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Accessing PCI device resources through sysfs
sysfs, usually mounted at /sys, provides access to PCI resources on platforms
that support it. For example, a given bus might look like this:
|-- 0000:17:00.0
| |-- class
| |-- config
| |-- device
| |-- irq
| |-- local_cpus
| |-- resource
| |-- resource0
| |-- resource1
| |-- resource2
| |-- rom
| |-- subsystem_device
| |-- subsystem_vendor
| `-- vendor
`-- ...
The topmost element describes the PCI domain and bus number. In this case,
the domain number is 0000 and the bus number is 17 (both values are in hex).
This bus contains a single function device in slot 0. The domain and bus
numbers are reproduced for convenience. Under the device directory are several
files, each with their own function.
file function
---- --------
class PCI class (ascii, ro)
config PCI config space (binary, rw)
device PCI device (ascii, ro)
irq IRQ number (ascii, ro)
local_cpus nearby CPU mask (cpumask, ro)
resource PCI resource host addresses (ascii, ro)
resource0..N PCI resource N, if present (binary, mmap)
rom PCI ROM resource, if present (binary, ro)
subsystem_device PCI subsystem device (ascii, ro)
subsystem_vendor PCI subsystem vendor (ascii, ro)
vendor PCI vendor (ascii, ro)
ro - read only file
rw - file is readable and writable
mmap - file is mmapable
ascii - file contains ascii text
binary - file contains binary data
cpumask - file contains a cpumask type
The read only files are informational, writes to them will be ignored, with
the exception of the 'rom' file. Writable files can be used to perform
actions on the device (e.g. changing config space, detaching a device).
mmapable files are available via an mmap of the file at offset 0 and can be
used to do actual device programming from userspace. Note that some platforms
don't support mmapping of certain resources, so be sure to check the return
value from any attempted mmap.
The 'rom' file is special in that it provides read-only access to the device's
ROM file, if available. It's disabled by default, however, so applications
should write the string "1" to the file to enable it before attempting a read
call, and disable it following the access by writing "0" to the file.
Accessing legacy resources through sysfs
Legacy I/O port and ISA memory resources are also provided in sysfs if the
underlying platform supports them. They're located in the PCI class heirarchy,
|-- bridge -> ../../../devices/pci0000:17
|-- cpuaffinity
|-- legacy_io
`-- legacy_mem
The legacy_io file is a read/write file that can be used by applications to
do legacy port I/O. The application should open the file, seek to the desired
port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes. The legacy_mem
file should be mmapped with an offset corresponding to the memory offset
desired, e.g. 0xa0000 for the VGA frame buffer. The application can then
simply dereference the returned pointer (after checking for errors of course)
to access legacy memory space.
Supporting PCI access on new platforms
In order to support PCI resource mapping as described above, Linux platform
code must define HAVE_PCI_MMAP and provide a pci_mmap_page_range function.
Platforms are free to only support subsets of the mmap functionality, but
useful return codes should be provided.
Legacy resources are protected by the HAVE_PCI_LEGACY define. Platforms
wishing to support legacy functionality should define it and provide
pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.