Documentation: better locations for sysfs-pci, sysfs-tagging

sysfs-pci and sysfs-tagging were mis-filed: their locations within
Documentation/ implied that they were related to file systems. Actually,
each topic is about a very specific *use* of sysfs, and sysfs *happens*
to be a (virtual) filesystem, so this is not really the right place.

It's jarring to be reading about filesystems in general and then come
across these specific details about PCI, and tagging...and then back to
general filesystems again.

Move sysfs-pci to PCI, and move sysfs-tagging to networking. (Thanks to
Jonathan Corbet for coming up with the final locations.)

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Link: https://lore.kernel.org/r/20201009070128.118639-1-jhubbard@nvidia.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

2 files changed, 139 insertions, 0 deletions

diff --git a/Documentation/PCI/index.rst b/Documentation/PCI/index.rst
index 8f66feaafd4f..c17c87af1968 100644
--- a/Documentation/PCI/index.rst
+++ b/Documentation/PCI/index.rst
@@ -12,6 +12,7 @@ Linux PCI Bus Subsystem
    pciebus-howto
    pci-iov-howto
    msi-howto
+   sysfs-pci
    acpi-info
    pci-error-recovery
    pcieaer-howto
diff --git a/Documentation/PCI/sysfs-pci.rst b/Documentation/PCI/sysfs-pci.rst
new file mode 100644
index 000000000000..742fbd21dc1f
--- /dev/null
+++ b/Documentation/PCI/sysfs-pci.rst
@@ -0,0 +1,138 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+============================================
+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
+     |   |-- revision
+     |   |-- 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)
+       revision		   PCI revision (ascii, ro)
+       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 IORESOURCE_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 should ideally define ARCH_GENERIC_PCI_MMAP_RESOURCE and use the generic
+implementation of that functionality. To support the historical interface of
+mmap() through files in /proc/bus/pci, platforms may also set HAVE_PCI_MMAP.
+
+Alternatively, platforms which set HAVE_PCI_MMAP may provide their own
+implementation of pci_mmap_page_range() instead of defining
+ARCH_GENERIC_PCI_MMAP_RESOURCE.
+
+Platforms which support write-combining maps of PCI resources must define
+arch_can_pci_mmap_wc() which shall evaluate to non-zero at runtime when
+write-combining is permitted. Platforms which support maps of I/O resources
+define arch_can_pci_mmap_io() similarly.
+
+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.