistration of the driver or when it's manually
 103                        pulled out of a hot-pluggable slot).
 104                        The remove function always gets called from process
 105                        context, so it can sleep.
 106
 107        suspend         Put device into low power state.
 108        suspend_late    Put device into low power state.
 109
 110        resume_early    Wake device from low power state.
 111        resume          Wake device from low power state.
 112
 113                (Please see Documentation/power/pci.txt for descriptions
 114                of PCI Power Management and the related functions.)
 115
 116        shutdown        Hook into reboot_notifier_list (kernel/sys.c).
 117                        Intended to stop any idling DMA operations.
 118                        Useful for enabling wake-on-lan (NIC) or changing
 119                        the power state of a device before reboot.
 120                        e.g. drivers/net/e100.c.
 121
 122        err_handler     See Documentation/PCI/pci-error-recovery.txt
 123
 124
 125The ID table is an array of struct pci_device_id entries ending with an
 126all-zero entry; use of the macro DEFINE_PCI_DEVICE_TABLE is the preferred
 127method of declaring the table.  Each entry consists of:
 128
 129        vendor,device   Vendor and device ID to match (or PCI_ANY_ID)
 130
 131        subvendor,      Subsystem vendor and device ID to match (or PCI_ANY_ID)
 132        subdevice,
 133
 134        class           Device class, subclass, and "interface" to match.
 135                        See Appendix D of the PCI Local Bus Spec or
 136                        include/linux/pci_ids.h for a full list of classes.
 137                        Most drivers do not need to specify class/class_mask
 138                        as vendor/device is normally sufficient.
 139
 140        class_mask      limit which sub-fields of the class field are compared.
 141                        See drivers/scsi/sym53c8xx_2/ for example of usage.
 142
 143        driver_data     Data private to the driver.
 144                        Most drivers don't need to use driver_data field.
 145                        Best practice is to use driver_data as an index
 146                        into a static list of equivalent device types,
 147                        instead of using it as a pointer.
 148
 149
 150Most drivers only need PCI_DEVICE() or PCI_DEVICE_CLASS() to set up
 151a pci_device_id table.
 152
 153New PCI IDs may be added to a device driver pci_ids table at runtime
 154as shown below:
 155
 156echo "vendor device subvendor subdevice class class_mask driver_data" > \
 157/sys/bus/pci/drivers/{driver}/new_id
 158
 159All fields are passed in as hexadecimal values (no leading 0x).
 160The vendor and device fields are mandatory, the others are optional. Users
 161need pass only as many optional fields as necessary:
 162        o subvendor and subdevice fields default to PCI_ANY_ID (FFFFFFFF)
 163        o class and classmask fields default to 0
 164        o driver_data defaults to 0UL.
 165
 166Note that driver_data must match the value used by any of the pci_device_id
 167entries defined in the driver. This makes the driver_data field mandatory
 168if all the pci_device_id entries have a non-zero driver_data value.
 169
 170Once added, the driver probe routine will be invoked for any unclaimed
 171PCI devices listed in its (newly updated) pci_ids list.
 172
 173When the driver exits, it just calls pci_unregister_driver() and the PCI layer
 174automatically calls the remove hook for all devices handled by the driver.
 175
 176
 1771.1 "Attributes" for driver functions/data
 178
 179Please mark the initialization and cleanup functions where appropriate
 180(the corresponding macros are defined in <linux/init.h>):
 181
 182        __init          Initialization code. Thrown away after the driver
 183                        initializes.
 184        __exit          Exit code. Ignored for non-modular drivers.
 185
 186
 187        __devinit       Device initialization code.
 188                        Identical to __init if the kernel is not compiled
 189                        with CONFIG_HOTPLUG, normal function otherwise.
 190        __devexit       The same for __exit.
 191
 192Tips on when/where to use the above attributes:
 193        o The module_init()/module_exit() functions (and all
 194          initialization functions called _only_ from these)
 195          should be marked __init/__exit.
 196
 197        o Do not mark the struct pci_driver.
 198
 199        o The ID table array should be marked __devinitconst; this is done
 200          automatically if the table is declared with DEFINE_PCI_DEVICE_TABLE().
 201
 202        o The probe() and remove() functions should be marked __devinit
 203          and __devexit respectively.  All initialization functions
 204          exclusively called by the probe() routine, can be marked __devinit.
 205          Ditto for remove() and __devexit.
 206
 207        o If mydriver_remove() is marked with __devexit(), then all address
 208          references to mydriver_remove must use __devexit_p(mydriver_remove)
 209          (in the struct pci_driver declaration for example).
 210          __devexit_p() will generate the function name _or_ NULL if the
 211          function will be discarded.  For an example, see drivers/net/tg3.c.
 212
 213        o Do NOT mark a function if you are not sure which mark to use.
 214          Better to not mark the function than mark the function wrong.
 215
 216
 217
 2182. How to find PCI devices manually
 219~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 220
 221PCI drivers should have a really good reason for not using the
 222pci_register_driver() interface to search for PCI devices.
 223The main reason PCI devices are controlled by multiple drivers
 224is because one PCI device implements several different HW services.
 225E.g. combined serial/parallel port/floppy controller.
 226
 227A manual search may be performed using the following constructs:
 228
 229Searching by vendor and device ID:
 230
 231        struct pci_dev *dev = NULL;
 232        while (dev = pci_get_device(VENDOR_ID, DEVICE_ID, dev))
 233                configure_device(dev);
 234
 235Searching by class ID (iterate in a similar way):
 236
 237        pci_get_class(CLASS_ID, dev)
 238
 239Searching by both vendor/device and subsystem vendor/device ID:
 240
 241        pci_get_subsys(VENDOR_ID,DEVICE_ID, SUBSYS_VENDOR_ID, SUBSYS_DEVICE_ID, dev).
 242
 243You can use the constant PCI_ANY_ID as a wildcard replacement for
 244VENDOR_ID or DEVICE_ID.  This allows searching for any device from a
 245specific vendor, for example.
 246
 247These functions are hotplug-safe. They increment the reference count on
 248the pci_dev that they return. You must eventually (possibly at module unload)
 249decrement the reference count on these devices by calling pci_dev_put().
 250
 251
 252
 2533. Device Initialization Steps
 254~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 255
 256As noted in the introduction, most PCI drivers need the following steps
 257for device initialization:
 258
 259        Enable the device
 260        Request MMIO/IOP resources
 261        Set the DMA mask size (for both coherent and streaming DMA)
 262        Allocate and initialize shared control data (pci_allocate_coherent())
 263        Access device configuration space (if needed)
 264        Register IRQ handler (request_irq())
 265        Initialize non-PCI (i.e. LAN/SCSI/etc parts of the chip)
 266        Enable DMA/processing engines.
 267
 268The driver can access PCI config space registers at any time.
 269(Well, almost. When running BIST, config space can go away...but
 270that will just result in a PCI Bus Master Abort and config reads
 271will return garbage).
 272
 273
 2743.1 Enable the PCI device
 275~~~~~~~~~~~~~~~~~~~~~~~~~
 276Before touching any device registers, the driver needs to enable
 277the PCI device by calling pci_enable_device(). This will:
 278        o wake up the device if it was in suspended state,
 279        o allocate I/O and memory regions of the device (if BIOS did not),
 280        o allocate an IRQ (if BIOS did not).
 281
 282NOTE: pci_enable_device() can fail! Check the return value.
 283
 284[ OS BUG: we don't check resource allocations before enabling those
 285  resources. The sequence would make more sense if we called
 286  pci_request_resources() before calling pci_enable_device().
 287  Currently, the device drivers can't detect the bug when when two
 288  devices have been allocated the same range. This is not a common
 289  problem and unlikely to get fixed soon.
 290
 291  This has been discussed before but not changed as of 2.6.19:
 292        http://lkml.org/lkml/2006/3/2/194
 293]
 294
 295pci_set_master() will enable DMA by setting the bus master bit
 296in the PCI_COMMAND register. It also fixes the latency timer value if
 297it's set to something bogus by the BIOS.  pci_clear_master() will
 298disable DMA by clearing the bus master bit.
 299
 300If the PCI device can use the PCI Memory-Write-Invalidate transaction,
 301call pci_set_mwi().  This enables the PCI_COMMAND bit for Mem-Wr-Inval
 302and also ensures that the cache line size register is set correctly.
 303Check the return value of pci_set_mwi() as not all architectures
 304or chip-sets may support Memory-Write-Invalidate.  Alternatively,
 305if Mem-Wr-Inval would be nice to have but is not required, call
 306pci_try_set_mwi() to have the system do its best effort at enabling
 307Mem-Wr-Inval.
 308
 309
 3103.2 Request MMIO/IOP resources
 311~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 312Memory (MMIO), and I/O port addresses should NOT be read directly
 313from the PCI device config space. Use the values in the pci_dev structure
 314as the PCI "bus address" might have been remapped to a "host physical"
 315address by the arch/chip-set specific kernel support.
 316
 317See Documentation/io-mapping.txt for how to access device registers
 318or device memory.
 319
 320The device driver needs to call pci_request_region() to verify
 321no other device is already using the same address resource.
 322Conversely, drivers should call pci_release_region() AFTER
 323calling pci_disable_device().
 324The idea is to prevent two devices colliding on the same address range.
 325
 326[ See OS BUG comment above. Currently (2.6.19), The driver can only
 327  determine MMIO and IO Port resource availability _after_ calling
 328  pci_enable_device(). ]
 329
 330Generic flavors of pci_request_region() are request_mem_region()
 331(for MMIO ranges) and request_region() (for IO Port ranges).
 332Use these for address resources that are not described by "normal" PCI
 333BARs.
 334
 335Also see pci_request_selected_regions() below.
 336
 337
 3383.3 Set the DMA mask size
 339~~~~~~~~~~~~~~~~~~~~~~~~~
 340[ If anything below doesn't make sense, please refer to
 341  Documentation/DMA-API.txt. This section is just a reminder that
 342  drivers need to indicate DMA capabilities of the device and is not
 343  an authoritative source for DMA interfaces. ]
 344
 345While all drivers should explicitly indicate the DMA capability
 346(e.g. 32 or 64 bit) of the PCI bus master, devices with more than
 34732-bit bus master capability for streaming data need the driver
 348to "register" this capability by calling pci_set_dma_mask() with
 349appropriate parameters.  In general this allows more efficient DMA
 350on systems where System RAM exists above 4G _physical_ address.
 351
 352Drivers for all PCI-X and PCIe compliant devices must call
 353pci_set_dma_mask() as they are 64-bit DMA devices.
 354
 355Similarly, drivers must also "register" this capability if the device
 356can directly address "consistent memory" in System RAM above 4G physical
 357address by calling pci_set_consistent_dma_mask().
 358Again, this includes drivers for all PCI-X and PCIe compliant devices.
 359Many 64-bit "PCI" devices (before PCI-X) and some PCI-X devices are
 36064-bit DMA capable for payload ("streaming") data but not control
 361("consistent") data.
 362
 363
 3643.4 Setup shared control data
 365~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 366Once the DMA masks are set, the driver can allocate "consistent" (a.k.a. shared)
 367memory.  See Documentation/DMA-API.txt for a full description of
 368the DMA APIs. This section is just a reminder that it needs to be done
 369before enabling DMA on the device.
 370
 371
 3723.5 Initialize device registers
 373~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 374Some drivers will need specific "capability" fields programmed
 375or other "vendor specific" register initialized or reset.
 376E.g. clearing pending interrupts.
 377
 378
 3793.6 Register IRQ handler
 380~~~~~~~~~~~~~~~~~~~~~~~~
 381While calling request_irq() is the last step described here,
 382this is often just another intermediate step to initialize a device.
 383This step can often be deferred until the device is opened for use.
 384
 385All interrupt handlers for IRQ lines should be registered with IRQF_SHARED
 386and use the devid to map IRQs to devices (remember that all PCI IRQ lines
 387can be shared).
 388
 389request_irq() will associate an interrupt handler and device handle
 390with an interrupt number. Historically interrupt numbers represent
 391IRQ lines which run from the PCI device to the Interrupt controller.
 392With MSI and MSI-X (more below) the interrupt number is a CPU "vector".
 393
 394request_irq() also enables the interrupt. Make sure the device is
 395quiesced and does not have any interrupts pending before registering
 396the interrupt handler.
 397
 398MSI and MSI-X are PCI capabilities. Both are "Message Signaled Interrupts"
 399which deliver interrupts to the CPU via a DMA write to a Local APIC.
 400The fundamental difference between MSI and MSI-X is how multiple
 401"vectors" get allocated. MSI requires contiguous blocks of vectors
 402while MSI-X can allocate several individual ones.
 403
 404MSI capability can be enabled by calling pci_enable_msi() or
 405pci_enable_msix() before calling request_irq(). This causes
 406the PCI support to program CPU vector data into the PCI device
 407capability registers.
 408
 409If your PCI device supports both, try to enable MSI-X first.
 410Only one can be enabled at a time.  Many architectures, chip-sets,
 411or BIOSes do NOT support MSI or MSI-X and the call to pci_enable_msi/msix
 412will fail. This is important to note since many drivers have
 413two (or more) interrupt handlers: one for MSI/MSI-X and another for IRQs.
 414They choose which handler to register with request_irq() based on the
 415return value from pci_enable_msi/msix().
 416
 417There are (at least) two really good reasons for using MSI:
 4181) MSI is an exclusive interrupt vector by definition.
 419   This means the interrupt handler doesn't have to verify
 420   its device caused the interrupt.
 421
 4222) MSI avoids DMA/IRQ race conditions. DMA to host memory is guaranteed
 423   to be visible to the host CPU(s) when the MSI is delivered. This
 424   is important for both data coherency and avoiding stale control data.
 425   This guarantee allows the driver to omit MMIO reads to flush
 426   the DMA stream.
 427
 428See drivers/infiniband/hw/mthca/ or drivers/net/tg3.c for examples
 429of MSI/MSI-X usage.
 430
 431
 432
 4334. PCI device shutdown
 434~~~~~~~~~~~~~~~~~~~~~~~
 435
 436When a PCI device driver is being unloaded, most of the following
 437steps need to be performed:
 438
 439        Disable the device from generating IRQs
 440        Release the IRQ (free_irq())
 441        Stop all DMA activity
 442        Release DMA buffers (both streaming and consistent)
 443        Unregister from other subsystems (e.g. scsi or netdev)
 444        Disable device from responding to MMIO/IO Port addresses
 445        Release MMIO/IO Port resource(s)
 446
 447
 4484.1 Stop IRQs on the device
 449~~~~~~~~~~~~~~~~~~~~~~~~~~~
 450How to do this is chip/device specific. If it's not done, it opens
 451the possibility of a "screaming interrupt" if (and only if)
 452the IRQ is shared with another device.
 453
 454When the shared IRQ handler is "unhooked", the remaining devices
 455using the same IRQ line will still need the IRQ enabled. Thus if the
 456"unhooked" device asserts IRQ line, the system will respond assuming
 457it was one of the remaining devices asserted the IRQ line. Since none
 458of the other devices will handle the IRQ, the system will "hang" until
 459it decides the IRQ isn't going to get handled and masks the IRQ (100,000
 460iterations later). Once the shared IRQ is masked, the remaining devices
 461will stop functioning properly. Not a nice situation.
 462
 463This is another reason to use MSI or MSI-X if it's available.
 464MSI and MSI-X are defined to be exclusive interrupts and thus
 465are not susceptible to the "screaming interrupt" problem.
 466
 467
 4684.2 Release the IRQ
 469~~~~~~~~~~~~~~~~~~~
 470Once the device is quiesced (no more IRQs), one can call free_irq().
 471This function will return control once any pending IRQs are handled,
 472"unhook" the drivers IRQ handler from that IRQ, and finally release
 473the IRQ if no one else is using it.
 474
 475
 4764.3 Stop all DMA activity
 477~~~~~~~~~~~~~~~~~~~~~~~~~
 478It's extremely important to stop all DMA operations BEFORE attempting
 479to deallocate DMA control data. Failure to do so can result in memory
 480corruption, hangs, and on some chip-sets a hard crash.
 481
 482Stopping DMA after stopping the IRQs can avoid races where the
 483IRQ handler might restart DMA engines.
 484
 485While this step sounds obvious and trivial, several "mature" drivers
 486didn't get this step right in the past.
 487
 488
 4894.4 Release DMA buffers
 490~~~~~~~~~~~~~~~~~~~~~~~
 491Once DMA is stopped, clean up streaming DMA first.
 492I.e. unmap data buffers and return buffers to "upstream"
 493owners if there is one.
 494
 495Then clean up "consistent" buffers which contain the control data.
 496
 497See Documentation/DMA-API.txt for details on unmapping interfaces.
 498
 499
 5004.5 Unregister from other subsystems
 501~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 502Most low level PCI device drivers support some other subsystem
 503like USB, ALSA, SCSI, NetDev, Infiniband, etc. Make sure your
 504driver isn't losing resources from that other subsystem.
 505If this happens, typically the symptom is an Oops (panic) when
 506the subsystem attempts to call into a driver that has been unloaded.
 507
 508
 5094.6 Disable Device from responding to MMIO/IO Port addresses
 510~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 511io_unmap() MMIO or IO Port resources and then call pci_disable_device().
 512This is the symmetric opposite of pci_enable_device().
 513Do not access device registers after calling pci_disable_device().
 514
 515
 5164.7 Release MMIO/IO Port Resource(s)
 517~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 518Call pci_release_region() to mark the MMIO or IO Port range as available.
 519Failure to do so usually results in the inability to reload the driver.
 520
 521
 522
 5235. How to access PCI config space
 524~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 525
 526You can use pci_(read|write)_config_(byte|word|dword) to access the config
 527space of a device represented by struct pci_dev *. All these functions return 0
 528when successful or an error code (PCIBIOS_...) which can be translated to a text
 529string by pcibios_strerror. Most drivers expect that accesses to valid PCI
 530devices don't fail.
 531
 532If you don't have a struct pci_dev available, you can call
 533pci_bus_(read|write)_config_(byte|word|dword) to access a given device
 534and function on that bus.
 535
 536If you access fields in the standard portion of the config header, please
 537use symbolic names of locations and bits declared in <linux/pci.h>.
 538
 539If you need to access Extended PCI Capability registers, just call
 540pci_find_capability() for the particular capability and it will find the
 541corresponding register block for you.
 542
 543
 544
 5456. Other interesting functions
 546~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 547
 548pci_find_slot()                 Find pci_dev corresponding to given bus and
 549                                slot numbers.
 550pci_set_power_state()           Set PCI Power Management state (0=D0 ... 3=D3)
 551pci_find_capability()           Find specified capability in device's capability
 552                                list.
 553pci_resource_start()            Returns bus start address for a given PCI region
 554pci_resource_end()              Returns bus end address for a given PCI region
 555pci_resource_len()              Returns the byte length of a PCI region
 556pci_set_drvdata()               Set private driver data pointer for a pci_dev
 557pci_get_drvdata()               Return private driver data pointer for a pci_dev
 558pci_set_mwi()                   Enable Memory-Write-Invalidate transactions.
 559pci_clear_mwi()                 Disable Memory-Write-Invalidate transactions.
 560
 561
 562
 5637. Miscellaneous hints
 564~~~~~~~~~~~~~~~~~~~~~~
 565
 566When displaying PCI device names to the user (for example when a driver wants
 567to tell the user what card has it found), please use pci_name(pci_dev).
 568
 569Always refer to the PCI devices by a pointer to the pci_dev structure.
 570All PCI layer functions use this identification and it's the only
 571reasonable one. Don't use bus/slot/function numbers except for very
 572special purposes -- on systems with multiple primary buses their semantics
 573can be pretty complex.
 574
 575Don't try to turn on Fast Back to Back writes in your driver.  All devices
 576on the bus need to be capable of doing it, so this is something which needs
 577to be handled by platform and generic code, not individual drivers.
 578
 579
 580
 5818. Vendor and device identifications
 582~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 583
 584One is not required to add new device ids to include/linux/pci_ids.h.
 585Please add PCI_VENDOR_ID_xxx for vendors and a hex constant for device ids.
 586
 587PCI_VENDOR_ID_xxx constants are re-used. The device ids are arbitrary
 588hex numbers (vendor controlled) and normally used only in a single
 589location, the pci_device_id table.
 590
 591Please DO submit new vendor/device ids to pciids.sourceforge.net project.
 592
 593
 594
 5959. Obsolete functions
 596~~~~~~~~~~~~~~~~~~~~~
 597
 598There are several functions which you might come across when trying to
 599port an old driver to the new PCI interface.  They are no longer present
 600in the kernel as they aren't compatible with hotplug or PCI domains or
 601having sane locking.
 602
 603pci_find_device()       Superseded by pci_get_device()
 604pci_find_subsys()       Superseded by pci_get_subsys()
 605pci_find_slot()         Superseded by pci_get_slot()
 606
 607
 608The alternative is the traditional PCI device driver that walks PCI
 609device lists. This is still possible but discouraged.
 610
 611
 612
 61310. MMIO Space and "Write Posting"
 614~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 615
 616Converting a driver from using I/O Port space to using MMIO space
 617often requires some additional changes. Specifically, "write posting"
 618needs to be handled. Many drivers (e.g. tg3, acenic, sym53c8xx_2)
 619already do this. I/O Port space guarantees write transactions reach the PCI
 620device before the CPU can continue. Writes to MMIO space allow the CPU
 621to continue before the transaction reaches the PCI device. HW weenies
 622call this "Write Posting" because the write completion is "posted" to
 623the CPU before the transaction has reached its destination.
 624
 625Thus, timing sensitive code should add readl() where the CPU is
 626expected to wait before doing other work.  The classic "bit banging"
 627sequence works fine for I/O Port space:
 628
 629       for (i = 8; --i; val >>= 1) {
 630               outb(val & 1, ioport_reg);      /* write bit */
 631               udelay(10);
 632       }
 633
 634The same sequence for MMIO space should be:
 635
 636       for (i = 8; --i; val >>= 1) {
 637               writeb(val & 1, mmio_reg);      /* write bit */
 638               readb(safe_mmio_reg);           /* flush posted write */
 639               udelay(10);
 640       }
 641
 642It is important that "safe_mmio_reg" not have any side effects that
 643interferes with the correct operation of the device.
 644
 645Another case to watch out for is when resetting a PCI device. Use PCI
 646Configuration space reads to flush the writel(). This will gracefully
 647handle the PCI master abort on all platforms if the PCI device is
 648expected to not respond to a readl().  Most x86 platforms will allow
 649MMIO reads to master abort (a.k.a. "Soft Fail") and return garbage
 650(e.g. ~0). But many RISC platforms will crash (a.k.a."Hard Fail").
 651
 652
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