139 lines
6.0 KiB
ReStructuredText
139 lines
6.0 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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============================================
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Accessing PCI device resources through sysfs
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============================================
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sysfs, usually mounted at /sys, provides access to PCI resources on platforms
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that support it. For example, a given bus might look like this::
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/sys/devices/pci0000:17
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|-- 0000:17:00.0
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| |-- class
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| |-- config
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| |-- device
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| |-- enable
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| |-- irq
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| |-- local_cpus
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| |-- remove
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| |-- resource
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| |-- resource0
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| |-- resource1
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| |-- resource2
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| |-- revision
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| |-- rom
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| |-- subsystem_device
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| |-- subsystem_vendor
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| `-- vendor
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`-- ...
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The topmost element describes the PCI domain and bus number. In this case,
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the domain number is 0000 and the bus number is 17 (both values are in hex).
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This bus contains a single function device in slot 0. The domain and bus
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numbers are reproduced for convenience. Under the device directory are several
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files, each with their own function.
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=================== =====================================================
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file function
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=================== =====================================================
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class PCI class (ascii, ro)
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config PCI config space (binary, rw)
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device PCI device (ascii, ro)
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enable Whether the device is enabled (ascii, rw)
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irq IRQ number (ascii, ro)
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local_cpus nearby CPU mask (cpumask, ro)
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remove remove device from kernel's list (ascii, wo)
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resource PCI resource host addresses (ascii, ro)
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resource0..N PCI resource N, if present (binary, mmap, rw\ [1]_)
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resource0_wc..N_wc PCI WC map resource N, if prefetchable (binary, mmap)
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revision PCI revision (ascii, ro)
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rom PCI ROM resource, if present (binary, ro)
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subsystem_device PCI subsystem device (ascii, ro)
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subsystem_vendor PCI subsystem vendor (ascii, ro)
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vendor PCI vendor (ascii, ro)
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=================== =====================================================
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::
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ro - read only file
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rw - file is readable and writable
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wo - write only file
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mmap - file is mmapable
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ascii - file contains ascii text
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binary - file contains binary data
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cpumask - file contains a cpumask type
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.. [1] rw for IORESOURCE_IO (I/O port) regions only
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The read only files are informational, writes to them will be ignored, with
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the exception of the 'rom' file. Writable files can be used to perform
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actions on the device (e.g. changing config space, detaching a device).
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mmapable files are available via an mmap of the file at offset 0 and can be
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used to do actual device programming from userspace. Note that some platforms
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don't support mmapping of certain resources, so be sure to check the return
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value from any attempted mmap. The most notable of these are I/O port
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resources, which also provide read/write access.
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The 'enable' file provides a counter that indicates how many times the device
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has been enabled. If the 'enable' file currently returns '4', and a '1' is
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echoed into it, it will then return '5'. Echoing a '0' into it will decrease
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the count. Even when it returns to 0, though, some of the initialisation
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may not be reversed.
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The 'rom' file is special in that it provides read-only access to the device's
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ROM file, if available. It's disabled by default, however, so applications
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should write the string "1" to the file to enable it before attempting a read
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call, and disable it following the access by writing "0" to the file. Note
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that the device must be enabled for a rom read to return data successfully.
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In the event a driver is not bound to the device, it can be enabled using the
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'enable' file, documented above.
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The 'remove' file is used to remove the PCI device, by writing a non-zero
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integer to the file. This does not involve any kind of hot-plug functionality,
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e.g. powering off the device. The device is removed from the kernel's list of
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PCI devices, the sysfs directory for it is removed, and the device will be
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removed from any drivers attached to it. Removal of PCI root buses is
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disallowed.
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Accessing legacy resources through sysfs
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----------------------------------------
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Legacy I/O port and ISA memory resources are also provided in sysfs if the
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underlying platform supports them. They're located in the PCI class hierarchy,
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e.g.::
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/sys/class/pci_bus/0000:17/
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|-- bridge -> ../../../devices/pci0000:17
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|-- cpuaffinity
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|-- legacy_io
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`-- legacy_mem
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The legacy_io file is a read/write file that can be used by applications to
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do legacy port I/O. The application should open the file, seek to the desired
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port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes. The legacy_mem
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file should be mmapped with an offset corresponding to the memory offset
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desired, e.g. 0xa0000 for the VGA frame buffer. The application can then
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simply dereference the returned pointer (after checking for errors of course)
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to access legacy memory space.
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Supporting PCI access on new platforms
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--------------------------------------
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In order to support PCI resource mapping as described above, Linux platform
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code should ideally define ARCH_GENERIC_PCI_MMAP_RESOURCE and use the generic
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implementation of that functionality. To support the historical interface of
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mmap() through files in /proc/bus/pci, platforms may also set HAVE_PCI_MMAP.
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Alternatively, platforms which set HAVE_PCI_MMAP may provide their own
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implementation of pci_mmap_resource_range() instead of defining
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ARCH_GENERIC_PCI_MMAP_RESOURCE.
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Platforms which support write-combining maps of PCI resources must define
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arch_can_pci_mmap_wc() which shall evaluate to non-zero at runtime when
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write-combining is permitted. Platforms which support maps of I/O resources
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define arch_can_pci_mmap_io() similarly.
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Legacy resources are protected by the HAVE_PCI_LEGACY define. Platforms
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wishing to support legacy functionality should define it and provide
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pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.
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