media: docs-rst: Document memory-to-memory video decoder interface

Due to complexity of the video decoding process, the V4L2 drivers of
stateful decoder hardware require specific sequences of V4L2 API calls
to be followed. These include capability enumeration, initialization,
decoding, seek, pause, dynamic resolution change, drain and end of
stream.

Specifics of the above have been discussed during Media Workshops at
LinuxCon Europe 2012 in Barcelona and then later Embedded Linux
Conference Europe 2014 in Düsseldorf. The de facto Codec API that
originated at those events was later implemented by the drivers we already
have merged in mainline, such as s5p-mfc or coda.

The only thing missing was the real specification included as a part of
Linux Media documentation. Fix it now and document the decoder part of
the Codec API.

Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

1 files changed, 27 insertions, 14 deletions

diff --git a/Documentation/media/uapi/v4l/vidioc-decoder-cmd.rst b/Documentation/media/uapi/v4l/vidioc-decoder-cmd.rst
index ccf83b05afa7..57f0066f4cff 100644
--- a/Documentation/media/uapi/v4l/vidioc-decoder-cmd.rst
+++ b/Documentation/media/uapi/v4l/vidioc-decoder-cmd.rst
@@ -56,14 +56,16 @@ The ``cmd`` field must contain the command code. Some commands use the
 
 A :ref:`write() <func-write>` or :ref:`VIDIOC_STREAMON`
 call sends an implicit START command to the decoder if it has not been
-started yet.
+started yet. Applies to both queues of mem2mem decoders.
 
 A :ref:`close() <func-close>` or :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`
 call of a streaming file descriptor sends an implicit immediate STOP
-command to the decoder, and all buffered data is discarded.
+command to the decoder, and all buffered data is discarded. Applies to both
+queues of mem2mem decoders.
 
-These ioctls are optional, not all drivers may support them. They were
-introduced in Linux 3.3.
+In principle, these ioctls are optional, not all drivers may support them. They were
+introduced in Linux 3.3. They are, however, mandatory for stateful mem2mem decoders
+(as further documented in :ref:`decoder`).
 
 
 .. tabularcolumns:: |p{1.1cm}|p{2.4cm}|p{1.2cm}|p{1.6cm}|p{10.6cm}|
@@ -167,26 +169,32 @@ introduced in Linux 3.3.
 	``V4L2_DEC_CMD_RESUME`` for that. This command has one flag:
 	``V4L2_DEC_CMD_START_MUTE_AUDIO``. If set, then audio will be
 	muted when playing back at a non-standard speed.
+
+	For a device implementing the :ref:`decoder`, once the drain sequence
+	is initiated with the ``V4L2_DEC_CMD_STOP`` command, it must be driven
+	to completion before this command can be invoked.  Any attempt to
+	invoke the command while the drain sequence is in progress will trigger
+	an ``EBUSY`` error code.  The command may be also used to restart the
+	decoder in case of an implicit stop initiated by the decoder itself,
+	without the ``V4L2_DEC_CMD_STOP`` being called explicitly. See
+	:ref:`decoder` for more details.
     * - ``V4L2_DEC_CMD_STOP``
       - 1
       - Stop the decoder. When the decoder is already stopped, this
 	command does nothing. This command has two flags: if
 	``V4L2_DEC_CMD_STOP_TO_BLACK`` is set, then the decoder will set
 	the picture to black after it stopped decoding. Otherwise the last
-	image will repeat. mem2mem decoders will stop producing new frames
-	altogether. They will send a ``V4L2_EVENT_EOS`` event when the
-	last frame has been decoded and all frames are ready to be
-	dequeued and will set the ``V4L2_BUF_FLAG_LAST`` buffer flag on
-	the last buffer of the capture queue to indicate there will be no
-	new buffers produced to dequeue. This buffer may be empty,
-	indicated by the driver setting the ``bytesused`` field to 0. Once
-	the ``V4L2_BUF_FLAG_LAST`` flag was set, the
-	:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl will not block anymore,
-	but return an ``EPIPE`` error code. If
+	image will repeat. If
 	``V4L2_DEC_CMD_STOP_IMMEDIATELY`` is set, then the decoder stops
 	immediately (ignoring the ``pts`` value), otherwise it will keep
 	decoding until timestamp >= pts or until the last of the pending
 	data from its internal buffers was decoded.
+
+	For a device implementing the :ref:`decoder`, the command will initiate
+	the drain sequence as documented in :ref:`decoder`.  No flags or other
+	arguments are accepted in this case. Any attempt to invoke the command
+	again before the sequence completes will trigger an ``EBUSY`` error
+	code.
     * - ``V4L2_DEC_CMD_PAUSE``
       - 2
       - Pause the decoder. When the decoder has not been started yet, the
@@ -209,6 +217,11 @@ On success 0 is returned, on error -1 and the ``errno`` variable is set
 appropriately. The generic error codes are described at the
 :ref:`Generic Error Codes <gen-errors>` chapter.
 
+EBUSY
+    A drain sequence of a device implementing the :ref:`decoder` is still in
+    progress. It is not allowed to issue another decoder command until it
+    completes.
+
 EINVAL
     The ``cmd`` field is invalid.