1 .. SPDX-License-Identifier: GPL-2.0 2 3 ============================================ 4 Accessing PCI device resources through sysfs 5 ============================================ 6 7 sysfs, usually mounted at /sys, provides access to PCI resources on platforms 8 that support it. For example, a given bus might look like this:: 9 10 /sys/devices/pci0000:17 11 |-- 0000:17:00.0 12 | |-- class 13 | |-- config 14 | |-- device 15 | |-- enable 16 | |-- irq 17 | |-- local_cpus 18 | |-- remove 19 | |-- resource 20 | |-- resource0 21 | |-- resource1 22 | |-- resource2 23 | |-- revision 24 | |-- rom 25 | |-- subsystem_device 26 | |-- subsystem_vendor 27 | `-- vendor 28 `-- ... 29 30 The topmost element describes the PCI domain and bus number. In this case, 31 the domain number is 0000 and the bus number is 17 (both values are in hex). 32 This bus contains a single function device in slot 0. The domain and bus 33 numbers are reproduced for convenience. Under the device directory are several 34 files, each with their own function. 35 36 =================== ===================================================== 37 file function 38 =================== ===================================================== 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 file 59 rw - file is readable and writable 60 wo - write only file 61 mmap - file is mmapable 62 ascii - file contains ascii text 63 binary - file contains binary data 64 cpumask - file contains a cpumask type 65 66 .. [1] rw for IORESOURCE_IO (I/O port) regions only 67 68 The read only files are informational, writes to them will be ignored, with 69 the exception of the 'rom' file. Writable files can be used to perform 70 actions on the device (e.g. changing config space, detaching a device). 71 mmapable files are available via an mmap of the file at offset 0 and can be 72 used to do actual device programming from userspace. Note that some platforms 73 don't support mmapping of certain resources, so be sure to check the return 74 value from any attempted mmap. The most notable of these are I/O port 75 resources, which also provide read/write access. 76 77 The 'enable' file provides a counter that indicates how many times the device 78 has been enabled. If the 'enable' file currently returns '4', and a '1' is 79 echoed into it, it will then return '5'. Echoing a '0' into it will decrease 80 the count. Even when it returns to 0, though, some of the initialisation 81 may not be reversed. 82 83 The 'rom' file is special in that it provides read-only access to the device's 84 ROM file, if available. It's disabled by default, however, so applications 85 should write the string "1" to the file to enable it before attempting a read 86 call, and disable it following the access by writing "0" to the file. Note 87 that the device must be enabled for a rom read to return data successfully. 88 In the event a driver is not bound to the device, it can be enabled using the 89 'enable' file, documented above. 90 91 The 'remove' file is used to remove the PCI device, by writing a non-zero 92 integer to the file. This does not involve any kind of hot-plug functionality, 93 e.g. powering off the device. The device is removed from the kernel's list of 94 PCI devices, the sysfs directory for it is removed, and the device will be 95 removed from any drivers attached to it. Removal of PCI root buses is 96 disallowed. 97 98 Accessing legacy resources through sysfs 99 ---------------------------------------- 100 101 Legacy I/O port and ISA memory resources are also provided in sysfs if the 102 underlying platform supports them. They're located in the PCI class hierarchy, 103 e.g.:: 104 105 /sys/class/pci_bus/0000:17/ 106 |-- bridge -> ../../../devices/pci0000:17 107 |-- cpuaffinity 108 |-- legacy_io 109 `-- legacy_mem 110 111 The legacy_io file is a read/write file that can be used by applications to 112 do legacy port I/O. The application should open the file, seek to the desired 113 port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes. The legacy_mem 114 file should be mmapped with an offset corresponding to the memory offset 115 desired, e.g. 0xa0000 for the VGA frame buffer. The application can then 116 simply dereference the returned pointer (after checking for errors of course) 117 to access legacy memory space. 118 119 Supporting PCI access on new platforms 120 -------------------------------------- 121 122 In order to support PCI resource mapping as described above, Linux platform 123 code should ideally define ARCH_GENERIC_PCI_MMAP_RESOURCE and use the generic 124 implementation of that functionality. To support the historical interface of 125 mmap() through files in /proc/bus/pci, platforms may also set HAVE_PCI_MMAP. 126 127 Alternatively, platforms which set HAVE_PCI_MMAP may provide their own 128 implementation of pci_mmap_resource_range() instead of defining 129 ARCH_GENERIC_PCI_MMAP_RESOURCE. 130 131 Platforms which support write-combining maps of PCI resources must define 132 arch_can_pci_mmap_wc() which shall evaluate to non-zero at runtime when 133 write-combining is permitted. Platforms which support maps of I/O resources 134 define arch_can_pci_mmap_io() similarly. 135 136 Legacy resources are protected by the HAVE_PCI_LEGACY define. Platforms 137 wishing to support legacy functionality should define it and provide 138 pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.