Documentation/sysfs-rules.txt: convert it to ReST markup

- Fix document title;
- use quote blocks where needed;
- use monotonic fonts for config options and file names;
- adjust whitespaces and blank lines;
- add it to the user's book.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/sysfs-rules.txt

 Rules on how to access information in the Linux kernel sysfs
+============================================================
 
 The kernel-exported sysfs exports internal kernel implementation details
 and depends on internal kernel structures and layout. It is agreed upon
@@ -18,36 +19,38 @@ the following rules and then your programs should work with future
 versions of the sysfs interface.
 
 - Do not use libsysfs
-  It makes assumptions about sysfs which are not true. Its API does not
-  offer any abstraction, it exposes all the kernel driver-core
-  implementation details in its own API. Therefore it is not better than
-  reading directories and opening the files yourself.
-  Also, it is not actively maintained, in the sense of reflecting the
-  current kernel development. The goal of providing a stable interface
-  to sysfs has failed; it causes more problems than it solves. It
-  violates many of the rules in this document.
-
-- sysfs is always at /sys
-  Parsing /proc/mounts is a waste of time. Other mount points are a
-  system configuration bug you should not try to solve. For test cases,
-  possibly support a SYSFS_PATH environment variable to overwrite the
-  application's behavior, but never try to search for sysfs. Never try
-  to mount it, if you are not an early boot script.
+    It makes assumptions about sysfs which are not true. Its API does not
+    offer any abstraction, it exposes all the kernel driver-core
+    implementation details in its own API. Therefore it is not better than
+    reading directories and opening the files yourself.
+    Also, it is not actively maintained, in the sense of reflecting the
+    current kernel development. The goal of providing a stable interface
+    to sysfs has failed; it causes more problems than it solves. It
+    violates many of the rules in this document.
+
+- sysfs is always at ``/sys``
+    Parsing ``/proc/mounts`` is a waste of time. Other mount points are a
+    system configuration bug you should not try to solve. For test cases,
+    possibly support a ``SYSFS_PATH`` environment variable to overwrite the
+    application's behavior, but never try to search for sysfs. Never try
+    to mount it, if you are not an early boot script.
 
 - devices are only "devices"
-  There is no such thing like class-, bus-, physical devices,
-  interfaces, and such that you can rely on in userspace. Everything is
-  just simply a "device". Class-, bus-, physical, ... types are just
-  kernel implementation details which should not be expected by
-  applications that look for devices in sysfs.
-
-  The properties of a device are:
-    o devpath (/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0)
+    There is no such thing like class-, bus-, physical devices,
+    interfaces, and such that you can rely on in userspace. Everything is
+    just simply a "device". Class-, bus-, physical, ... types are just
+    kernel implementation details which should not be expected by
+    applications that look for devices in sysfs.
+
+    The properties of a device are:
+
+    - devpath (``/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0``)
+
       - identical to the DEVPATH value in the event sent from the kernel
         at device creation and removal
       - the unique key to the device at that point in time
       - the kernel's path to the device directory without the leading
-        /sys, and always starting with a slash
+        ``/sys``, and always starting with a slash
       - all elements of a devpath must be real directories. Symlinks
         pointing to /sys/devices must always be resolved to their real
         target and the target path must be used to access the device.
@@ -56,17 +59,20 @@ versions of the sysfs interface.
       - using or exposing symlink values as elements in a devpath string
         is a bug in the application
 
-    o kernel name (sda, tty, 0000:00:1f.2, ...)
+    - kernel name (``sda``, ``tty``, ``0000:00:1f.2``, ...)
+
       - a directory name, identical to the last element of the devpath
-      - applications need to handle spaces and characters like '!' in
+      - applications need to handle spaces and characters like ``!`` in
         the name
 
-    o subsystem (block, tty, pci, ...)
+    - subsystem (``block``, ``tty``, ``pci``, ...)
+
       - simple string, never a path or a link
       - retrieved by reading the "subsystem"-link and using only the
         last element of the target path
 
-    o driver (tg3, ata_piix, uhci_hcd)
+    - driver (``tg3``, ``ata_piix``, ``uhci_hcd``)
+
       - a simple string, which may contain spaces, never a path or a
         link
       - it is retrieved by reading the "driver"-link and using only the
@@ -75,110 +81,112 @@ versions of the sysfs interface.
         driver; copying the driver value in a child device context is a
         bug in the application
 
-    o attributes
+    - attributes
+
       - the files in the device directory or files below subdirectories
         of the same device directory
       - accessing attributes reached by a symlink pointing to another device,
         like the "device"-link, is a bug in the application
 
-  Everything else is just a kernel driver-core implementation detail
-  that should not be assumed to be stable across kernel releases.
+    Everything else is just a kernel driver-core implementation detail
+    that should not be assumed to be stable across kernel releases.
 
 - Properties of parent devices never belong into a child device.
-  Always look at the parent devices themselves for determining device
-  context properties. If the device 'eth0' or 'sda' does not have a
-  "driver"-link, then this device does not have a driver. Its value is empty.
-  Never copy any property of the parent-device into a child-device. Parent
-  device properties may change dynamically without any notice to the
-  child device.
+    Always look at the parent devices themselves for determining device
+    context properties. If the device ``eth0`` or ``sda`` does not have a
+    "driver"-link, then this device does not have a driver. Its value is empty.
+    Never copy any property of the parent-device into a child-device. Parent
+    device properties may change dynamically without any notice to the
+    child device.
 
 - Hierarchy in a single device tree
-  There is only one valid place in sysfs where hierarchy can be examined
-  and this is below: /sys/devices.
-  It is planned that all device directories will end up in the tree
-  below this directory.
+    There is only one valid place in sysfs where hierarchy can be examined
+    and this is below: ``/sys/devices.``
+    It is planned that all device directories will end up in the tree
+    below this directory.
 
 - Classification by subsystem
-  There are currently three places for classification of devices:
-  /sys/block, /sys/class and /sys/bus. It is planned that these will
-  not contain any device directories themselves, but only flat lists of
-  symlinks pointing to the unified /sys/devices tree.
-  All three places have completely different rules on how to access
-  device information. It is planned to merge all three
-  classification directories into one place at /sys/subsystem,
-  following the layout of the bus directories. All buses and
-  classes, including the converted block subsystem, will show up
-  there.
-  The devices belonging to a subsystem will create a symlink in the
-  "devices" directory at /sys/subsystem/<name>/devices.
-
-  If /sys/subsystem exists, /sys/bus, /sys/class and /sys/block can be
-  ignored. If it does not exist, you always have to scan all three
-  places, as the kernel is free to move a subsystem from one place to
-  the other, as long as the devices are still reachable by the same
-  subsystem name.
-
-  Assuming /sys/class/<subsystem> and /sys/bus/<subsystem>, or
-  /sys/block and /sys/class/block are not interchangeable is a bug in
-  the application.
+    There are currently three places for classification of devices:
+    ``/sys/block,`` ``/sys/class`` and ``/sys/bus.`` It is planned that these will
+    not contain any device directories themselves, but only flat lists of
+    symlinks pointing to the unified ``/sys/devices`` tree.
+    All three places have completely different rules on how to access
+    device information. It is planned to merge all three
+    classification directories into one place at ``/sys/subsystem``,
+    following the layout of the bus directories. All buses and
+    classes, including the converted block subsystem, will show up
+    there.
+    The devices belonging to a subsystem will create a symlink in the
+    "devices" directory at ``/sys/subsystem/<name>/devices``,
+
+    If ``/sys/subsystem`` exists, ``/sys/bus``, ``/sys/class`` and ``/sys/block``
+    can be ignored. If it does not exist, you always have to scan all three
+    places, as the kernel is free to move a subsystem from one place to
+    the other, as long as the devices are still reachable by the same
+    subsystem name.
+
+    Assuming ``/sys/class/<subsystem>`` and ``/sys/bus/<subsystem>``, or
+    ``/sys/block`` and ``/sys/class/block`` are not interchangeable is a bug in
+    the application.
 
 - Block
-  The converted block subsystem at /sys/class/block or
-  /sys/subsystem/block will contain the links for disks and partitions
-  at the same level, never in a hierarchy. Assuming the block subsystem to
-  contain only disks and not partition devices in the same flat list is
-  a bug in the application.
+    The converted block subsystem at ``/sys/class/block`` or
+    ``/sys/subsystem/block`` will contain the links for disks and partitions
+    at the same level, never in a hierarchy. Assuming the block subsystem to
+    contain only disks and not partition devices in the same flat list is
+    a bug in the application.
 
 - "device"-link and <subsystem>:<kernel name>-links
-  Never depend on the "device"-link. The "device"-link is a workaround
-  for the old layout, where class devices are not created in
-  /sys/devices/ like the bus devices. If the link-resolving of a
-  device directory does not end in /sys/devices/, you can use the
-  "device"-link to find the parent devices in /sys/devices/. That is the
-  single valid use of the "device"-link; it must never appear in any
-  path as an element. Assuming the existence of the "device"-link for
-  a device in /sys/devices/ is a bug in the application.
-  Accessing /sys/class/net/eth0/device is a bug in the application.
-
-  Never depend on the class-specific links back to the /sys/class
-  directory.  These links are also a workaround for the design mistake
-  that class devices are not created in /sys/devices. If a device
-  directory does not contain directories for child devices, these links
-  may be used to find the child devices in /sys/class. That is the single
-  valid use of these links; they must never appear in any path as an
-  element. Assuming the existence of these links for devices which are
-  real child device directories in the /sys/devices tree is a bug in
-  the application.
-
-  It is planned to remove all these links when all class device
-  directories live in /sys/devices.
+    Never depend on the "device"-link. The "device"-link is a workaround
+    for the old layout, where class devices are not created in
+    ``/sys/devices/`` like the bus devices. If the link-resolving of a
+    device directory does not end in ``/sys/devices/``, you can use the
+    "device"-link to find the parent devices in ``/sys/devices/``, That is the
+    single valid use of the "device"-link; it must never appear in any
+    path as an element. Assuming the existence of the "device"-link for
+    a device in ``/sys/devices/`` is a bug in the application.
+    Accessing ``/sys/class/net/eth0/device`` is a bug in the application.
+
+    Never depend on the class-specific links back to the ``/sys/class``
+    directory.  These links are also a workaround for the design mistake
+    that class devices are not created in ``/sys/devices.`` If a device
+    directory does not contain directories for child devices, these links
+    may be used to find the child devices in ``/sys/class.`` That is the single
+    valid use of these links; they must never appear in any path as an
+    element. Assuming the existence of these links for devices which are
+    real child device directories in the ``/sys/devices`` tree is a bug in
+    the application.
+
+    It is planned to remove all these links when all class device
+    directories live in ``/sys/devices.``
 
 - Position of devices along device chain can change.
-  Never depend on a specific parent device position in the devpath,
-  or the chain of parent devices. The kernel is free to insert devices into
-  the chain. You must always request the parent device you are looking for
-  by its subsystem value. You need to walk up the chain until you find
-  the device that matches the expected subsystem. Depending on a specific
-  position of a parent device or exposing relative paths using "../" to
-  access the chain of parents is a bug in the application.
+    Never depend on a specific parent device position in the devpath,
+    or the chain of parent devices. The kernel is free to insert devices into
+    the chain. You must always request the parent device you are looking for
+    by its subsystem value. You need to walk up the chain until you find
+    the device that matches the expected subsystem. Depending on a specific
+    position of a parent device or exposing relative paths using ``../`` to
+    access the chain of parents is a bug in the application.
 
 - When reading and writing sysfs device attribute files, avoid dependency
-  on specific error codes wherever possible. This minimizes coupling to
-  the error handling implementation within the kernel.
+    on specific error codes wherever possible. This minimizes coupling to
+    the error handling implementation within the kernel.
 
-  In general, failures to read or write sysfs device attributes shall
-  propagate errors wherever possible. Common errors include, but are not
-  limited to:
+    In general, failures to read or write sysfs device attributes shall
+    propagate errors wherever possible. Common errors include, but are not
+    limited to:
 
-  -EIO: The read or store operation is not supported, typically returned by
-        the sysfs system itself if the read or store pointer is NULL.
+	``-EIO``: The read or store operation is not supported, typically
+	returned by the sysfs system itself if the read or store pointer
+	is ``NULL``.
 
-  -ENXIO: The read or store operation failed
+	``-ENXIO``: The read or store operation failed
 
-  Error codes will not be changed without good reason, and should a change
-  to error codes result in user-space breakage, it will be fixed, or the
-  the offending change will be reverted.
+    Error codes will not be changed without good reason, and should a change
+    to error codes result in user-space breakage, it will be fixed, or the
+    the offending change will be reverted.
 
-  Userspace applications can, however, expect the format and contents of
-  the attribute files to remain consistent in the absence of a version
-  attribute change in the context of a given attribute.
+    Userspace applications can, however, expect the format and contents of
+    the attribute files to remain consistent in the absence of a version
+    attribute change in the context of a given attribute.