Re: [PATCH v7 1/6] [media] vb2: add is_unordered callback for drivers

[Gustavo Padovan <gustavo@padovan.org>]

2018-01-15 Alexandre Courbot <acourbot@chromium.org>:

> On Thu, Jan 11, 2018 at 1:07 AM, Gustavo Padovan <gustavo@padovan.org> wrote:
> > From: Gustavo Padovan <gustavo.padovan@collabora.com>
> >
> > Explicit synchronization benefits a lot from ordered queues, they fit
> > better in a pipeline with DRM for example so create a opt-in way for
> > drivers notify videobuf2 that the queue is unordered.
> >
> > Drivers don't need implement it if the queue is ordered.
> 
> This is going to make user-space believe that *all* vb2 drivers use
> ordered queues by default, at least until non-ordered drivers catch up
> with this change. Wouldn't it be less dangerous to do the opposite
> (make queues non-ordered by default)?

The rational behind this decision was because most formats/drivers are
ordered so only a small amount of drivers need to changed. I think this
was proposed by Hans on the Media Summit.

I understand your concern. My question is how dangerous will it be. If
you are building a product you will make the changes in the driver if
they are not there yet, or if it is a distribution you'd never know
which driver/format you are using so you should be prepared for
everything.

AFAIK all Capture drivers are ordered and that is where I think fences
is most useful.

Gustavo