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1.. SPDX-License-Identifier: GPL-2.02 3============================================4Accessing PCI device resources through sysfs5============================================6 7sysfs, usually mounted at /sys, provides access to PCI resources on platforms8that support it. For example, a given bus might look like this::9 10 /sys/devices/pci0000:1711 |-- 0000:17:00.012 | |-- class13 | |-- config14 | |-- device15 | |-- enable16 | |-- irq17 | |-- local_cpus18 | |-- remove19 | |-- resource20 | |-- resource021 | |-- resource122 | |-- resource223 | |-- revision24 | |-- rom25 | |-- subsystem_device26 | |-- subsystem_vendor27 | `-- vendor28 `-- ...29 30The topmost element describes the PCI domain and bus number. In this case,31the domain number is 0000 and the bus number is 17 (both values are in hex).32This bus contains a single function device in slot 0. The domain and bus33numbers are reproduced for convenience. Under the device directory are several34files, each with their own function.35 36 =================== =====================================================37 file function38 =================== =====================================================39 class PCI class (ascii, ro)40 config PCI config space (binary, rw)41 device PCI device (ascii, ro)42 enable Whether the device is enabled (ascii, rw)43 irq IRQ number (ascii, ro)44 local_cpus nearby CPU mask (cpumask, ro)45 remove remove device from kernel's list (ascii, wo)46 resource PCI resource host addresses (ascii, ro)47 resource0..N PCI resource N, if present (binary, mmap, rw\ [1]_)48 resource0_wc..N_wc PCI WC map resource N, if prefetchable (binary, mmap)49 revision PCI revision (ascii, ro)50 rom PCI ROM resource, if present (binary, ro)51 subsystem_device PCI subsystem device (ascii, ro)52 subsystem_vendor PCI subsystem vendor (ascii, ro)53 vendor PCI vendor (ascii, ro)54 =================== =====================================================55 56::57 58 ro - read only file59 rw - file is readable and writable60 wo - write only file61 mmap - file is mmapable62 ascii - file contains ascii text63 binary - file contains binary data64 cpumask - file contains a cpumask type65 66.. [1] rw for IORESOURCE_IO (I/O port) regions only67 68The read only files are informational, writes to them will be ignored, with69the exception of the 'rom' file. Writable files can be used to perform70actions on the device (e.g. changing config space, detaching a device).71mmapable files are available via an mmap of the file at offset 0 and can be72used to do actual device programming from userspace. Note that some platforms73don't support mmapping of certain resources, so be sure to check the return74value from any attempted mmap. The most notable of these are I/O port75resources, which also provide read/write access.76 77The 'enable' file provides a counter that indicates how many times the device78has been enabled. If the 'enable' file currently returns '4', and a '1' is79echoed into it, it will then return '5'. Echoing a '0' into it will decrease80the count. Even when it returns to 0, though, some of the initialisation81may not be reversed.82 83The 'rom' file is special in that it provides read-only access to the device's84ROM file, if available. It's disabled by default, however, so applications85should write the string "1" to the file to enable it before attempting a read86call, and disable it following the access by writing "0" to the file. Note87that the device must be enabled for a rom read to return data successfully.88In the event a driver is not bound to the device, it can be enabled using the89'enable' file, documented above.90 91The 'remove' file is used to remove the PCI device, by writing a non-zero92integer to the file. This does not involve any kind of hot-plug functionality,93e.g. powering off the device. The device is removed from the kernel's list of94PCI devices, the sysfs directory for it is removed, and the device will be95removed from any drivers attached to it. Removal of PCI root buses is96disallowed.97 98Accessing legacy resources through sysfs99----------------------------------------100 101Legacy I/O port and ISA memory resources are also provided in sysfs if the102underlying platform supports them. They're located in the PCI class hierarchy,103e.g.::104 105 /sys/class/pci_bus/0000:17/106 |-- bridge -> ../../../devices/pci0000:17107 |-- cpuaffinity108 |-- legacy_io109 `-- legacy_mem110 111The legacy_io file is a read/write file that can be used by applications to112do legacy port I/O. The application should open the file, seek to the desired113port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes. The legacy_mem114file should be mmapped with an offset corresponding to the memory offset115desired, e.g. 0xa0000 for the VGA frame buffer. The application can then116simply dereference the returned pointer (after checking for errors of course)117to access legacy memory space.118 119Supporting PCI access on new platforms120--------------------------------------121 122In order to support PCI resource mapping as described above, Linux platform123code should ideally define ARCH_GENERIC_PCI_MMAP_RESOURCE and use the generic124implementation of that functionality. To support the historical interface of125mmap() through files in /proc/bus/pci, platforms may also set HAVE_PCI_MMAP.126 127Alternatively, platforms which set HAVE_PCI_MMAP may provide their own128implementation of pci_mmap_resource_range() instead of defining129ARCH_GENERIC_PCI_MMAP_RESOURCE.130 131Platforms which support write-combining maps of PCI resources must define132arch_can_pci_mmap_wc() which shall evaluate to non-zero at runtime when133write-combining is permitted. Platforms which support maps of I/O resources134define arch_can_pci_mmap_io() similarly.135 136Legacy resources are protected by the HAVE_PCI_LEGACY define. Platforms137wishing to support legacy functionality should define it and provide138pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.139