On 02-12-2011 08:31, Kamil Debski wrote:
Hi, Yesterday we had a chat about video codecs in V4L2 and how to change the interface to accommodate the needs of GStreamer (and possibly other media players and applications using video codecs). The problem that many hardware codecs need a fixed number of pre-allocated buffers should be resolved when gstreamer 0.11 will be released. The main issue that we are facing is the resolution change and how it should be handled by the driver and the application. The resolution change is particularly common in digital TV. It occurs when content on a single channel is changed. For example there is the transition from a TV series to a commercials block. Other stream parameters may also change. The minimum number of buffers required for decoding is of particular interest of us. It is connected with how many old buffers are used for picture prediction. When this occurs there are two possibilities: resolution can be increased or decreased. In the first case it is very likely that the current buffers are too small to fit the decoded frames. In the latter there is the choice to use the existing buffers or allocate new set of buffer with reduced size. Same applies to the number of buffers - it can be decreased or increased. On the OUTPUT queue there is not much to be done. A buffer that contains a frame with the new resolution will not be dequeued until it is fully processed. On the CAPTURE queue the application has to be notified about the resolution change. The idea proposed during the chat is to introduce a new flag V4L2_BUF_FLAG_WRONGFORMAT.
IMO, a bad name. I would call it as V4L2_BUF_FLAG_FORMATCHANGED.
1) After all the frames with the old resolution are dequeued a buffer with the following flags V4L2_BUF_FLAG_ERROR | V4L2_BUF_FLAG_WRONGFORMAT is returned. 2) To acknowledge the resolution change the application should STREAMOFF, check what has changed and then STREAMON.
I don't think this is needed, if the buffer size is enough to support the new format. Btw, a few drivers (bttv comes into my mind) properly handles format changes. This were there in order to support a bad behavior found on a few V4L1 applications, where the applications were first calling STREAMON and then setting the buffer. If I'm not mistaken, the old vlc V4L1 driver used to do that. What bttv used to do is to allocate a buffer big enough to support the max resolution. So, any format changes (size increase or decrease) would fit into the allocated buffers. Depending on how applications want to handle format changes, and how big is the amount of memory on the system, a similar approach may be done with CREATE_BUFFERS: just allocate enough space for the maximum supported resolution for that stream, and let the resolution changes, as required. I see two possible scenarios here: 1) new format size is smaller than the buffers. Just V4L2_BUF_FLAG_FORMATCHANGED should be rised. No need to stop DMA transfers with STREAMOFF. 2) new requirement is for a bigger buffer. DMA transfers need to be stopped before actually writing inside the buffer (otherwise, memory will be corrupted). In this case, all queued buffers should be marked with an error flag. So, both V4L2_BUF_FLAG_FORMATCHANGED and V4L2_BUF_FLAG_ERROR should raise. The new format should be available via G_FMT.
3) The application should check with G_FMT how did the format change and the V4L2_CID_MIN_BUFFERS_FOR_CAPTURE control to check how many buffers are required. 4) Now it is necessary to resume processing: A. If there is no need to change the size of buffers or their number the application needs only to STREAMON. B. If it is necessary to allocate bigger buffers the application should use CREATE_BUFS to allocate new buffers, the old should be left until the application is finished and frees them with the DESTROY_BUFS call. S_FMT should be used to adjust the new format (if necessary and possible in HW).
If the application already cleaned the DMA transfers with STREAMOFF, it can also just re-queue the buffers with REQBUFS, e. g. vb2 should be smart enough to accept both ways to allocate buffers. Also, if the format changed, applications should use G_FMT to get the new buffer requirements. Using S_FMT here doesn't seem to be the right thing to do, as the format may have changed again, while the DMA transfers were stopped by STREAMOFF.
C. If only the number of buffers has changed then it is possible to add buffers with CREATE_BUF or remove spare buffers with DESTROY_BUFS (not yet implemented to my knowledge).
I don't see why a new format would require more buffers. The minimal number of buffers is more related to latency issues and processing speed at userspace than to any driver or format-dependent hardware constraints. On the other hand, the maximum number of buffers might eventually have some constraint, e. g. a hardware might support less buffers, if the resolution is too high. I prefer to not add anything to the V4L2 API with regards to changes at max/min number of buffers, except if we actually have any limitation at the supported hardware. In that case, it will likely need a separate flag, to indicate userspace that buffer constraints have changed, and that audio buffers will also need to be re-negotiated, in order to preserve A/V synch.
5) After the application does STREMON the processing should continue. Old buffers can still be used by the application (as CREATE_BUFS was used), but they should not be queued (error shall be returned in this case). After the application is finished with the old buffers it should free them with DESTROY_BUFS.
If the buffers are bigger, there's no issue on not allowing queuing them. Enforcing it will likely break drivers and eventually applications.
I hope I haven't missed anything from our chat. Also please feel free to share your comments or suggestions. The log from our chat can be found here http://www.retiisi.org.uk/v4l2/v4l2codecs-2011-12-01.txt (thanks Sakari). In a couple of words I would like describe how it is done now - in the MFC driver that has been posted and accepted in V4L2. Currently the approach is very similar, but there is no special flag to indicate that the format has changed. Instead a buffer with no error and bytesused=0 is returned to notify the application that the resolution/number of buffers has changed. After that the application does STREAMOFF on the CAPTURE queue, checks the new format with G_FMT and then reallocates the buffers. To do this the buffers are unmapped and reqbufs(0) is called. Next step is allocating the new number/size of buffers. Finally STREAMON is used to resume processing. When this API was implemented there was no CREATE_BUFS/DESTROY_BUF implementation and plans to implement it. Handling resolution change with CREATE_BUFS/DESTORY_BUFS and notifying the application with a dedicated flag seems like a much better solution. Best wishes, -- Kamil Debski Linux Platform Group Samsung Poland R&D Center
-- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
