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

Mauro Carvalho Chehab
2017-06-08T20:17:36Z

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

Currently, nothing warrants that buffers will be returned in order,
but that should be the case of most drivers. I guess the main
exception would be mem2mem codec drivers. On those, the driver
or the hardware may opt to reorder the buffers.

If this is a mandatory requirement for explicit fences to work, then
we'll need to be able to explicitly enable it, per driver, and
clearly document that drivers using it *should* warrant that the
dequeued buffer will follow the queued order.

We may need to modify VB2 in order to enforce it or return an
error if the order doesn't match.

--
Thanks,
Mauro