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

Due to complexity of the video encoding process, the V4L2 drivers of
stateful encoder hardware require specific sequences of V4L2 API calls
to be followed. These include capability enumeration, initialization,
encoding, encode parameters change, drain and reset.

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 encoder part of
the Codec API.

Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

1 files changed, 31 insertions, 20 deletions

diff --git a/Documentation/userspace-api/media/v4l/vidioc-encoder-cmd.rst b/Documentation/userspace-api/media/v4l/vidioc-encoder-cmd.rst
index 16269b3b1715..d0eacce5485e 100644
--- a/Documentation/userspace-api/media/v4l/vidioc-encoder-cmd.rst
+++ b/Documentation/userspace-api/media/v4l/vidioc-encoder-cmd.rst
@@ -51,25 +51,26 @@ To send a command applications must initialize all fields of a struct
 ``VIDIOC_ENCODER_CMD`` or ``VIDIOC_TRY_ENCODER_CMD`` with a pointer to
 this structure.
 
-The ``cmd`` field must contain the command code. The ``flags`` field is
-currently only used by the STOP command and contains one bit: If the
-``V4L2_ENC_CMD_STOP_AT_GOP_END`` flag is set, encoding will continue
-until the end of the current *Group Of Pictures*, otherwise it will stop
-immediately.
+The ``cmd`` field must contain the command code. Some commands use the
+``flags`` field for additional information.
 
-A :ref:`read() <func-read>` or :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`
-call sends an implicit START command to the encoder if it has not been
-started yet. After a STOP command, :ref:`read() <func-read>` calls will read
+After a STOP command, :ref:`read() <func-read>` calls will read
 the remaining data buffered by the driver. When the buffer is empty,
 :ref:`read() <func-read>` will return zero and the next :ref:`read() <func-read>`
 call will restart the encoder.
 
+A :ref:`read() <func-read>` or :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`
+call sends an implicit START command to the encoder if it has not been
+started yet. Applies to both queues of mem2mem encoders.
+
 A :ref:`close() <func-close>` or :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`
 call of a streaming file descriptor sends an implicit immediate STOP to
-the encoder, and all buffered data is discarded.
+the encoder, and all buffered data is discarded. Applies to both queues of
+mem2mem encoders.
 
 These ioctls are optional, not all drivers may support them. They were
-introduced in Linux 2.6.21.
+introduced in Linux 2.6.21. They are, however, mandatory for stateful mem2mem
+encoders (as further documented in :ref:`encoder`).
 
 
 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
@@ -109,21 +110,24 @@ introduced in Linux 2.6.21.
       - 0
       - Start the encoder. When the encoder is already running or paused,
 	this command does nothing. No flags are defined for this command.
+
+	For a device implementing the :ref:`encoder`, once the drain sequence
+	is initiated with the ``V4L2_ENC_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. See :ref:`encoder` for more details.
     * - ``V4L2_ENC_CMD_STOP``
       - 1
       - Stop the encoder. When the ``V4L2_ENC_CMD_STOP_AT_GOP_END`` flag
 	is set, encoding will continue until the end of the current *Group
 	Of Pictures*, otherwise encoding will stop immediately. When the
-	encoder is already stopped, this command does nothing. mem2mem
-	encoders will send a ``V4L2_EVENT_EOS`` event when the last frame
-	has been encoded 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.
+	encoder is already stopped, this command does nothing.
+
+	For a device implementing the :ref:`encoder`, the command will initiate
+	the drain sequence as documented in :ref:`encoder`. 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_ENC_CMD_PAUSE``
       - 2
       - Pause the encoder. When the encoder has not been started yet, the
@@ -152,6 +156,8 @@ introduced in Linux 2.6.21.
       - Stop encoding at the end of the current *Group Of Pictures*,
 	rather than immediately.
 
+        Does not apply to :ref:`encoder`.
+
 
 Return Value
 ============
@@ -160,6 +166,11 @@ 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:`encoder` is still in
+    progress. It is not allowed to issue another encoder command until it
+    completes.
+
 EINVAL
     The ``cmd`` field is invalid.