1Accessing PCI device resources through sysfs
  2--------------------------------------------
  3
  4sysfs, usually mounted at /sys, provides access to PCI resources on platforms
  5that support it.  For example, a given bus might look like this:
  6
  7     /sys/devices/pci0000:17
  8     |-- 0000:17:00.0
  9     |   |-- class
 10     |   |-- config
 11     |   |-- device
 12     |   |-- enable
 13     |   |-- irq
 14     |   |-- local_cpus
 15     |   |-- remove
 16     |   |-- resource
 17     |   |-- resource0
 18     |   |-- resource1
 19     |   |-- resource2
 20     |   |-- revision
 21     |   |-- rom
 22     |   |-- subsystem_device
 23     |   |-- subsystem_vendor
 24     |   `-- vendor
 25     `-- ...
 26
 27The topmost element describes the PCI domain and bus number.  In this case,
 28the domain number is 0000 and the bus number is 17 (both values are in hex).
 29This bus contains a single function device in slot 0.  The domain and bus
 30numbers are reproduced for convenience.  Under the device directory are several
 31files, each with their own function.
 32
 33       file		   function
 34       ----		   --------
 35       class		   PCI class (ascii, ro)
 36       config		   PCI config space (binary, rw)
 37       device		   PCI device (ascii, ro)
 38       enable	           Whether the device is enabled (ascii, rw)
 39       irq		   IRQ number (ascii, ro)
 40       local_cpus	   nearby CPU mask (cpumask, ro)
 41       remove		   remove device from kernel's list (ascii, wo)
 42       resource		   PCI resource host addresses (ascii, ro)
 43       resource0..N	   PCI resource N, if present (binary, mmap, rw[1])
 44       resource0_wc..N_wc  PCI WC map resource N, if prefetchable (binary, mmap)
 45       revision		   PCI revision (ascii, ro)
 46       rom		   PCI ROM resource, if present (binary, ro)
 47       subsystem_device	   PCI subsystem device (ascii, ro)
 48       subsystem_vendor	   PCI subsystem vendor (ascii, ro)
 49       vendor		   PCI vendor (ascii, ro)
 50
 51  ro - read only file
 52  rw - file is readable and writable
 53  wo - write only file
 54  mmap - file is mmapable
 55  ascii - file contains ascii text
 56  binary - file contains binary data
 57  cpumask - file contains a cpumask type
 58
 59[1] rw for RESOURCE_IO (I/O port) regions only
 60
 61The read only files are informational, writes to them will be ignored, with
 62the exception of the 'rom' file.  Writable files can be used to perform
 63actions on the device (e.g. changing config space, detaching a device).
 64mmapable files are available via an mmap of the file at offset 0 and can be
 65used to do actual device programming from userspace.  Note that some platforms
 66don't support mmapping of certain resources, so be sure to check the return
 67value from any attempted mmap.  The most notable of these are I/O port
 68resources, which also provide read/write access.
 69
 70The 'enable' file provides a counter that indicates how many times the device 
 71has been enabled.  If the 'enable' file currently returns '4', and a '1' is
 72echoed into it, it will then return '5'.  Echoing a '0' into it will decrease
 73the count.  Even when it returns to 0, though, some of the initialisation
 74may not be reversed.  
 75
 76The 'rom' file is special in that it provides read-only access to the device's
 77ROM file, if available.  It's disabled by default, however, so applications
 78should write the string "1" to the file to enable it before attempting a read
 79call, and disable it following the access by writing "0" to the file.  Note
 80that the device must be enabled for a rom read to return data successfully.
 81In the event a driver is not bound to the device, it can be enabled using the
 82'enable' file, documented above.
 83
 84The 'remove' file is used to remove the PCI device, by writing a non-zero
 85integer to the file.  This does not involve any kind of hot-plug functionality,
 86e.g. powering off the device.  The device is removed from the kernel's list of
 87PCI devices, the sysfs directory for it is removed, and the device will be
 88removed from any drivers attached to it. Removal of PCI root buses is
 89disallowed.
 90
 91Accessing legacy resources through sysfs
 92----------------------------------------
 93
 94Legacy I/O port and ISA memory resources are also provided in sysfs if the
 95underlying platform supports them.  They're located in the PCI class hierarchy,
 96e.g.
 97
 98	/sys/class/pci_bus/0000:17/
 99	|-- bridge -> ../../../devices/pci0000:17
100	|-- cpuaffinity
101	|-- legacy_io
102	`-- legacy_mem
103
104The legacy_io file is a read/write file that can be used by applications to
105do legacy port I/O.  The application should open the file, seek to the desired
106port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes.  The legacy_mem
107file should be mmapped with an offset corresponding to the memory offset
108desired, e.g. 0xa0000 for the VGA frame buffer.  The application can then
109simply dereference the returned pointer (after checking for errors of course)
110to access legacy memory space.
111
112Supporting PCI access on new platforms
113--------------------------------------
114
115In order to support PCI resource mapping as described above, Linux platform
116code should ideally define ARCH_GENERIC_PCI_MMAP_RESOURCE and use the generic
117implementation of that functionality. To support the historical interface of
118mmap() through files in /proc/bus/pci, platforms may also set HAVE_PCI_MMAP.
119
120Alternatively, platforms which set HAVE_PCI_MMAP may provide their own
121implementation of pci_mmap_page_range() instead of defining
122ARCH_GENERIC_PCI_MMAP_RESOURCE.
123
124Platforms which support write-combining maps of PCI resources must define
125arch_can_pci_mmap_wc() which shall evaluate to non-zero at runtime when
126write-combining is permitted. Platforms which support maps of I/O resources
127define arch_can_pci_mmap_io() similarly.
128
129Legacy resources are protected by the HAVE_PCI_LEGACY define.  Platforms
130wishing to support legacy functionality should define it and provide
131pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.