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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.
-
-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").
-