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Accessing PCI device resources through sysfs
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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.
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Writable files can be used to perform actions on the device (e.g. changing
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config space, detaching a device). mmapable files are available via an
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mmap of the file at offset 0 and can be used to do actual device programming
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from userspace. Note that some platforms don't support mmapping of certain
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resources, so be sure to check the return value from any attempted mmap.
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Accessing legacy resources through sysfs
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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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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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