On 08/25/2017 05:35 PM, wm4 wrote:
+static inline int buffer_ops_v4l2buf_to_bufref(V4L2Buffer *in, int plane,
AVBufferRef **buf)
+{
+ if (plane >= in->num_planes)
+ return AVERROR(EINVAL);
+
+ /* even though most encoders return 0 in data_offset encoding vp8 does
require this value*/
+ *buf = av_buffer_create((char *)in->plane_info[plane].mm_addr +
in->planes[plane].data_offset,
+ in->plane_info[plane].lengths, free_v4l2buf_cb,
in, 0);
+ if (!*buf)
+ return AVERROR(ENOMEM);
+
+ in->status = V4L2BUF_RET_USER;
+
+ return 0;
+}
This looks like it would trigger massive UB if you keep a frame after
the decoder is closed. This should not happen, an AVBufferRef must
stay valid forever.
Yes you are right.
v4l2 uses a limited number of buffers in physical memory that it reuses
during the video operation.
these buffers must be mapped to the process address space before they
can be used.
for decoding, ffmpeg maps the V4L2 physical buffers, memcpies the input
data to the output queue and uses the _references_ from the capture
queue to create AVBufferRefs (these references are mmaped addresses of
physical memory buffers).
In the current design buffers are not be reused until their
corresponding AVBufferRefs are released (notice that freeing an
AVBufferRef executes the callback that sets the V4L2BUF_AVAILABLE flag
making it ready to be enqueued again).
So:
1. would it be acceptable if I stopped the codec from ummaping its
buffers until all AVBufferRefs have been freed?
From what you are saying I think this has to be done.
2. also notice that if ffmpeg keeps all the AVBufferRefs for 'too long'
this would cause the v4l2 pipeline to starve.
I have not seen this case (I have tested all decoders with ffplay) but
from what you are saying it could be possible (typical hardware can
handle 32 physical buffers although v4l2 applications rarely request so
many).
Is there any way in ffmpeg to catch this condition and force the release
of some of the AVBuffers?
_______________________________________________
ffmpeg-devel mailing list
ffmpeg-devel@ffmpeg.org
http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
