media: vidioc-enum-fmt.rst: clarify format preference

It has been decided to use the ENUM_FMT index value
as a hint for driver preference. This is defined purposedly
in a very liberal way, letting drivers define what "preference"
means.

For instance, the Hantro VPU driver indicates additional
processing to output a given format, and thus implicates
more CPU usage, which is enumerated after native (non-processed)
formats.
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/media/uapi/v4l/vidioc-enum-fmt.rst

@@ -44,7 +44,9 @@ To enumerate image formats applications initialize the ``type`` and
 the :ref:`VIDIOC_ENUM_FMT` ioctl with a pointer to this structure. Drivers
 fill the rest of the structure or return an ``EINVAL`` error code. All
 formats are enumerable by beginning at index zero and incrementing by
-one until ``EINVAL`` is returned.
+one until ``EINVAL`` is returned. If applicable, drivers shall return
+formats in preference order, where preferred formats are returned before
+(that is, with lower ``index`` value) less-preferred formats.
 
 .. note::