Revert "[CPUFREQ] constify cpufreq_driver where possible."

[linux-2.6/linux-acpi-2.6/ibm-acpi-2.6.git] / 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.)

        enable_wake     Enable device to generate wake events from a low power
                        state.

        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

        multithread_probe       Enable multi-threaded probe/scan. Driver must
                        provide its own locking/syncronization for init
                        operations if this is enabled.


The ID table is an array of struct pci_device_id entries ending with an
all-zero entry.  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).
Users need pass only as many fields as necessary:
        o vendor, device, 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 __devinitdata.

        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.
NOTE2: Also see pci_enable_device_bars() below. Drivers can
    attempt to enable only a subset of BARs they need.

[ 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.


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 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_module_init()               Inline helper function for ensuring correct
                                pci_driver initialization and error handling.
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. pci_enable_device_bars() and Legacy I/O Port space
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Large servers may not be able to provide I/O port resources to all PCI
devices. I/O Port space is only 64KB on Intel Architecture[1] and is
likely also fragmented since the I/O base register of PCI-to-PCI
bridge will usually be aligned to a 4KB boundary[2]. On such systems,
pci_enable_device() and pci_request_region() will fail when
attempting to enable I/O Port regions that don't have I/O Port
resources assigned.

Fortunately, many PCI devices which request I/O Port resources also
provide access to the same registers via MMIO BARs. These devices can
be handled without using I/O port space and the drivers typically
offer a CONFIG_ option to only use MMIO regions
(e.g. CONFIG_TULIP_MMIO). PCI devices typically provide I/O port
interface for legacy OSes and will work when I/O port resources are not
assigned. The "PCI Local Bus Specification Revision 3.0" discusses
this on p.44, "IMPLEMENTATION NOTE".

If your PCI device driver doesn't need I/O port resources assigned to
I/O Port BARs, you should use pci_enable_device_bars() instead of
pci_enable_device() in order not to enable I/O port regions for the
corresponding devices. In addition, you should use
pci_request_selected_regions() and pci_release_selected_regions()
instead of pci_request_regions()/pci_release_regions() in order not to
request/release I/O port regions for the corresponding devices.

[1] Some systems support 64KB I/O port space per PCI segment.
[2] Some PCI-to-PCI bridges support optional 1KB aligned I/O base.



11. 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").