272 lines
12 KiB
Plaintext
272 lines
12 KiB
Plaintext
How To Write Linux PCI Drivers
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by Martin Mares <mj@ucw.cz> on 07-Feb-2000
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The world of PCI is vast and it's full of (mostly unpleasant) surprises.
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Different PCI devices have different requirements and different bugs --
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because of this, the PCI support layer in Linux kernel is not as trivial
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as one would wish. This short pamphlet tries to help all potential driver
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authors find their way through the deep forests of PCI handling.
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0. Structure of PCI drivers
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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There exist two kinds of PCI drivers: new-style ones (which leave most of
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probing for devices to the PCI layer and support online insertion and removal
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of devices [thus supporting PCI, hot-pluggable PCI and CardBus in a single
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driver]) and old-style ones which just do all the probing themselves. Unless
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you have a very good reason to do so, please don't use the old way of probing
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in any new code. After the driver finds the devices it wishes to operate
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on (either the old or the new way), it needs to perform the following steps:
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Enable the device
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Access device configuration space
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Discover resources (addresses and IRQ numbers) provided by the device
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Allocate these resources
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Communicate with the device
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Disable the device
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Most of these topics are covered by the following sections, for the rest
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look at <linux/pci.h>, it's hopefully well commented.
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If the PCI subsystem is not configured (CONFIG_PCI is not set), most of
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the functions described below are defined as inline functions either completely
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empty or just returning an appropriate error codes to avoid lots of ifdefs
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in the drivers.
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1. New-style drivers
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~~~~~~~~~~~~~~~~~~~~
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The new-style drivers just call pci_register_driver during their initialization
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with a pointer to a structure describing the driver (struct pci_driver) which
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contains:
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name Name of the driver
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id_table Pointer to table of device ID's the driver is
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interested in. Most drivers should export this
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table using MODULE_DEVICE_TABLE(pci,...).
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probe Pointer to a probing function which gets called (during
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execution of pci_register_driver for already existing
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devices or later if a new device gets inserted) for all
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PCI devices which match the ID table and are not handled
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by the other drivers yet. This function gets passed a
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pointer to the pci_dev structure representing the device
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and also which entry in the ID table did the device
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match. It returns zero when the driver has accepted the
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device or an error code (negative number) otherwise.
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This function always gets called from process context,
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so it can sleep.
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remove Pointer to a function which gets called whenever a
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device being handled by this driver is removed (either
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during deregistration of the driver or when it's
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manually pulled out of a hot-pluggable slot). This
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function always gets called from process context, so it
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can sleep.
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save_state Save a device's state before it's suspend.
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suspend Put device into low power state.
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resume Wake device from low power state.
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enable_wake Enable device to generate wake events from a low power
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state.
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(Please see Documentation/power/pci.txt for descriptions
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of PCI Power Management and the related functions)
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The ID table is an array of struct pci_device_id ending with a all-zero entry.
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Each entry consists of:
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vendor, device Vendor and device ID to match (or PCI_ANY_ID)
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subvendor, Subsystem vendor and device ID to match (or PCI_ANY_ID)
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subdevice
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class, Device class to match. The class_mask tells which bits
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class_mask of the class are honored during the comparison.
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driver_data Data private to the driver.
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Most drivers don't need to use the driver_data field. Best practice
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for use of driver_data is to use it as an index into a static list of
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equivalant device types, not to use it as a pointer.
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Have a table entry {PCI_ANY_ID, PCI_ANY_ID, PCI_ANY_ID, PCI_ANY_ID}
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to have probe() called for every PCI device known to the system.
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New PCI IDs may be added to a device driver at runtime by writing
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to the file /sys/bus/pci/drivers/{driver}/new_id. When added, the
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driver will probe for all devices it can support.
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echo "vendor device subvendor subdevice class class_mask driver_data" > \
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/sys/bus/pci/drivers/{driver}/new_id
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where all fields are passed in as hexadecimal values (no leading 0x).
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Users need pass only as many fields as necessary; vendor, device,
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subvendor, and subdevice fields default to PCI_ANY_ID (FFFFFFFF),
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class and classmask fields default to 0, and driver_data defaults to
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0UL. Device drivers must initialize use_driver_data in the dynids struct
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in their pci_driver struct prior to calling pci_register_driver in order
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for the driver_data field to get passed to the driver. Otherwise, only a
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0 is passed in that field.
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When the driver exits, it just calls pci_unregister_driver() and the PCI layer
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automatically calls the remove hook for all devices handled by the driver.
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Please mark the initialization and cleanup functions where appropriate
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(the corresponding macros are defined in <linux/init.h>):
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__init Initialization code. Thrown away after the driver
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initializes.
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__exit Exit code. Ignored for non-modular drivers.
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__devinit Device initialization code. Identical to __init if
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the kernel is not compiled with CONFIG_HOTPLUG, normal
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function otherwise.
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__devexit The same for __exit.
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Tips:
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The module_init()/module_exit() functions (and all initialization
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functions called only from these) should be marked __init/exit.
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The struct pci_driver shouldn't be marked with any of these tags.
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The ID table array should be marked __devinitdata.
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The probe() and remove() functions (and all initialization
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functions called only from these) should be marked __devinit/exit.
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If you are sure the driver is not a hotplug driver then use only
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__init/exit __initdata/exitdata.
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Pointers to functions marked as __devexit must be created using
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__devexit_p(function_name). That will generate the function
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name or NULL if the __devexit function will be discarded.
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2. How to find PCI devices manually (the old style)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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PCI drivers not using the pci_register_driver() interface search
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for PCI devices manually using the following constructs:
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Searching by vendor and device ID:
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struct pci_dev *dev = NULL;
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while (dev = pci_get_device(VENDOR_ID, DEVICE_ID, dev))
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configure_device(dev);
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Searching by class ID (iterate in a similar way):
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pci_get_class(CLASS_ID, dev)
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Searching by both vendor/device and subsystem vendor/device ID:
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pci_get_subsys(VENDOR_ID, DEVICE_ID, SUBSYS_VENDOR_ID, SUBSYS_DEVICE_ID, dev).
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You can use the constant PCI_ANY_ID as a wildcard replacement for
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VENDOR_ID or DEVICE_ID. This allows searching for any device from a
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specific vendor, for example.
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These functions are hotplug-safe. They increment the reference count on
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the pci_dev that they return. You must eventually (possibly at module unload)
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decrement the reference count on these devices by calling pci_dev_put().
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3. Enabling and disabling devices
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Before you do anything with the device you've found, you need to enable
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it by calling pci_enable_device() which enables I/O and memory regions of
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the device, allocates an IRQ if necessary, assigns missing resources if
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needed and wakes up the device if it was in suspended state. Please note
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that this function can fail.
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If you want to use the device in bus mastering mode, call pci_set_master()
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which enables the bus master bit in PCI_COMMAND register and also fixes
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the latency timer value if it's set to something bogus by the BIOS.
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If you want to use the PCI Memory-Write-Invalidate transaction,
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call pci_set_mwi(). This enables the PCI_COMMAND bit for Mem-Wr-Inval
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and also ensures that the cache line size register is set correctly.
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Make sure to check the return value of pci_set_mwi(), not all architectures
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may support Memory-Write-Invalidate.
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If your driver decides to stop using the device (e.g., there was an
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error while setting it up or the driver module is being unloaded), it
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should call pci_disable_device() to deallocate any IRQ resources, disable
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PCI bus-mastering, etc. You should not do anything with the device after
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calling pci_disable_device().
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4. How to access PCI config space
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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You can use pci_(read|write)_config_(byte|word|dword) to access the config
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space of a device represented by struct pci_dev *. All these functions return 0
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when successful or an error code (PCIBIOS_...) which can be translated to a text
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string by pcibios_strerror. Most drivers expect that accesses to valid PCI
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devices don't fail.
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If you don't have a struct pci_dev available, you can call
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pci_bus_(read|write)_config_(byte|word|dword) to access a given device
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and function on that bus.
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If you access fields in the standard portion of the config header, please
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use symbolic names of locations and bits declared in <linux/pci.h>.
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If you need to access Extended PCI Capability registers, just call
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pci_find_capability() for the particular capability and it will find the
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corresponding register block for you.
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5. Addresses and interrupts
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Memory and port addresses and interrupt numbers should NOT be read from the
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config space. You should use the values in the pci_dev structure as they might
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have been remapped by the kernel.
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See Documentation/IO-mapping.txt for how to access device memory.
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You still need to call request_region() for I/O regions and
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request_mem_region() for memory regions to make sure nobody else is using the
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same device.
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All interrupt handlers should be registered with SA_SHIRQ and use the devid
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to map IRQs to devices (remember that all PCI interrupts are shared).
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6. Other interesting functions
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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pci_find_slot() Find pci_dev corresponding to given bus and
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slot numbers.
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pci_set_power_state() Set PCI Power Management state (0=D0 ... 3=D3)
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pci_find_capability() Find specified capability in device's capability
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list.
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pci_module_init() Inline helper function for ensuring correct
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pci_driver initialization and error handling.
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pci_resource_start() Returns bus start address for a given PCI region
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pci_resource_end() Returns bus end address for a given PCI region
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pci_resource_len() Returns the byte length of a PCI region
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pci_set_drvdata() Set private driver data pointer for a pci_dev
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pci_get_drvdata() Return private driver data pointer for a pci_dev
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pci_set_mwi() Enable Memory-Write-Invalidate transactions.
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pci_clear_mwi() Disable Memory-Write-Invalidate transactions.
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7. Miscellaneous hints
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~~~~~~~~~~~~~~~~~~~~~~
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When displaying PCI slot names to the user (for example when a driver wants
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to tell the user what card has it found), please use pci_name(pci_dev)
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for this purpose.
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Always refer to the PCI devices by a pointer to the pci_dev structure.
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All PCI layer functions use this identification and it's the only
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reasonable one. Don't use bus/slot/function numbers except for very
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special purposes -- on systems with multiple primary buses their semantics
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can be pretty complex.
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If you're going to use PCI bus mastering DMA, take a look at
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Documentation/DMA-mapping.txt.
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Don't try to turn on Fast Back to Back writes in your driver. All devices
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on the bus need to be capable of doing it, so this is something which needs
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to be handled by platform and generic code, not individual drivers.
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8. Obsolete functions
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~~~~~~~~~~~~~~~~~~~~~
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There are several functions which you might come across when trying to
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port an old driver to the new PCI interface. They are no longer present
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in the kernel as they aren't compatible with hotplug or PCI domains or
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having sane locking.
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pci_find_device() Superseded by pci_get_device()
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pci_find_subsys() Superseded by pci_get_subsys()
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pci_find_slot() Superseded by pci_get_slot()
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