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
 186Tips on when/where to use the above attributes:
 187        o The module_init()/module_exit() functions (and all
 188          initialization functions called _only_ from these)
 189          should be marked __init/__exit.
 190
 191        o Do not mark the struct pci_driver.
 192
 193        o Do NOT mark a function if you are not sure which mark to use.
 194          Better to not mark the function than mark the function wrong.
 195
 196
 197
 1982. How to find PCI devices manually
 199~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 200
 201PCI drivers should have a really good reason for not using the
 202pci_register_driver() interface to search for PCI devices.
 203The main reason PCI devices are controlled by multiple drivers
 204is because one PCI device implements several different HW services.
 205E.g. combined serial/parallel port/floppy controller.
 206
 207A manual search may be performed using the following constructs:
 208
 209Searching by vendor and device ID:
 210
 211        struct pci_dev *dev = NULL;
 212        while (dev = pci_get_device(VENDOR_ID, DEVICE_ID, dev))
 213                configure_device(dev);
 214
 215Searching by class ID (iterate in a similar way):
 216
 217        pci_get_class(CLASS_ID, dev)
 218
 219Searching by both vendor/device and subsystem vendor/device ID:
 220
 221        pci_get_subsys(VENDOR_ID,DEVICE_ID, SUBSYS_VENDOR_ID, SUBSYS_DEVICE_ID, dev).
 222
 223You can use the constant PCI_ANY_ID as a wildcard replacement for
 224VENDOR_ID or DEVICE_ID.  This allows searching for any device from a
 225specific vendor, for example.
 226
 227These functions are hotplug-safe. They increment the reference count on
 228the pci_dev that they return. You must eventually (possibly at module unload)
 229decrement the reference count on these devices by calling pci_dev_put().
 230
 231
 232
 2333. Device Initialization Steps
 234~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 235
 236As noted in the introduction, most PCI drivers need the following steps
 237for device initialization:
 238
 239        Enable the device
 240        Request MMIO/IOP resources
 241        Set the DMA mask size (for both coherent and streaming DMA)
 242        Allocate and initialize shared control data (pci_allocate_coherent())
 243        Access device configuration space (if needed)
 244        Register IRQ handler (request_irq())
 245        Initialize non-PCI (i.e. LAN/SCSI/etc parts of the chip)
 246        Enable DMA/processing engines.
 247
 248The driver can access PCI config space registers at any time.
 249(Well, almost. When running BIST, config space can go away...but
 250that will just result in a PCI Bus Master Abort and config reads
 251will return garbage).
 252
 253
 2543.1 Enable the PCI device
 255~~~~~~~~~~~~~~~~~~~~~~~~~
 256Before touching any device registers, the driver needs to enable
 257the PCI device by calling pci_enable_device(). This will:
 258        o wake up the device if it was in suspended state,
 259        o allocate I/O and memory regions of the device (if BIOS did not),
 260        o allocate an IRQ (if BIOS did not).
 261
 262NOTE: pci_enable_device() can fail! Check the return value.
 263
 264[ OS BUG: we don't check resource allocations before enabling those
 265  resources. The sequence would make more sense if we called
 266  pci_request_resources() before calling pci_enable_device().
 267  Currently, the device drivers can't detect the bug when when two
 268  devices have been allocated the same range. This is not a common
 269  problem and unlikely to get fixed soon.
 270
 271  This has been discussed before but not changed as of 2.6.19:
 272        http://lkml.org/lkml/2006/3/2/194
 273]
 274
 275pci_set_master() will enable DMA by setting the bus master bit
 276in the PCI_COMMAND register. It also fixes the latency timer value if
 277it's set to something bogus by the BIOS.  pci_clear_master() will
 278disable DMA by clearing the bus master bit.
 279
 280If the PCI device can use the PCI Memory-Write-Invalidate transaction,
 281call pci_set_mwi().  This enables the PCI_COMMAND bit for Mem-Wr-Inval
 282and also ensures that the cache line size register is set correctly.
 283Check the return value of pci_set_mwi() as not all architectures
 284or chip-sets may support Memory-Write-Invalidate.  Alternatively,
 285if Mem-Wr-Inval would be nice to have but is not required, call
 286pci_try_set_mwi() to have the system do its best effort at enabling
 287Mem-Wr-Inval.
 288
 289
 2903.2 Request MMIO/IOP resources
 291~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 292Memory (MMIO), and I/O port addresses should NOT be read directly
 293from the PCI device config space. Use the values in the pci_dev structure
 294as the PCI "bus address" might have been remapped to a "host physical"
 295address by the arch/chip-set specific kernel support.
 296
 297See Documentation/io-mapping.txt for how to access device registers
 298or device memory.
 299
 300The device driver needs to call pci_request_region() to verify
 301no other device is already using the same address resource.
 302Conversely, drivers should call pci_release_region() AFTER
 303calling pci_disable_device().
 304The idea is to prevent two devices colliding on the same address range.
 305
 306[ See OS BUG comment above. Currently (2.6.19), The driver can only
 307  determine MMIO and IO Port resource availability _after_ calling
 308  pci_enable_device(). ]
 309
 310Generic flavors of pci_request_region() are request_mem_region()
 311(for MMIO ranges) and request_region() (for IO Port ranges).
 312Use these for address resources that are not described by "normal" PCI
 313BARs.
 314
 315Also see pci_request_selected_regions() below.
 316
 317
 3183.3 Set the DMA mask size
 319~~~~~~~~~~~~~~~~~~~~~~~~~
 320[ If anything below doesn't make sense, please refer to
 321  Documentation/DMA-API.txt. This section is just a reminder that
 322  drivers need to indicate DMA capabilities of the device and is not
 323  an authoritative source for DMA interfaces. ]
 324
 325While all drivers should explicitly indicate the DMA capability
 326(e.g. 32 or 64 bit) of the PCI bus master, devices with more than
 32732-bit bus master capability for streaming data need the driver
 328to "register" this capability by calling pci_set_dma_mask() with
 329appropriate parameters.  In general this allows more efficient DMA
 330on systems where System RAM exists above 4G _physical_ address.
 331
 332Drivers for all PCI-X and PCIe compliant devices must call
 333pci_set_dma_mask() as they are 64-bit DMA devices.
 334
 335Similarly, drivers must also "register" this capability if the device
 336can directly address "consistent memory" in System RAM above 4G physical
 337address by calling pci_set_consistent_dma_mask().
 338Again, this includes drivers for all PCI-X and PCIe compliant devices.
 339Many 64-bit "PCI" devices (before PCI-X) and some PCI-X devices are
 34064-bit DMA capable for payload ("streaming") data but not control
 341("consistent") data.
 342
 343
 3443.4 Setup shared control data
 345~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 346Once the DMA masks are set, the driver can allocate "consistent" (a.k.a. shared)
 347memory.  See Documentation/DMA-API.txt for a full description of
 348the DMA APIs. This section is just a reminder that it needs to be done
 349before enabling DMA on the device.
 350
 351
 3523.5 Initialize device registers
 353~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 354Some drivers will need specific "capability" fields programmed
 355or other "vendor specific" register initialized or reset.
 356E.g. clearing pending interrupts.
 357
 358
 3593.6 Register IRQ handler
 360~~~~~~~~~~~~~~~~~~~~~~~~
 361While calling request_irq() is the last step described here,
 362this is often just another intermediate step to initialize a device.
 363This step can often be deferred until the device is opened for use.
 364
 365All interrupt handlers for IRQ lines should be registered with IRQF_SHARED
 366and use the devid to map IRQs to devices (remember that all PCI IRQ lines
 367can be shared).
 368
 369request_irq() will associate an interrupt handler and device handle
 370with an interrupt number. Historically interrupt numbers represent
 371IRQ lines which run from the PCI device to the Interrupt controller.
 372With MSI and MSI-X (more below) the interrupt number is a CPU "vector".
 373
 374request_irq() also enables the interrupt. Make sure the device is
 375quiesced and does not have any interrupts pending before registering
 376the interrupt handler.
 377
 378MSI and MSI-X are PCI capabilities. Both are "Message Signaled Interrupts"
 379which deliver interrupts to the CPU via a DMA write to a Local APIC.
 380The fundamental difference between MSI and MSI-X is how multiple
 381"vectors" get allocated. MSI requires contiguous blocks of vectors
 382while MSI-X can allocate several individual ones.
 383
 384MSI capability can be enabled by calling pci_enable_msi() or
 385pci_enable_msix() before calling request_irq(). This causes
 386the PCI support to program CPU vector data into the PCI device
 387capability registers.
 388
 389If your PCI device supports both, try to enable MSI-X first.
 390Only one can be enabled at a time.  Many architectures, chip-sets,
 391or BIOSes do NOT support MSI or MSI-X and the call to pci_enable_msi/msix
 392will fail. This is important to note since many drivers have
 393two (or more) interrupt handlers: one for MSI/MSI-X and another for IRQs.
 394They choose which handler to register with request_irq() based on the
 395return value from pci_enable_msi/msix().
 396
 397There are (at least) two really good reasons for using MSI:
 3981) MSI is an exclusive interrupt vector by definition.
 399   This means the interrupt handler doesn't have to verify
 400   its device caused the interrupt.
 401
 4022) MSI avoids DMA/IRQ race conditions. DMA to host memory is guaranteed
 403   to be visible to the host CPU(s) when the MSI is delivered. This
 404   is important for both data coherency and avoiding stale control data.
 405   This guarantee allows the driver to omit MMIO reads to flush
 406   the DMA stream.
 407
 408See drivers/infiniband/hw/mthca/ or drivers/net/tg3.c for examples
 409of MSI/MSI-X usage.
 410
 411
 412
 4134. PCI device shutdown
 414~~~~~~~~~~~~~~~~~~~~~~~
 415
 416When a PCI device driver is being unloaded, most of the following
 417steps need to be performed:
 418
 419        Disable the device from generating IRQs
 420        Release the IRQ (free_irq())
 421        Stop all DMA activity
 422        Release DMA buffers (both streaming and consistent)
 423        Unregister from other subsystems (e.g. scsi or netdev)
 424        Disable device from responding to MMIO/IO Port addresses
 425        Release MMIO/IO Port resource(s)
 426
 427
 4284.1 Stop IRQs on the device
 429~~~~~~~~~~~~~~~~~~~~~~~~~~~
 430How to do this is chip/device specific. If it's not done, it opens
 431the possibility of a "screaming interrupt" if (and only if)
 432the IRQ is shared with another device.
 433
 434When the shared IRQ handler is "unhooked", the remaining devices
 435using the same IRQ line will still need the IRQ enabled. Thus if the
 436"unhooked" device asserts IRQ line, the system will respond assuming
 437it was one of the remaining devices asserted the IRQ line. Since none
 438of the other devices will handle the IRQ, the system will "hang" until
 439it decides the IRQ isn't going to get handled and masks the IRQ (100,000
 440iterations later). Once the shared IRQ is masked, the remaining devices
 441will stop functioning properly. Not a nice situation.
 442
 443This is another reason to use MSI or MSI-X if it's available.
 444MSI and MSI-X are defined to be exclusive interrupts and thus
 445are not susceptible to the "screaming interrupt" problem.
 446
 447
 4484.2 Release the IRQ
 449~~~~~~~~~~~~~~~~~~~
 450Once the device is quiesced (no more IRQs), one can call free_irq().
 451This function will return control once any pending IRQs are handled,
 452"unhook" the drivers IRQ handler from that IRQ, and finally release
 453the IRQ if no one else is using it.
 454
 455
 4564.3 Stop all DMA activity
 457~~~~~~~~~~~~~~~~~~~~~~~~~
 458It's extremely important to stop all DMA operations BEFORE attempting
 459to deallocate DMA control data. Failure to do so can result in memory
 460corruption, hangs, and on some chip-sets a hard crash.
 461
 462Stopping DMA after stopping the IRQs can avoid races where the
 463IRQ handler might restart DMA engines.
 464
 465While this step sounds obvious and trivial, several "mature" drivers
 466didn't get this step right in the past.
 467
 468
 4694.4 Release DMA buffers
 470~~~~~~~~~~~~~~~~~~~~~~~
 471Once DMA is stopped, clean up streaming DMA first.
 472I.e. unmap data buffers and return buffers to "upstream"
 473owners if there is one.
 474
 475Then clean up "consistent" buffers which contain the control data.
 476
 477See Documentation/DMA-API.txt for details on unmapping interfaces.
 478
 479
 4804.5 Unregister from other subsystems
 481~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 482Most low level PCI device drivers support some other subsystem
 483like USB, ALSA, SCSI, NetDev, Infiniband, etc. Make sure your
 484driver isn't losing resources from that other subsystem.
 485If this happens, typically the symptom is an Oops (panic) when
 486the subsystem attempts to call into a driver that has been unloaded.
 487
 488
 4894.6 Disable Device from responding to MMIO/IO Port addresses
 490~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 491io_unmap() MMIO or IO Port resources and then call pci_disable_device().
 492This is the symmetric opposite of pci_enable_device().
 493Do not access device registers after calling pci_disable_device().
 494
 495
 4964.7 Release MMIO/IO Port Resource(s)
 497~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 498Call pci_release_region() to mark the MMIO or IO Port range as available.
 499Failure to do so usually results in the inability to reload the driver.
 500
 501
 502
 5035. How to access PCI config space
 504~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 505
 506You can use pci_(read|write)_config_(byte|word|dword) to access the config
 507space of a device represented by struct pci_dev *. All these functions return 0
 508when successful or an error code (PCIBIOS_...) which can be translated to a text
 509string by pcibios_strerror. Most drivers expect that accesses to valid PCI
 510devices don't fail.
 511
 512If you don't have a struct pci_dev available, you can call
 513pci_bus_(read|write)_config_(byte|word|dword) to access a given device
 514and function on that bus.
 515
 516If you access fields in the standard portion of the config header, please
 517use symbolic names of locations and bits declared in <linux/pci.h>.
 518
 519If you need to access Extended PCI Capability registers, just call
 520pci_find_capability() for the particular capability and it will find the
 521corresponding register block for you.
 522
 523
 524
 5256. Other interesting functions
 526~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 527
 528pci_find_slot()                 Find pci_dev corresponding to given bus and
 529                                slot numbers.
 530pci_set_power_state()           Set PCI Power Management state (0=D0 ... 3=D3)
 531pci_find_capability()           Find specified capability in device's capability
 532                                list.
 533pci_resource_start()            Returns bus start address for a given PCI region
 534pci_resource_end()              Returns bus end address for a given PCI region
 535pci_resource_len()              Returns the byte length of a PCI region
 536pci_set_drvdata()               Set private driver data pointer for a pci_dev
 537pci_get_drvdata()               Return private driver data pointer for a pci_dev
 538pci_set_mwi()                   Enable Memory-Write-Invalidate transactions.
 539pci_clear_mwi()                 Disable Memory-Write-Invalidate transactions.
 540
 541
 542
 5437. Miscellaneous hints
 544~~~~~~~~~~~~~~~~~~~~~~
 545
 546When displaying PCI device names to the user (for example when a driver wants
 547to tell the user what card has it found), please use pci_name(pci_dev).
 548
 549Always refer to the PCI devices by a pointer to the pci_dev structure.
 550All PCI layer functions use this identification and it's the only
 551reasonable one. Don't use bus/slot/function numbers except for very
 552special purposes -- on systems with multiple primary buses their semantics
 553can be pretty complex.
 554
 555Don't try to turn on Fast Back to Back writes in your driver.  All devices
 556on the bus need to be capable of doing it, so this is something which needs
 557to be handled by platform and generic code, not individual drivers.
 558
 559
 560
 5618. Vendor and device identifications
 562~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 563
 564One is not required to add new device ids to include/linux/pci_ids.h.
 565Please add PCI_VENDOR_ID_xxx for vendors and a hex constant for device ids.
 566
 567PCI_VENDOR_ID_xxx constants are re-used. The device ids are arbitrary
 568hex numbers (vendor controlled) and normally used only in a single
 569location, the pci_device_id table.
 570
 571Please DO submit new vendor/device ids to pciids.sourceforge.net project.
 572
 573
 574
 5759. Obsolete functions
 576~~~~~~~~~~~~~~~~~~~~~
 577
 578There are several functions which you might come across when trying to
 579port an old driver to the new PCI interface.  They are no longer present
 580in the kernel as they aren't compatible with hotplug or PCI domains or
 581having sane locking.
 582
 583pci_find_device()       Superseded by pci_get_device()
 584pci_find_subsys()       Superseded by pci_get_subsys()
 585pci_find_slot()         Superseded by pci_get_slot()
 586
 587
 588The alternative is the traditional PCI device driver that walks PCI
 589device lists. This is still possible but discouraged.
 590
 591
 592
 59310. MMIO Space and "Write Posting"
 594~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 595
 596Converting a driver from using I/O Port space to using MMIO space
 597often requires some additional changes. Specifically, "write posting"
 598needs to be handled. Many drivers (e.g. tg3, acenic, sym53c8xx_2)
 599already do this. I/O Port space guarantees write transactions reach the PCI
 600device before the CPU can continue. Writes to MMIO space allow the CPU
 601to continue before the transaction reaches the PCI device. HW weenies
 602call this "Write Posting" because the write completion is "posted" to
 603the CPU before the transaction has reached its destination.
 604
 605Thus, timing sensitive code should add readl() where the CPU is
 606expected to wait before doing other work.  The classic "bit banging"
 607sequence works fine for I/O Port space:
 608
 609       for (i = 8; --i; val >>= 1) {
 610               outb(val & 1, ioport_reg);      /* write bit */
 611               udelay(10);
 612       }
 613
 614The same sequence for MMIO space should be:
 615
 616       for (i = 8; --i; val >>= 1) {
 617               writeb(val & 1, mmio_reg);      /* write bit */
 618               readb(safe_mmio_reg);           /* flush posted write */
 619               udelay(10);
 620       }
 621
 622It is important that "safe_mmio_reg" not have any side effects that
 623interferes with the correct operation of the device.
 624
 625Another case to watch out for is when resetting a PCI device. Use PCI
 626Configuration space reads to flush the writel(). This will gracefully
 627handle the PCI master abort on all platforms if the PCI device is
 628expected to not respond to a readl().  Most x86 platforms will allow
 629MMIO reads to master abort (a.k.a. "Soft Fail") and return garbage
 630(e.g. ~0). But many RISC platforms will crash (a.k.a."Hard Fail").
 631
 632
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