[FFmpeg-devel] [PATCH 1/5] avutil/buffer: add av_buffer_pool_flush()

[Jonas Karlman]

On 2020-12-09 23:09, Lynne wrote:
> Dec 9, 2020, 21:25 by jonas at kwiboo.se:
> 
>> Signed-off-by: Jonas Karlman <jonas at kwiboo.se>
>> ---
>>  doc/APIchanges      |  3 +++
>>  libavutil/buffer.c  | 13 +++++++++++++
>>  libavutil/buffer.h  |  5 +++++
>>  libavutil/version.h |  2 +-
>>  4 files changed, 22 insertions(+), 1 deletion(-)
>>
>> diff --git a/doc/APIchanges b/doc/APIchanges
>> index 3fb9e12525..4a739ce453 100644
>> --- a/doc/APIchanges
>> +++ b/doc/APIchanges
>> @@ -15,6 +15,9 @@ libavutil:     2017-10-21
>>  
>>  API changes, most recent first:
>>  
>> +2020-xx-xx - xxxxxxxxxx - lavu 56.63.100 - buffer.h
>> +  Add av_buffer_pool_flush().
>> +
>>  2020-12-03 - xxxxxxxxxx - lavu 56.62.100 - timecode.h
>>  Add av_timecode_init_from_components.
>>  
>> diff --git a/libavutil/buffer.c b/libavutil/buffer.c
>> index d67b4bbdaf..a0683664cf 100644
>> --- a/libavutil/buffer.c
>> +++ b/libavutil/buffer.c
>> @@ -300,6 +300,19 @@ static void buffer_pool_free(AVBufferPool *pool)
>>  av_freep(&pool);
>>  }
>>  
>> +void av_buffer_pool_flush(AVBufferPool *pool)
>> +{
>> +    ff_mutex_lock(&pool->mutex);
>> +    while (pool->pool) {
>> +        BufferPoolEntry *buf = pool->pool;
>> +        pool->pool = buf->next;
>> +
>> +        buf->free(buf->opaque, buf->data);
>> +        av_freep(&buf);
>> +    }
>> +    ff_mutex_unlock(&pool->mutex);
>> +}
>>
> 
> This frees all buffers from the pool, which an API user probably has references to,
> leading to very nasty crashes. I can't see how this is even useful nor needed.
> 

This function should only free buffers that has been returned to the pool and
is once again part of the pool->pool linked list.

Main use-case is to flush and free all returned buffers in a pool used by a
hwaccel ctx while a new hwaccel ctx is being initialized, i.e. during seek
of H.264 video and a SPS change triggers initialization of a new hwaccel ctx.

For devices with memory constraints keeping all previously allocated buffers
for the old hwaccel ctx around while trying to allocate new buffers for a new
hwaccel ctx tend to cause to much memory usage, especially for 2160p video.

This function works around this limitation by freeing all buffers that has
already been returned to the pool during hwaccel uninit, before the last
buffer is returned, the last buffer is most of the time being displayed and
is not returned to the pool until next frame has been decoded and displayed
from next hwaccel ctx.
Without this the pool buffers is only freed once the last buffer is returned.

Please note that I may have mixed up hwaccel and hw frames ctx above :-)

This function is being called from ff_v4l2_request_uninit and
v4l2_request_hwframe_ctx_free in next patch.

Best regards,
Jonas