Re: [RFC 00/10] V4L2 explicit synchronization support

Shuah Khan
2017-06-09T16:10:16Z

On 06/09/2017 12:25 AM, Gustavo Padovan wrote:
> 2017-06-08 Shuah Khan <shuahkhan@gmail.com>:
>
>> Hi Gustavo,
>>
>> On Thu, Jun 8, 2017 at 2:17 PM, Mauro Carvalho Chehab
>> <mchehab@osg.samsung.com> wrote:
>>> Hi Gustavo,
>>>
>>> Em Wed, 24 May 2017 21:31:01 -0300
>>> Gustavo Padovan <gustavo@padovan.org> escreveu:
>>>
>>>> Hi all,
>>>>
>>>> I've been working on the v2 of this series, but I think I hit a blocker
>>>> when trying to cover the case where the driver asks to requeue the
>>>> buffer. It is related to the out-fence side.
>>>>
>>>> In the current implementation we return on QBUF an out-fence fd that is not
>>>> tied to any buffer, because we don't know the queueing order until the
>>>> buffer is queued to the driver. Then when the buffer is queued we use
>>>> the BUF_QUEUED event to notify userspace of the index of the buffer,
>>>> so now userspace knows the buffer associated to the out-fence fd
>>>> received earlier.
>>>>
>>>> Userspace goes ahead and send a DRM Atomic Request to the kernel to
>>>> display that buffer on the screen once the fence signals. If it is
>>>> a nonblocking request the fence waiting is past the check phase, thus
>>>> it isn't allowed to fail anymore.
>>>>
>>>> But now, what happens if the V4L2 driver calls buffer_done() asking
>>>> to requeue the buffer. That means the operation failed and can't
>>>> signal the fence, starving the DRM side.
>>>>
>>>> We need to fix that. The only way I can see is to guarantee ordering of
>>>> buffers when out-fences are used. Ordering is something that HAL3 needs
>>>> to so maybe there is more than one reason to do it like this. I'm not
>>>> a V4L2 expert, so I don't know all the consequences of such a change.
>>>>
>>>> Any other ideas?
>>>>
>>>> The current patchset is at:
>>>>
>>>> https://git.kernel.org/pub/scm/linux/kernel/git/padovan/linux.git/log/?h=v4l2-fences
>>
>> Do you plan to send the v2 out? I did a quick review and have a few comments.
>>
>> [media] vb2: split out queueing from vb_core_qbuf()
>>
>> It changes the sequence a bit.
>>
>> /* Fill buffer information for the userspace */
>> if (pb)
>> call_void_bufop(q, fill_user_buffer, vb, pb);
>>
>> With the changes - user information is filled before __enqueue_in_driver(vb);
>
> Without my changes it also fills it before __enqueue_in_driver() when
> start_streaming wasn't called yet. So I don't think it really matters.

Right, with this change, it fills the buffer before __enqueue_in_driver()
when start_streaming is called. Is that an issue, I don't know for sure.
It might not be necessary perhaps if buffer is filled in the path when
stream is already called.

>
>>
>> Anyway, it might be a good idea to send the v2 out for review and we can review
>> patches in detail. I am hoping to test your patch series on odroid-xu4
>> next week.
>> Could you please add me to the thread as well as include me when you send
>> v2 and subsequent versions.
>
> I will send a v2 as soon as I can, but from Thursday next week until
> the 25th I'll be on vacation.

okay sounds good.

thanks,
-- Shuah