[media] DocBook media: document codec draining flow

Document the interaction between VIDIOC_DECODER_CMD V4L2_DEC_CMD_STOP and
VIDIOC_ENCODER_CMD V4L2_ENC_CMD_STOP to start the draining, the V4L2_EVENT_EOS
event signalling all capture buffers are finished and ready to be dequeud,
the new V4L2_BUF_FLAG_LAST buffer flag indicating the last buffer being dequeued
from the capture queue, and the poll and VIDIOC_DQBUF ioctl return values once
the queue is drained.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/Documentation/DocBook/media/v4l/vidioc-encoder-cmd.xml b/Documentation/DocBook/media/v4l/vidioc-encoder-cmd.xml
index 0619ca5d2d36..fc1d4625a78c 100644
--- a/Documentation/DocBook/media/v4l/vidioc-encoder-cmd.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-encoder-cmd.xml
@@ -129,7 +129,15 @@ this command.</entry>
 encoding will continue until the end of the current <wordasword>Group
 Of Pictures</wordasword>, otherwise encoding will stop immediately.
 When the encoder is already stopped, this command does
-nothing.</entry>
+nothing. mem2mem encoders will send a <constant>V4L2_EVENT_EOS</constant> event
+when the last frame has been decoded and all frames are ready to be dequeued and
+will set the <constant>V4L2_BUF_FLAG_LAST</constant> 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
+<structfield>bytesused</structfield> field to 0. Once the
+<constant>V4L2_BUF_FLAG_LAST</constant> flag was set, the
+<link linkend="vidioc-qbuf">VIDIOC_DQBUF</link> ioctl will not block anymore,
+but return an &EPIPE;.</entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_ENC_CMD_PAUSE</constant></entry>