media: docs: move uAPI book to userspace-api/media

Since 2017, there is an space reserved for userspace API,
created by changeset 1d596dee38 ("docs: Create a user-space API guide").

As the media subsystem was one of the first subsystems to use
Sphinx, until this patch, we were keeping things on a separate
place.

Let's just use the new location, as having all uAPI altogether
will likely make things easier for developers.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/userspace-api/media/gen-errors.rst

@@ -0,0 +1,103 @@
+.. Permission is granted to copy, distribute and/or modify this
+.. document under the terms of the GNU Free Documentation License,
+.. Version 1.1 or any later version published by the Free Software
+.. Foundation, with no Invariant Sections, no Front-Cover Texts
+.. and no Back-Cover Texts. A copy of the license is included at
+.. Documentation/userspace-api/media/fdl-appendix.rst.
+..
+.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
+
+.. _gen_errors:
+
+*******************
+Generic Error Codes
+*******************
+
+
+.. _gen-errors:
+
+.. tabularcolumns:: |p{2.5cm}|p{15.0cm}|
+
+.. flat-table:: Generic error codes
+    :header-rows:  0
+    :stub-columns: 0
+    :widths: 1 16
+
+
+    -  -  ``EAGAIN`` (aka ``EWOULDBLOCK``)
+
+       -  The ioctl can't be handled because the device is in state where it
+	  can't perform it. This could happen for example in case where
+	  device is sleeping and ioctl is performed to query statistics. It
+	  is also returned when the ioctl would need to wait for an event,
+	  but the device was opened in non-blocking mode.
+
+    -  -  ``EBADF``
+
+       -  The file descriptor is not a valid.
+
+    -  -  ``EBUSY``
+
+       -  The ioctl can't be handled because the device is busy. This is
+	  typically return while device is streaming, and an ioctl tried to
+	  change something that would affect the stream, or would require
+	  the usage of a hardware resource that was already allocated. The
+	  ioctl must not be retried without performing another action to fix
+	  the problem first (typically: stop the stream before retrying).
+
+    -  -  ``EFAULT``
+
+       -  There was a failure while copying data from/to userspace, probably
+	  caused by an invalid pointer reference.
+
+    -  -  ``EINVAL``
+
+       -  One or more of the ioctl parameters are invalid or out of the
+	  allowed range. This is a widely used error code. See the
+	  individual ioctl requests for specific causes.
+
+    -  -  ``ENODEV``
+
+       -  Device not found or was removed.
+
+    -  -  ``ENOMEM``
+
+       -  There's not enough memory to handle the desired operation.
+
+    -  -  ``ENOTTY``
+
+       -  The ioctl is not supported by the driver, actually meaning that
+	  the required functionality is not available, or the file
+	  descriptor is not for a media device.
+
+    -  -  ``ENOSPC``
+
+       -  On USB devices, the stream ioctl's can return this error, meaning
+	  that this request would overcommit the usb bandwidth reserved for
+	  periodic transfers (up to 80% of the USB bandwidth).
+
+    -  -  ``EPERM``
+
+       -  Permission denied. Can be returned if the device needs write
+	  permission, or some special capabilities is needed (e. g. root)
+
+    -  -  ``EIO``
+
+       -  I/O error. Typically used when there are problems communicating with
+          a hardware device. This could indicate broken or flaky hardware.
+	  It's a 'Something is wrong, I give up!' type of error.
+
+    -  - ``ENXIO``
+
+       -  No device corresponding to this device special file exists.
+
+
+.. note::
+
+  #. This list is not exhaustive; ioctls may return other error codes.
+     Since errors may have side effects such as a driver reset,
+     applications should abort on unexpected errors, or otherwise
+     assume that the device is in a bad state.
+
+  #. Request-specific error codes are listed in the individual
+     requests descriptions.