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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:
- /sys/devices/pci0000:17
- |-- 0000:17:00.0
- | |-- class
- | |-- config
- | |-- detach_state
- | |-- device
- | |-- irq
- | |-- local_cpus
- | |-- resource
- | |-- resource0
- | |-- resource1
- | |-- resource2
- | |-- rom
- | |-- subsystem_device
- | |-- subsystem_vendor
- | `-- vendor
- `-- detach_state
- 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)
- detach_state connection status (bool, 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.
- 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.
- 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,
- e.g.
- /sys/class/pci_bus/0000:17/
- |-- 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.
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