| 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 |
| | |-- device |
| | |-- enable |
| | |-- irq |
| | |-- local_cpus |
| | |-- remove |
| | |-- resource |
| | |-- resource0 |
| | |-- resource1 |
| | |-- resource2 |
| | |-- rom |
| | |-- subsystem_device |
| | |-- subsystem_vendor |
| | `-- vendor |
| `-- ... |
| |
| 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) |
| device PCI device (ascii, ro) |
| enable Whether the device is enabled (ascii, rw) |
| irq IRQ number (ascii, ro) |
| local_cpus nearby CPU mask (cpumask, ro) |
| remove remove device from kernel's list (ascii, wo) |
| resource PCI resource host addresses (ascii, ro) |
| resource0..N PCI resource N, if present (binary, mmap, rw[1]) |
| resource0_wc..N_wc PCI WC map resource N, if prefetchable (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 |
| wo - write only file |
| mmap - file is mmapable |
| ascii - file contains ascii text |
| binary - file contains binary data |
| cpumask - file contains a cpumask type |
| |
| [1] rw for RESOURCE_IO (I/O port) regions only |
| |
| The read only files are informational, writes to them will be ignored, with |
| the exception of the 'rom' file. 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. The most notable of these are I/O port |
| resources, which also provide read/write access. |
| |
| The 'enable' file provides a counter that indicates how many times the device |
| has been enabled. If the 'enable' file currently returns '4', and a '1' is |
| echoed into it, it will then return '5'. Echoing a '0' into it will decrease |
| the count. Even when it returns to 0, though, some of the initialisation |
| may not be reversed. |
| |
| The 'rom' file is special in that it provides read-only access to the device's |
| ROM file, if available. It's disabled by default, however, so applications |
| should write the string "1" to the file to enable it before attempting a read |
| call, and disable it following the access by writing "0" to the file. Note |
| that the device must be enabled for a rom read to return data successfully. |
| In the event a driver is not bound to the device, it can be enabled using the |
| 'enable' file, documented above. |
| |
| The 'remove' file is used to remove the PCI device, by writing a non-zero |
| integer to the file. This does not involve any kind of hot-plug functionality, |
| e.g. powering off the device. The device is removed from the kernel's list of |
| PCI devices, the sysfs directory for it is removed, and the device will be |
| removed from any drivers attached to it. Removal of PCI root buses is |
| disallowed. |
| |
| 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 hierarchy, |
| 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. |