2018-09-17 16:51 GMT+02:00 Hans Verkuil [off-list ref]:

On 09/11/2018 05:09 PM, Maxime Jourdan wrote:

quoted

 - Moved the single instance check (returning -EBUSY) to start/stop streaming
 The check was previously in queue_setup but there was no great location to
 clear it except for .close().

Actually, you can clear it by called VIDIOC_REQBUFS with count set to 0. That
freed all buffers and clears this.

Now, the difference between queue_setup and start/stop streaming is that if you
do this in queue_setup you'll know early on that the device is busy. It is
reasonable to assume that you only allocate buffers when you also want to start
streaming, so that it a good place to know this quickly.

Whereas with start_streaming you won't know until you call STREAMON, or even later
if you start streaming with no buffers queued, since start_streaming won't
be called until you have at least 'min_buffers_needed' buffers queued (1 for this
driver). So in that case EBUSY won't be returned until the first VIDIOC_QBUF.

My preference is to check this in queue_setup, but it is up to you to decide.
Just be aware of the difference between the two options.

Regards,

        Hans

I could for instance keep track of which queue(s) have been called
with queue_setup, catch calls to VIDIOC_REQBUFS with count set to 0,
and clear the current session once both queues have been reset ?

You leverage another issue with min_buffers_needed. It's indeed set to
1 but this value is wrong for the CAPTURE queue. The problem is that
this value changes depending on the codec and the amount of CAPTURE
buffers requested by userspace.
Ultimately I want it set to the total amount of CAPTURE buffers,
because the hardware needs the full buffer list before starting a
decode job.
Am I free to change this queue parameter later, or is m2m_queue_init
the only place to do it ?

Thanks,
Maxime