How To Write Linux PCI Drivers
  
  		   by Martin Mares <mj@ucw.cz> on 07-Feb-2000
  
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  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.
  
  
  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:
  
  	Enable the device
  	Access device configuration space
  	Discover resources (addresses and IRQ numbers) provided by the device
  	Allocate these resources
  	Communicate with the device
  	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.
  
  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.
  
  
  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:
  
  	name		Name of the driver
  	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.
  	suspend		Put device into low power state.
  	resume		Wake device from low power state.
  	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)
  
  The ID table is an array of struct pci_device_id ending with a 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 to match. The class_mask tells which bits
  	class_mask	of the class are honored during the comparison.
  	driver_data	Data private to the driver.
  
  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
  equivalant 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.
  
  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.
  
  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.
  
  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.
  
  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:
  	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.
  
          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.
  
  
  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:
  
  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. 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.
  
     If you want to 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.
  
     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
  calling pci_disable_device().
  
  4. 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.
  
  
  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.
  
     You still need to call request_region() for I/O regions and
  request_mem_region() for memory regions to make sure nobody else is using the
  same device.
  
     All interrupt handlers should be registered with SA_SHIRQ 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)
  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 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.
  
  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.
  
  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. 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.
  
  pcibios_present() and		Since ages, you don't need to test presence
  pci_present()			of PCI subsystem when trying to talk to it.
  				If it's not there, the list of PCI devices
  				is empty and all functions for searching for
  				devices just return NULL.
  pcibios_(read|write)_*		Superseded by their pci_(read|write)_*
  				counterparts.
  pcibios_find_*			Superseded by their pci_get_* counterparts.
  pci_for_each_dev()		Superseded by pci_get_device()
  pci_for_each_dev_reverse()	Superseded by pci_find_device_reverse()
  pci_for_each_bus()		Superseded by pci_find_next_bus()
  pci_find_device()		Superseded by pci_get_device()
  pci_find_subsys()		Superseded by pci_get_subsys()
  pcibios_find_class()		Superseded by pci_get_class()
  pci_find_class()		Superseded by pci_get_class()
  pci_(read|write)_*_nodev()	Superseded by pci_bus_(read|write)_*()