Re: [PATCH 08/12] [media] vb2: add 'ordered' property to queues

[Gustavo Padovan <gustavo@padovan.org>]

Hi Nicolas,

2017-06-16 Nicolas Dufresne <nicolas@ndufresne.ca>:

> Le vendredi 16 juin 2017 à 16:39 +0900, Gustavo Padovan a écrit :
> > > From: Gustavo Padovan <gustavo.padovan@collabora.com>
> > 
> > For explicit synchronization (and soon for HAL3/Request API) we need
> > the v4l2-driver to guarantee the ordering which the buffer were queued
> > by userspace. This is already true for many drivers, but we never had
> > the need to say it.
> 
> Phrased this way, that sound like a statement that a m2m decoder
> handling b-frame will just never be supported. I think decoders are a
> very important use case for explicit synchronization.
> 
> What I believe happens with decoders is simply that the allocation
> order (the order in which empty buffers are retrieved from the queue)
> will be different then the actual presentation order. Also, multiple
> buffers endup being filled at the same time. Some firmware may inform
> of the new order at the last minute, making indeed the fence useless,
> but these are firmware and the information can be known earlier. Also,
> this information would be known by userspace for the case (up-coming,
> see STM patches and Rockchip comments [0]) or state-less decoder,
> because it is available while parsing the bitstream. For this last
> scenarios, the fact that ordering is not the same should disable the
> fences since userspace can know which fences to wait for first. Those
> drivers would need to set "ordered" to 0, which would be counter
> intuitive.
> 
> I think this use case is too important to just ignore it. I would
> expect that we at least have a todo with something sensible as a plan
> to cover this.

We definitely need to cover these usecases, I sent the patchset in a
hurry just before going on vacation and forget to lay down any plan for
other things. But for now, I believe we need refine the implementation
of the most common case and then look at expanding it.

	Gustavo