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+
+                        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.
+
+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.
+
+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").
+