• Hans Verkuil's avatar
    [media] vb2: only call start_streaming if sufficient buffers are queued · b3379c62
    Hans Verkuil authored
    In commit 02f142ec support was added to
    start_streaming to return -ENOBUFS if insufficient buffers were queued
    for the DMA engine to start. The vb2 core would attempt calling
    start_streaming again if another buffer would be queued up.
    
    Later analysis uncovered problems with the queue management if start_streaming
    would return an error: the buffers are enqueued to the driver before the
    start_streaming op is called, so after an error they are never returned to
    the vb2 core. The solution for this is to let the driver return them to
    the vb2 core in case of an error while starting the DMA engine. However,
    in the case of -ENOBUFS that would be weird: it is not a real error, it
    just says that more buffers are needed. Requiring start_streaming to give
    them back only to have them requeued again the next time the application
    calls QBUF is inefficient.
    
    This patch changes this mechanism: it adds a 'min_buffers_needed' field
    to vb2_queue that drivers can set with the minimum number of buffers
    required to start the DMA engine. The start_streaming op is only called
    if enough buffers are queued. The -ENOBUFS handling has been dropped in
    favor of this new method.
    
    Drivers are expected to return buffers back to vb2 core with state QUEUED
    if start_streaming would return an error. The vb2 core checks for this
    and produces a warning if that didn't happen and it will forcefully
    reclaim such buffers to ensure that the internal vb2 core state remains
    consistent and all buffer-related resources have been correctly freed
    and all op calls have been balanced.
    
    __reqbufs() has been updated to check that at least min_buffers_needed
    buffers could be allocated. If fewer buffers were allocated then __reqbufs
    will free what was allocated and return -ENOMEM. Based on a suggestion from
    Pawel Osciak.
    
    __create_bufs() doesn't do that check, since the use of __create_bufs
    assumes some advance scenario where the user might want more control.
    Instead streamon will check if enough buffers were allocated to prevent
    streaming with fewer than the minimum required number of buffers.
    Signed-off-by: default avatarHans Verkuil <hans.verkuil@cisco.com>
    Signed-off-by: default avatarMauro Carvalho Chehab <m.chehab@samsung.com>
    b3379c62
vpfe_video.c 44.6 KB