FreeBSD/Linux Kernel Cross Reference
sys/Documentation/pci.txt

     
                             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-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 DECLARE_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.
   
   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 DECLARE_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.
   
   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 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").
   

Cache object: ec2a9088aab70ff038ea85d9fa56efb7