How To Write Linux PCI Drivers
  
  		by Martin Mares <mj@ucw.cz> on 07-Feb-2000
  	updated by Grant Grundler <grundler@parisc-linux.org> on 23-Dec-2006

  
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  The world of PCI is vast and full of (mostly unpleasant) surprises.
  Since each CPU architecture implements different chip-sets and PCI devices
  have different requirements (erm, "features"), the result is the PCI support
  in the Linux kernel is not as trivial as one would wish. This short paper
  tries to introduce all potential driver authors to Linux APIs for
  PCI device drivers.
  
  A more complete resource is the third edition of "Linux Device Drivers"
  by Jonathan Corbet, Alessandro Rubini, and Greg Kroah-Hartman.
  LDD3 is available for free (under Creative Commons License) from:
  
  	http://lwn.net/Kernel/LDD3/
  
  However, keep in mind that all documents are subject to "bit rot".
  Refer to the source code if things are not working as described here.
  
  Please send questions/comments/patches about Linux PCI API to the
  "Linux PCI" <linux-pci@atrey.karlin.mff.cuni.cz> mailing list.

  
  
  0. Structure of PCI drivers
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~

  PCI drivers "discover" PCI devices in a system via pci_register_driver().
  Actually, it's the other way around. When the PCI generic code discovers
  a new device, the driver with a matching "description" will be notified.
  Details on this below.
  
  pci_register_driver() leaves most of the probing for devices to
  the PCI layer and supports online insertion/removal of devices [thus
  supporting hot-pluggable PCI, CardBus, and Express-Card in a single driver].
  pci_register_driver() call requires passing in a table of function
  pointers and thus dictates the high level structure of a driver.
  
  Once the driver knows about a PCI device and takes ownership, the
  driver generally needs to perform the following initialization:

  
  	Enable the device

  	Request MMIO/IOP resources
  	Set the DMA mask size (for both coherent and streaming DMA)
  	Allocate and initialize shared control data (pci_allocate_coherent())
  	Access device configuration space (if needed)
  	Register IRQ handler (request_irq())
  	Initialize non-PCI (i.e. LAN/SCSI/etc parts of the chip)
  	Enable DMA/processing engines
  
  When done using the device, and perhaps the module needs to be unloaded,
  the driver needs to take the follow steps:
  	Disable the device from generating IRQs
  	Release the IRQ (free_irq())
  	Stop all DMA activity
  	Release DMA buffers (both streaming and coherent)
  	Unregister from other subsystems (e.g. scsi or netdev)
  	Release MMIO/IOP resources

  	Disable the device

  Most of these topics are covered in the following sections.
  For the rest look at LDD3 or <linux/pci.h> .

  
  If the PCI subsystem is not configured (CONFIG_PCI is not set), most of

  the PCI functions described below are defined as inline functions either
  completely empty or just returning an appropriate error codes to avoid
  lots of ifdefs in the drivers.

  1. pci_register_driver() call
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  PCI device drivers call pci_register_driver() during their
  initialization with a pointer to a structure describing the driver
  (struct pci_driver):
  
  	field name	Description
  	----------	------------------------------------------------------

  	id_table	Pointer to table of device ID's the driver is
  			interested in.  Most drivers should export this
  			table using MODULE_DEVICE_TABLE(pci,...).

  
  	probe		This probing function gets called (during execution
  			of pci_register_driver() for already existing
  			devices or later if a new device gets inserted) for
  			all PCI devices which match the ID table and are not
  			"owned" by the other drivers yet. This function gets
  			passed a "struct pci_dev *" for each device whose
  			entry in the ID table matches the device. The probe
  			function returns zero when the driver chooses to
  			take "ownership" of the device or an error code
  			(negative number) otherwise.
  			The probe function always gets called from process
  			context, so it can sleep.
  
  	remove		The remove() function gets called whenever a device
  			being handled by this driver is removed (either during
  			deregistration of the driver or when it's manually
  			pulled out of a hot-pluggable slot).
  			The remove function always gets called from process
  			context, so it can sleep.

  	suspend		Put device into low power state.

  	suspend_late	Put device into low power state.
  
  	resume_early	Wake device from low power state.

  	resume		Wake device from low power state.

  
  		(Please see Documentation/power/pci.txt for descriptions
  		of PCI Power Management and the related functions.)

  	shutdown	Hook into reboot_notifier_list (kernel/sys.c).
  			Intended to stop any idling DMA operations.
  			Useful for enabling wake-on-lan (NIC) or changing
  			the power state of a device before reboot.
  			e.g. drivers/net/e100.c.

  	err_handler	See Documentation/PCI/pci-error-recovery.txt

  The ID table is an array of struct pci_device_id entries ending with an

  all-zero entry; use of the macro DEFINE_PCI_DEVICE_TABLE is the preferred

  method of declaring the table.  Each entry consists of:

  
  	vendor,device	Vendor and device ID to match (or PCI_ANY_ID)

  	subvendor,	Subsystem vendor and device ID to match (or PCI_ANY_ID)

  	subdevice,
  
  	class		Device class, subclass, and "interface" to match.
  			See Appendix D of the PCI Local Bus Spec or
  			include/linux/pci_ids.h for a full list of classes.
  			Most drivers do not need to specify class/class_mask
  			as vendor/device is normally sufficient.
  
  	class_mask	limit which sub-fields of the class field are compared.
  			See drivers/scsi/sym53c8xx_2/ for example of usage.

  	driver_data	Data private to the driver.

  			Most drivers don't need to use driver_data field.
  			Best practice is to use driver_data as an index
  			into a static list of equivalent device types,
  			instead of using it as a pointer.

  Most drivers only need PCI_DEVICE() or PCI_DEVICE_CLASS() to set up
  a pci_device_id table.

  New PCI IDs may be added to a device driver pci_ids table at runtime
  as shown below:

  
  echo "vendor device subvendor subdevice class class_mask driver_data" > \

  /sys/bus/pci/drivers/{driver}/new_id
  
  All fields are passed in as hexadecimal values (no leading 0x).

  The vendor and device fields are mandatory, the others are optional. Users
  need pass only as many optional fields as necessary:
  	o subvendor and subdevice fields default to PCI_ANY_ID (FFFFFFFF)

  	o class and classmask fields default to 0
  	o driver_data defaults to 0UL.

  Note that driver_data must match the value used by any of the pci_device_id
  entries defined in the driver. This makes the driver_data field mandatory
  if all the pci_device_id entries have a non-zero driver_data value.

  Once added, the driver probe routine will be invoked for any unclaimed
  PCI devices listed in its (newly updated) pci_ids list.

  
  When the driver exits, it just calls pci_unregister_driver() and the PCI layer
  automatically calls the remove hook for all devices handled by the driver.

  
  1.1 "Attributes" for driver functions/data

  Please mark the initialization and cleanup functions where appropriate
  (the corresponding macros are defined in <linux/init.h>):
  
  	__init		Initialization code. Thrown away after the driver
  			initializes.
  	__exit		Exit code. Ignored for non-modular drivers.

  
  
  	__devinit	Device initialization code.
  			Identical to __init if the kernel is not compiled
  			with CONFIG_HOTPLUG, normal function otherwise.

  	__devexit	The same for __exit.

  Tips on when/where to use the above attributes:
  	o The module_init()/module_exit() functions (and all
  	  initialization functions called _only_ from these)
  	  should be marked __init/__exit.

  	o Do not mark the struct pci_driver.

  	o The ID table array should be marked __devinitconst; this is done

  	  automatically if the table is declared with DEFINE_PCI_DEVICE_TABLE().

  	o The probe() and remove() functions should be marked __devinit
  	  and __devexit respectively.  All initialization functions
  	  exclusively called by the probe() routine, can be marked __devinit.
  	  Ditto for remove() and __devexit.

  	o If mydriver_remove() is marked with __devexit(), then all address
  	  references to mydriver_remove must use __devexit_p(mydriver_remove)

  	  (in the struct pci_driver declaration for example).
  	  __devexit_p() will generate the function name _or_ NULL if the
  	  function will be discarded.  For an example, see drivers/net/tg3.c.
  
  	o Do NOT mark a function if you are not sure which mark to use.
  	  Better to not mark the function than mark the function wrong.
  
  
  
  2. How to find PCI devices manually
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  PCI drivers should have a really good reason for not using the
  pci_register_driver() interface to search for PCI devices.
  The main reason PCI devices are controlled by multiple drivers
  is because one PCI device implements several different HW services.
  E.g. combined serial/parallel port/floppy controller.
  
  A manual search may be performed using the following constructs:

  
  Searching by vendor and device ID:
  
  	struct pci_dev *dev = NULL;
  	while (dev = pci_get_device(VENDOR_ID, DEVICE_ID, dev))
  		configure_device(dev);
  
  Searching by class ID (iterate in a similar way):
  
  	pci_get_class(CLASS_ID, dev)
  
  Searching by both vendor/device and subsystem vendor/device ID:

  	pci_get_subsys(VENDOR_ID,DEVICE_ID, SUBSYS_VENDOR_ID, SUBSYS_DEVICE_ID, dev).

  You can use the constant PCI_ANY_ID as a wildcard replacement for

  VENDOR_ID or DEVICE_ID.  This allows searching for any device from a
  specific vendor, for example.

  These functions are hotplug-safe. They increment the reference count on

  the pci_dev that they return. You must eventually (possibly at module unload)
  decrement the reference count on these devices by calling pci_dev_put().

  3. Device Initialization Steps
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  As noted in the introduction, most PCI drivers need the following steps
  for device initialization:

  	Enable the device
  	Request MMIO/IOP resources
  	Set the DMA mask size (for both coherent and streaming DMA)
  	Allocate and initialize shared control data (pci_allocate_coherent())
  	Access device configuration space (if needed)
  	Register IRQ handler (request_irq())
  	Initialize non-PCI (i.e. LAN/SCSI/etc parts of the chip)
  	Enable DMA/processing engines.
  
  The driver can access PCI config space registers at any time.
  (Well, almost. When running BIST, config space can go away...but
  that will just result in a PCI Bus Master Abort and config reads
  will return garbage).
  
  
  3.1 Enable the PCI device
  ~~~~~~~~~~~~~~~~~~~~~~~~~
  Before touching any device registers, the driver needs to enable
  the PCI device by calling pci_enable_device(). This will:
  	o wake up the device if it was in suspended state,
  	o allocate I/O and memory regions of the device (if BIOS did not),
  	o allocate an IRQ (if BIOS did not).
  
  NOTE: pci_enable_device() can fail! Check the return value.

  
  [ OS BUG: we don't check resource allocations before enabling those
    resources. The sequence would make more sense if we called
    pci_request_resources() before calling pci_enable_device().
    Currently, the device drivers can't detect the bug when when two
    devices have been allocated the same range. This is not a common
    problem and unlikely to get fixed soon.
  
    This has been discussed before but not changed as of 2.6.19:
  	http://lkml.org/lkml/2006/3/2/194
  ]
  
  pci_set_master() will enable DMA by setting the bus master bit
  in the PCI_COMMAND register. It also fixes the latency timer value if

  it's set to something bogus by the BIOS.  pci_clear_master() will
  disable DMA by clearing the bus master bit.

  
  If the PCI device can use the PCI Memory-Write-Invalidate transaction,

  call pci_set_mwi().  This enables the PCI_COMMAND bit for Mem-Wr-Inval
  and also ensures that the cache line size register is set correctly.

  Check the return value of pci_set_mwi() as not all architectures

  or chip-sets may support Memory-Write-Invalidate.  Alternatively,
  if Mem-Wr-Inval would be nice to have but is not required, call
  pci_try_set_mwi() to have the system do its best effort at enabling
  Mem-Wr-Inval.

  
  
  3.2 Request MMIO/IOP resources
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Memory (MMIO), and I/O port addresses should NOT be read directly
  from the PCI device config space. Use the values in the pci_dev structure
  as the PCI "bus address" might have been remapped to a "host physical"
  address by the arch/chip-set specific kernel support.

  See Documentation/io-mapping.txt for how to access device registers

  or device memory.
  
  The device driver needs to call pci_request_region() to verify
  no other device is already using the same address resource.
  Conversely, drivers should call pci_release_region() AFTER

  calling pci_disable_device().

  The idea is to prevent two devices colliding on the same address range.
  
  [ See OS BUG comment above. Currently (2.6.19), The driver can only
    determine MMIO and IO Port resource availability _after_ calling
    pci_enable_device(). ]
  
  Generic flavors of pci_request_region() are request_mem_region()
  (for MMIO ranges) and request_region() (for IO Port ranges).
  Use these for address resources that are not described by "normal" PCI
  BARs.
  
  Also see pci_request_selected_regions() below.
  
  
  3.3 Set the DMA mask size
  ~~~~~~~~~~~~~~~~~~~~~~~~~
  [ If anything below doesn't make sense, please refer to
    Documentation/DMA-API.txt. This section is just a reminder that
    drivers need to indicate DMA capabilities of the device and is not
    an authoritative source for DMA interfaces. ]
  
  While all drivers should explicitly indicate the DMA capability
  (e.g. 32 or 64 bit) of the PCI bus master, devices with more than
  32-bit bus master capability for streaming data need the driver
  to "register" this capability by calling pci_set_dma_mask() with
  appropriate parameters.  In general this allows more efficient DMA
  on systems where System RAM exists above 4G _physical_ address.
  
  Drivers for all PCI-X and PCIe compliant devices must call
  pci_set_dma_mask() as they are 64-bit DMA devices.
  
  Similarly, drivers must also "register" this capability if the device
  can directly address "consistent memory" in System RAM above 4G physical
  address by calling pci_set_consistent_dma_mask().
  Again, this includes drivers for all PCI-X and PCIe compliant devices.
  Many 64-bit "PCI" devices (before PCI-X) and some PCI-X devices are
  64-bit DMA capable for payload ("streaming") data but not control
  ("consistent") data.
  
  
  3.4 Setup shared control data
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Once the DMA masks are set, the driver can allocate "consistent" (a.k.a. shared)
  memory.  See Documentation/DMA-API.txt for a full description of
  the DMA APIs. This section is just a reminder that it needs to be done
  before enabling DMA on the device.
  
  
  3.5 Initialize device registers
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Some drivers will need specific "capability" fields programmed
  or other "vendor specific" register initialized or reset.
  E.g. clearing pending interrupts.
  
  
  3.6 Register IRQ handler
  ~~~~~~~~~~~~~~~~~~~~~~~~

  While calling request_irq() is the last step described here,

  this is often just another intermediate step to initialize a device.
  This step can often be deferred until the device is opened for use.
  
  All interrupt handlers for IRQ lines should be registered with IRQF_SHARED
  and use the devid to map IRQs to devices (remember that all PCI IRQ lines
  can be shared).
  
  request_irq() will associate an interrupt handler and device handle
  with an interrupt number. Historically interrupt numbers represent
  IRQ lines which run from the PCI device to the Interrupt controller.
  With MSI and MSI-X (more below) the interrupt number is a CPU "vector".
  
  request_irq() also enables the interrupt. Make sure the device is
  quiesced and does not have any interrupts pending before registering
  the interrupt handler.
  
  MSI and MSI-X are PCI capabilities. Both are "Message Signaled Interrupts"
  which deliver interrupts to the CPU via a DMA write to a Local APIC.
  The fundamental difference between MSI and MSI-X is how multiple
  "vectors" get allocated. MSI requires contiguous blocks of vectors
  while MSI-X can allocate several individual ones.
  
  MSI capability can be enabled by calling pci_enable_msi() or
  pci_enable_msix() before calling request_irq(). This causes
  the PCI support to program CPU vector data into the PCI device
  capability registers.
  
  If your PCI device supports both, try to enable MSI-X first.
  Only one can be enabled at a time.  Many architectures, chip-sets,
  or BIOSes do NOT support MSI or MSI-X and the call to pci_enable_msi/msix
  will fail. This is important to note since many drivers have
  two (or more) interrupt handlers: one for MSI/MSI-X and another for IRQs.
  They choose which handler to register with request_irq() based on the
  return value from pci_enable_msi/msix().
  
  There are (at least) two really good reasons for using MSI:
  1) MSI is an exclusive interrupt vector by definition.
     This means the interrupt handler doesn't have to verify
     its device caused the interrupt.
  
  2) MSI avoids DMA/IRQ race conditions. DMA to host memory is guaranteed
     to be visible to the host CPU(s) when the MSI is delivered. This
     is important for both data coherency and avoiding stale control data.
     This guarantee allows the driver to omit MMIO reads to flush
     the DMA stream.
  
  See drivers/infiniband/hw/mthca/ or drivers/net/tg3.c for examples
  of MSI/MSI-X usage.
  
  
  
  4. PCI device shutdown
  ~~~~~~~~~~~~~~~~~~~~~~~
  
  When a PCI device driver is being unloaded, most of the following
  steps need to be performed:
  
  	Disable the device from generating IRQs
  	Release the IRQ (free_irq())
  	Stop all DMA activity
  	Release DMA buffers (both streaming and consistent)
  	Unregister from other subsystems (e.g. scsi or netdev)
  	Disable device from responding to MMIO/IO Port addresses
  	Release MMIO/IO Port resource(s)
  
  
  4.1 Stop IRQs on the device
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~
  How to do this is chip/device specific. If it's not done, it opens
  the possibility of a "screaming interrupt" if (and only if)
  the IRQ is shared with another device.
  
  When the shared IRQ handler is "unhooked", the remaining devices
  using the same IRQ line will still need the IRQ enabled. Thus if the
  "unhooked" device asserts IRQ line, the system will respond assuming
  it was one of the remaining devices asserted the IRQ line. Since none
  of the other devices will handle the IRQ, the system will "hang" until
  it decides the IRQ isn't going to get handled and masks the IRQ (100,000
  iterations later). Once the shared IRQ is masked, the remaining devices
  will stop functioning properly. Not a nice situation.
  
  This is another reason to use MSI or MSI-X if it's available.
  MSI and MSI-X are defined to be exclusive interrupts and thus
  are not susceptible to the "screaming interrupt" problem.
  
  
  4.2 Release the IRQ
  ~~~~~~~~~~~~~~~~~~~
  Once the device is quiesced (no more IRQs), one can call free_irq().
  This function will return control once any pending IRQs are handled,
  "unhook" the drivers IRQ handler from that IRQ, and finally release
  the IRQ if no one else is using it.
  
  
  4.3 Stop all DMA activity
  ~~~~~~~~~~~~~~~~~~~~~~~~~
  It's extremely important to stop all DMA operations BEFORE attempting
  to deallocate DMA control data. Failure to do so can result in memory
  corruption, hangs, and on some chip-sets a hard crash.

  Stopping DMA after stopping the IRQs can avoid races where the
  IRQ handler might restart DMA engines.
  
  While this step sounds obvious and trivial, several "mature" drivers
  didn't get this step right in the past.
  
  
  4.4 Release DMA buffers
  ~~~~~~~~~~~~~~~~~~~~~~~
  Once DMA is stopped, clean up streaming DMA first.
  I.e. unmap data buffers and return buffers to "upstream"
  owners if there is one.
  
  Then clean up "consistent" buffers which contain the control data.
  
  See Documentation/DMA-API.txt for details on unmapping interfaces.
  
  
  4.5 Unregister from other subsystems
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Most low level PCI device drivers support some other subsystem
  like USB, ALSA, SCSI, NetDev, Infiniband, etc. Make sure your
  driver isn't losing resources from that other subsystem.
  If this happens, typically the symptom is an Oops (panic) when
  the subsystem attempts to call into a driver that has been unloaded.
  
  
  4.6 Disable Device from responding to MMIO/IO Port addresses
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  io_unmap() MMIO or IO Port resources and then call pci_disable_device().
  This is the symmetric opposite of pci_enable_device().
  Do not access device registers after calling pci_disable_device().
  
  
  4.7 Release MMIO/IO Port Resource(s)
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Call pci_release_region() to mark the MMIO or IO Port range as available.
  Failure to do so usually results in the inability to reload the driver.
  
  
  
  5. How to access PCI config space

  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  
  You can use pci_(read|write)_config_(byte|word|dword) to access the config

  space of a device represented by struct pci_dev *. All these functions return 0
  when successful or an error code (PCIBIOS_...) which can be translated to a text
  string by pcibios_strerror. Most drivers expect that accesses to valid PCI
  devices don't fail.

  If you don't have a struct pci_dev available, you can call

  pci_bus_(read|write)_config_(byte|word|dword) to access a given device
  and function on that bus.

  If you access fields in the standard portion of the config header, please

  use symbolic names of locations and bits declared in <linux/pci.h>.

  If you need to access Extended PCI Capability registers, just call

  pci_find_capability() for the particular capability and it will find the
  corresponding register block for you.

  
  6. Other interesting functions
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  pci_find_slot()			Find pci_dev corresponding to given bus and
  				slot numbers.
  pci_set_power_state()		Set PCI Power Management state (0=D0 ... 3=D3)
  pci_find_capability()		Find specified capability in device's capability
  				list.

  pci_resource_start()		Returns bus start address for a given PCI region
  pci_resource_end()		Returns bus end address for a given PCI region
  pci_resource_len()		Returns the byte length of a PCI region
  pci_set_drvdata()		Set private driver data pointer for a pci_dev
  pci_get_drvdata()		Return private driver data pointer for a pci_dev
  pci_set_mwi()			Enable Memory-Write-Invalidate transactions.
  pci_clear_mwi()			Disable Memory-Write-Invalidate transactions.

  7. Miscellaneous hints
  ~~~~~~~~~~~~~~~~~~~~~~

  
  When displaying PCI device names to the user (for example when a driver wants
  to tell the user what card has it found), please use pci_name(pci_dev).

  
  Always refer to the PCI devices by a pointer to the pci_dev structure.
  All PCI layer functions use this identification and it's the only
  reasonable one. Don't use bus/slot/function numbers except for very
  special purposes -- on systems with multiple primary buses their semantics
  can be pretty complex.

  Don't try to turn on Fast Back to Back writes in your driver.  All devices
  on the bus need to be capable of doing it, so this is something which needs
  to be handled by platform and generic code, not individual drivers.

  8. Vendor and device identifications
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  One is not required to add new device ids to include/linux/pci_ids.h.

  Please add PCI_VENDOR_ID_xxx for vendors and a hex constant for device ids.
  
  PCI_VENDOR_ID_xxx constants are re-used. The device ids are arbitrary
  hex numbers (vendor controlled) and normally used only in a single
  location, the pci_device_id table.
  
  Please DO submit new vendor/device ids to pciids.sourceforge.net project.

  
  9. Obsolete functions

  ~~~~~~~~~~~~~~~~~~~~~

  There are several functions which you might come across when trying to
  port an old driver to the new PCI interface.  They are no longer present
  in the kernel as they aren't compatible with hotplug or PCI domains or
  having sane locking.

  pci_find_device()	Superseded by pci_get_device()
  pci_find_subsys()	Superseded by pci_get_subsys()
  pci_find_slot()		Superseded by pci_get_slot()
  
  
  The alternative is the traditional PCI device driver that walks PCI
  device lists. This is still possible but discouraged.

  10. MMIO Space and "Write Posting"

  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  Converting a driver from using I/O Port space to using MMIO space
  often requires some additional changes. Specifically, "write posting"
  needs to be handled. Many drivers (e.g. tg3, acenic, sym53c8xx_2)
  already do this. I/O Port space guarantees write transactions reach the PCI
  device before the CPU can continue. Writes to MMIO space allow the CPU
  to continue before the transaction reaches the PCI device. HW weenies
  call this "Write Posting" because the write completion is "posted" to
  the CPU before the transaction has reached its destination.
  
  Thus, timing sensitive code should add readl() where the CPU is
  expected to wait before doing other work.  The classic "bit banging"
  sequence works fine for I/O Port space:
  
         for (i = 8; --i; val >>= 1) {
                 outb(val & 1, ioport_reg);      /* write bit */
                 udelay(10);
         }
  
  The same sequence for MMIO space should be:
  
         for (i = 8; --i; val >>= 1) {
                 writeb(val & 1, mmio_reg);      /* write bit */
                 readb(safe_mmio_reg);           /* flush posted write */
                 udelay(10);
         }
  
  It is important that "safe_mmio_reg" not have any side effects that
  interferes with the correct operation of the device.
  
  Another case to watch out for is when resetting a PCI device. Use PCI
  Configuration space reads to flush the writel(). This will gracefully
  handle the PCI master abort on all platforms if the PCI device is
  expected to not respond to a readl().  Most x86 platforms will allow
  MMIO reads to master abort (a.k.a. "Soft Fail") and return garbage
  (e.g. ~0). But many RISC platforms will crash (a.k.a."Hard Fail").