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5d135dff53
idr gently pointed out today that not only is the sysfs rom file interface somewhat unintuitive (despite my efforts and initial implementation), but it's also undocumented! This patch to Documentation/filesystems/sysfs-pci.txt corrects the latter problem; the former is a userland ABI now though, so we're stuck with it for awhile at least. Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
96 lines
3.9 KiB
Plaintext
96 lines
3.9 KiB
Plaintext
Accessing PCI device resources through sysfs
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--------------------------------------------
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sysfs, usually mounted at /sys, provides access to PCI resources on platforms
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that support it. For example, a given bus might look like this:
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/sys/devices/pci0000:17
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|-- 0000:17:00.0
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| |-- class
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| |-- config
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| |-- device
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| |-- irq
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| |-- local_cpus
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| |-- resource
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| |-- resource0
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| |-- resource1
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| |-- resource2
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| |-- rom
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| |-- subsystem_device
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| |-- subsystem_vendor
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| `-- vendor
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`-- ...
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The topmost element describes the PCI domain and bus number. In this case,
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the domain number is 0000 and the bus number is 17 (both values are in hex).
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This bus contains a single function device in slot 0. The domain and bus
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numbers are reproduced for convenience. Under the device directory are several
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files, each with their own function.
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file function
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---- --------
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class PCI class (ascii, ro)
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config PCI config space (binary, rw)
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device PCI device (ascii, ro)
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irq IRQ number (ascii, ro)
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local_cpus nearby CPU mask (cpumask, ro)
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resource PCI resource host addresses (ascii, ro)
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resource0..N PCI resource N, if present (binary, mmap)
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rom PCI ROM resource, if present (binary, ro)
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subsystem_device PCI subsystem device (ascii, ro)
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subsystem_vendor PCI subsystem vendor (ascii, ro)
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vendor PCI vendor (ascii, ro)
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ro - read only file
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rw - file is readable and writable
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mmap - file is mmapable
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ascii - file contains ascii text
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binary - file contains binary data
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cpumask - file contains a cpumask type
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The read only files are informational, writes to them will be ignored, with
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the exception of the 'rom' file. Writable files can be used to perform
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actions on the device (e.g. changing config space, detaching a device).
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mmapable files are available via an mmap of the file at offset 0 and can be
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used to do actual device programming from userspace. Note that some platforms
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don't support mmapping of certain resources, so be sure to check the return
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value from any attempted mmap.
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The 'rom' file is special in that it provides read-only access to the device's
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ROM file, if available. It's disabled by default, however, so applications
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should write the string "1" to the file to enable it before attempting a read
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call, and disable it following the access by writing "0" to the file.
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Accessing legacy resources through sysfs
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----------------------------------------
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Legacy I/O port and ISA memory resources are also provided in sysfs if the
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underlying platform supports them. They're located in the PCI class heirarchy,
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e.g.
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/sys/class/pci_bus/0000:17/
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|-- bridge -> ../../../devices/pci0000:17
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|-- cpuaffinity
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|-- legacy_io
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`-- legacy_mem
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The legacy_io file is a read/write file that can be used by applications to
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do legacy port I/O. The application should open the file, seek to the desired
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port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes. The legacy_mem
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file should be mmapped with an offset corresponding to the memory offset
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desired, e.g. 0xa0000 for the VGA frame buffer. The application can then
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simply dereference the returned pointer (after checking for errors of course)
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to access legacy memory space.
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Supporting PCI access on new platforms
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--------------------------------------
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In order to support PCI resource mapping as described above, Linux platform
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code must define HAVE_PCI_MMAP and provide a pci_mmap_page_range function.
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Platforms are free to only support subsets of the mmap functionality, but
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useful return codes should be provided.
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Legacy resources are protected by the HAVE_PCI_LEGACY define. Platforms
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wishing to support legacy functionality should define it and provide
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pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.
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