--- /dev/null
+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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