1.. SPDX-License-Identifier: GPL-2.0 2 3============================================ 4Accessing PCI device resources through sysfs 5============================================ 6 7sysfs, usually mounted at /sys, provides access to PCI resources on platforms 8that 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 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 bus 33numbers are reproduced for convenience. Under the device directory are several 34files, 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 68The read only files are informational, writes to them will be ignored, with 69the exception of the 'rom' file. Writable files can be used to perform 70actions 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 be 72used to do actual device programming from userspace. Note that some platforms 73don't support mmapping of certain resources, so be sure to check the return 74value from any attempted mmap. The most notable of these are I/O port 75resources, which also provide read/write access. 76 77The 'enable' file provides a counter that indicates how many times the device 78has been enabled. If the 'enable' file currently returns '4', and a '1' is 79echoed into it, it will then return '5'. Echoing a '0' into it will decrease 80the count. Even when it returns to 0, though, some of the initialisation 81may not be reversed. 82 83The 'rom' file is special in that it provides read-only access to the device's 84ROM file, if available. It's disabled by default, however, so applications 85should write the string "1" to the file to enable it before attempting a read 86call, and disable it following the access by writing "0" to the file. Note 87that 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 the 89'enable' file, documented above. 90 91The 'remove' file is used to remove the PCI device, by writing a non-zero 92integer 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 of 94PCI devices, the sysfs directory for it is removed, and the device will be 95removed from any drivers attached to it. Removal of PCI root buses is 96disallowed. 97 98Accessing legacy resources through sysfs 99---------------------------------------- 100 101Legacy I/O port and ISA memory resources are also provided in sysfs if the 102underlying platform supports them. They're located in the PCI class hierarchy, 103e.g.:: 104 105 /sys/class/pci_bus/0000:17/ 106 |-- bridge -> ../../../devices/pci0000:17 107 |-- cpuaffinity 108 |-- legacy_io 109 `-- legacy_mem 110 111The legacy_io file is a read/write file that can be used by applications to 112do legacy port I/O. The application should open the file, seek to the desired 113port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes. The legacy_mem 114file should be mmapped with an offset corresponding to the memory offset 115desired, e.g. 0xa0000 for the VGA frame buffer. The application can then 116simply dereference the returned pointer (after checking for errors of course) 117to access legacy memory space. 118 119Supporting PCI access on new platforms 120-------------------------------------- 121 122In order to support PCI resource mapping as described above, Linux platform 123code should ideally define ARCH_GENERIC_PCI_MMAP_RESOURCE and use the generic 124implementation of that functionality. To support the historical interface of 125mmap() through files in /proc/bus/pci, platforms may also set HAVE_PCI_MMAP. 126 127Alternatively, platforms which set HAVE_PCI_MMAP may provide their own 128implementation of pci_mmap_page_range() instead of defining 129ARCH_GENERIC_PCI_MMAP_RESOURCE. 130 131Platforms which support write-combining maps of PCI resources must define 132arch_can_pci_mmap_wc() which shall evaluate to non-zero at runtime when 133write-combining is permitted. Platforms which support maps of I/O resources 134define arch_can_pci_mmap_io() similarly. 135 136Legacy resources are protected by the HAVE_PCI_LEGACY define. Platforms 137wishing to support legacy functionality should define it and provide 138pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions. 139