[PATCH v2 0/3] Add Amlogic video decoder driver

Maxime Jourdan mjourdan at baylibre.com
Mon Sep 17 09:36:33 PDT 2018

2018-09-17 16:51 GMT+02:00 Hans Verkuil <hverkuil at xs4all.nl>:
> On 09/11/2018 05:09 PM, Maxime Jourdan wrote:
>>  - 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 ?


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