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    SubjectRe: [PATCH] Update Documentation/pci.txt v7
    On Mon, Dec 25, 2006 at 01:06:35AM -0700, Grant Grundler wrote:
    > On Sat, Dec 23, 2006 at 11:07:26PM -0700, Grant Grundler wrote:
    > > "final" patch v7 and commit log entry appended below. :)
    >
    > v8 adds 2cd round of feedback from Randy Dunlap.

    Obviously the subject line is stale...it's really v8 now.
    It's just way past my bedtime again.

    grant

    > Going once...twice... ;)
    >
    > > Full pci.txt text is easier to review at:
    > > http://www.parisc-linux.org/~grundler/Documentation/
    >
    > Same place.
    >
    > grant
    >
    >
    > Rewrite Documentation/pci.txt:
    > o restructure document to match how API is used when writing init code.
    > o update to reflect changes in struct pci_driver function pointers.
    > o removed language on "new style vs old style" device discovery.
    > "Old style" is now deprecated. Don't use it. Left description in
    > to document existing driver behaviors.
    > o add section "Legacy I/O Port free driver" by Kenji Kaneshige
    > http://lkml.org/lkml/2006/11/22/25
    > (renamed to "pci_enable_device_bars() and Legacy I/O Port space")
    > o add "MMIO space and write posting" section to help avoid common pitfall
    > when converting drivers from IO Port space to MMIO space.
    > Orignally posted http://lkml.org/lkml/2006/2/27/24
    > o many typo/grammer/spelling corrections from Randy Dunlap
    > o two more spelling corrections from Stephan Richter
    > o fix CodingStyle as per Randy Dunlap
    >
    >
    > Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
    >
    >
    > diff --git a/Documentation/pci.txt b/Documentation/pci.txt
    > index 2b395e4..0d33d80 100644
    > --- a/Documentation/pci.txt
    > +++ b/Documentation/pci.txt
    > @@ -1,142 +1,231 @@
    > - How To Write Linux PCI Drivers
    >
    > - by Martin Mares <mj@ucw.cz> on 07-Feb-2000
    > + 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 it's full of (mostly unpleasant) surprises.
    > -Different PCI devices have different requirements and different bugs --
    > -because of this, the PCI support layer in Linux kernel is not as trivial
    > -as one would wish. This short pamphlet tries to help all potential driver
    > -authors find their way through the deep forests of PCI handling.
    > +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
    > ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    > -There exist two kinds of PCI drivers: new-style ones (which leave most of
    > -probing for devices to the PCI layer and support online insertion and removal
    > -of devices [thus supporting PCI, hot-pluggable PCI and CardBus in a single
    > -driver]) and old-style ones which just do all the probing themselves. Unless
    > -you have a very good reason to do so, please don't use the old way of probing
    > -in any new code. After the driver finds the devices it wishes to operate
    > -on (either the old or the new way), it needs to perform the following steps:
    > +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
    > - Access device configuration space
    > - Discover resources (addresses and IRQ numbers) provided by the device
    > - Allocate these resources
    > - Communicate with 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 by the following sections, for the rest
    > -look at <linux/pci.h>, it's hopefully well commented.
    > +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 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.
    > +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. New-style drivers
    > -~~~~~~~~~~~~~~~~~~~~
    > -The new-style drivers just call pci_register_driver during their initialization
    > -with a pointer to a structure describing the driver (struct pci_driver) which
    > -contains:
    > +1. pci_register_driver() call
    > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    >
    > - name Name of the driver
    > +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 Pointer to a probing function which 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 handled
    > - by the other drivers yet. This function gets passed a
    > - pointer to the pci_dev structure representing the device
    > - and also which entry in the ID table did the device
    > - match. It returns zero when the driver has accepted the
    > - device or an error code (negative number) otherwise.
    > - This function always gets called from process context,
    > - so it can sleep.
    > - remove Pointer to a function which 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). This
    > - function always gets called from process context, so it
    > - can sleep.
    > - save_state Save a device's state before it's suspend.
    > +
    > + 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.
    >
    > - (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
    > +
    > + 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 ending with a all-zero entry.
    > -Each entry consists of:
    > +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)
    >
    > - 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 to match. The class_mask tells which bits
    > - class_mask of the class are honored during the comparison.
    > + 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 don't need to use the driver_data field. Best practice
    > -for use of driver_data is to use it as an index into a static list of
    > -equivalent device types, not to use it as a pointer.
    >
    > -Have a table entry {PCI_ANY_ID, PCI_ANY_ID, PCI_ANY_ID, PCI_ANY_ID}
    > -to have probe() called for every PCI device known to the system.
    > +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 at runtime by writing
    > -to the file /sys/bus/pci/drivers/{driver}/new_id. When added, the
    > -driver will probe for all devices it can support.
    > +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
    > -where all fields are passed in as hexadecimal values (no leading 0x).
    > -Users need pass only as many fields as necessary; vendor, device,
    > -subvendor, and subdevice fields default to PCI_ANY_ID (FFFFFFFF),
    > -class and classmask fields default to 0, and driver_data defaults to
    > -0UL. Device drivers must initialize use_driver_data in the dynids struct
    > -in their pci_driver struct prior to calling pci_register_driver in order
    > -for the driver_data field to get passed to the driver. Otherwise, only a
    > -0 is passed in that field.
    > +/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.
    > +
    > +
    > + __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:
    > - The module_init()/module_exit() functions (and all initialization
    > - functions called only from these) should be marked __init/exit.
    > - The struct pci_driver shouldn't be marked with any of these tags.
    > - The ID table array should be marked __devinitdata.
    > - The probe() and remove() functions (and all initialization
    > - functions called only from these) should be marked __devinit/exit.
    > - If you are sure the driver is not a hotplug driver then use only
    > - __init/exit __initdata/exitdata.
    > +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.
    >
    > - Pointers to functions marked as __devexit must be created using
    > - __devexit_p(function_name). That will generate the function
    > - name or NULL if the __devexit function will be discarded.
    > + o Do not mark the struct pci_driver.
    >
    > + o The ID table array should be marked __devinitdata.
    >
    > -2. How to find PCI devices manually (the old style)
    > -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    > -PCI drivers not using the pci_register_driver() interface search
    > -for PCI devices manually using the following constructs:
    > + 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_probe() is marked with __devinit(), then all address
    > + references to mydriver_probe must use __devexit_p(mydriver_probe)
    > + (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:
    >
    > @@ -150,87 +239,311 @@ Searching by class ID (iterate in a similar way):
    >
    > Searching by both vendor/device and subsystem vendor/device ID:
    >
    > - pci_get_subsys(VENDOR_ID, DEVICE_ID, SUBSYS_VENDOR_ID, SUBSYS_DEVICE_ID, dev).
    > + 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
    > +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
    > +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. Enabling and disabling devices
    > -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    > - Before you do anything with the device you've found, you need to enable
    > -it by calling pci_enable_device() which enables I/O and memory regions of
    > -the device, allocates an IRQ if necessary, assigns missing resources if
    > -needed and wakes up the device if it was in suspended state. Please note
    > -that this function can fail.
    >
    > - If you want to use the device in bus mastering mode, call pci_set_master()
    > -which enables the bus master bit in PCI_COMMAND register and also fixes
    > -the latency timer value if it's set to something bogus by the BIOS.
    > +3. Device Initialization Steps
    > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    > +
    > +As noted in the introduction, most PCI drivers need the following steps
    > +for device initialization:
    >
    > - If you want to use the PCI Memory-Write-Invalidate transaction,
    > + 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.
    > -Make sure to check the return value of pci_set_mwi(), not all architectures
    > -may support Memory-Write-Invalidate.
    > +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.
    >
    > - If your driver decides to stop using the device (e.g., there was an
    > -error while setting it up or the driver module is being unloaded), it
    > -should call pci_disable_device() to deallocate any IRQ resources, disable
    > -PCI bus-mastering, etc. You should not do anything with the device after
    > +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.
    >
    > -4. How to access PCI config space
    > +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
    > +
    > +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
    > +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
    > +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
    > +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.
    >
    >
    > -5. Addresses and interrupts
    > -~~~~~~~~~~~~~~~~~~~~~~~~~~~
    > - Memory and port addresses and interrupt numbers should NOT be read from the
    > -config space. You should use the values in the pci_dev structure as they might
    > -have been remapped by the kernel.
    > -
    > - See Documentation/IO-mapping.txt for how to access device memory.
    > -
    > - The device driver needs to call pci_request_region() to make sure
    > -no other device is already using the same resource. The driver is expected
    > -to determine MMIO and IO Port resource availability _before_ calling
    > -pci_enable_device(). 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.
    > -
    > -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
    > -interfaces (e.g. BAR).
    > -
    > - All interrupt handlers should be registered with IRQF_SHARED and use the devid
    > -to map IRQs to devices (remember that all PCI interrupts are shared).
    > -
    >
    > 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)
    > @@ -247,11 +560,12 @@ pci_set_mwi() Enable Memory-Write-Invalidate transactions.
    > pci_clear_mwi() Disable Memory-Write-Invalidate transactions.
    >
    >
    > +
    > 7. Miscellaneous hints
    > ~~~~~~~~~~~~~~~~~~~~~~
    > -When displaying PCI slot 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)
    > -for this purpose.
    > +
    > +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
    > @@ -259,31 +573,113 @@ 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.
    >
    > -If you're going to use PCI bus mastering DMA, take a look at
    > -Documentation/DMA-mapping.txt.
    > -
    > 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
    > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    > -For the future, let's avoid adding device ids to include/linux/pci_ids.h.
    >
    > -PCI_VENDOR_ID_xxx for vendors, and a hex constant for device ids.
    > +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.
    > +
    >
    > -Rationale: PCI_VENDOR_ID_xxx constants are re-used, but device ids are not.
    > - Further, device ids are arbitrary hex numbers, normally used only in a
    > - single location, the pci_device_id table.
    >
    > 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()
    > +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").
    > +
    -
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