On 02-12-2011 11:57, Sakari Ailus wrote:
Hi Mauro,
On Fri, Dec 02, 2011 at 10:35:40AM -0200, Mauro Carvalho Chehab wrote:
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.
The alternative is to return a specific error code to the user --- the frame
would not be decoded in either case. See below.
As I said, some drivers work with buffers with a bigger size than the format, and
does allow setting a smaller format without the need of streamoff.
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.
Sometimes not, but sometimes there are buffer line alignment requirements
which must be communicated to the driver using S_FMT. If the frames are
decoded using a driver-decided format, it might be impossible to actually
use these decoded frames.
That's why there's streamoff and streamon.
Don't get me wrong. What I'm saying is that there are valid cases where there's no
need to streamoff/streamon. What I'm saying is that, when there's no need to do it,
just don't rise the V4L2_BUF_FLAG_ERROR flag. The V4L2_BUF_FLAG_FORMATCHANGED still
makes sense.
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.
The buffers do not have a format, the video device queue has. If the format
changes during streaming it is impossible to find that out using the current
API.
Yes, but extending it to proper support it is the scope of this RFC. Yet, several
drivers allow to resize the "format" (e. g. the resolution of the image) without
streamon/streamoff, via an explicit call to S_FMT.
So, whatever change at the API is done, it should keep supporting format changes
(in practice, resolution changes) without the need of re-initializing the DMA engine,
of course when such change won't break the capability for userspace to decode
the new frames.
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'm not fully certain it is always possible to find out the largest stream
resolution. I'd like an answer from someone knowing more about video codecs
than I do.
When the input or output is some hardware device, there is a maximum resolution
(sensor resolution, monitor resolution, pixel sampling rate, etc). A pure DSP block
doesn't have it, but, anyway, it should be up to the userspace application to decide
if it wants to over-allocate a buffer, in order to cover a scenario where the
resolution may change or not.
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.
In memory-to-memory devices, I assume that the processing stops immediately
when it's not possible to further process the frames. The capture queue
would be stopped.
In all cases, when it is not possible to further process the frames, the
stream needs to be stopped (whatever it means ;) ).
The decision of stopping the streaming should be taken by the driver. API needs
to properly support it and provide some way for userspace to re-start streaming
with the correct format, when this occurs.
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.
No need to REQBUFS after streaming has been stopped. STREAMOFF won't harm
the buffers in any way anymore --- as it did in videobuf1.
OK, but userspace applications may use REQBUFS instead of CREATE_BUFFERS, as REQBUFS
is part of the API.
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.
S_FMT is needed to communicate line alignment to the driver. Not every time
but sometimes, depending on the hardware.
OK, in such case.
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.
That's a good point. It's more related to changes in stream properties ---
the frame rate of the stream could change, too. That might be when you could
like to have more buffers in the queue. I don't think this is critical
either.
This could also depend on the properties of the codec. Again, I'd wish a
comment from someone who knows codecs well. Some codecs need to be able to
access buffers which have already been decoded to decode more buffers. Key
frames, simply.
Ok, but let's not add unneeded things at the API if you're not sure. If we have
such need for a given hardware, then add it. Otherwise, keep it simple.
The user space still wants to be able to show these buffers, so a new flag
would likely be required --- V4L2_BUF_FLAG_READ_ONLY, for example.
Huh? Assuming a capture device, when kernel makes a buffer available to userspace,
kernel should not touch on it anymore (not even for read - although reading from
it probably won't cause any issues, as video applications in general don't write
into those buffers). The opposite is true for output devices: once userspace fills it,
and queues, it should not touch that buffer again.
This is part of the queue/dequeue logic. I can't see any need for an extra
flag to explicitly say that.
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.
I think that boils down to the properties of the codec and possibly also the
stream.
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 think this means buffers that are too small for the new format. They are
no longer needed after they have been displayed --- remember there must also
be no interruption in displaying the video.
Kind regards,
Regards,
Mauro
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