linux-kernel - Re: [PATCH 10/10] venus: dec: make decoder compliant with stateful codec API

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Message-ID: <e8a90694c306fde24928a569b7bcb231b86ec73b.camel@ndufresne.ca>
Date:   Tue, 29 Jan 2019 23:21:24 -0500
From:   Nicolas Dufresne <nicolas@...fresne.ca>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil@...all.nl>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Vikash Garodia <vgarodia@...eaurora.org>,
        Alexandre Courbot <acourbot@...omium.org>,
        Malathi Gottam <mgottam@...eaurora.org>
Subject: Re: [PATCH 10/10] venus: dec: make decoder compliant with stateful
 codec API

Le mercredi 30 janvier 2019 à 12:38 +0900, Tomasz Figa a écrit :
> > Yes, unfortunately, GStreamer still rely on G_FMT waiting a minimal
> > amount of time of the headers to be processed. This was how things was
> > created back in 2011, I could not program GStreamer for the future. If
> > we stop doing this, we do break GStreamer as a valid userspace
> > application.
> 
> Does it? Didn't you say earlier that you end up setting the OUTPUT
> format with the stream resolution as parsed on your own? If so, that
> would actually expose a matching framebuffer format on the CAPTURE
> queue, so there is no need to wait for the real parsing to happen.

I don't remember saying that, maybe I meant to say there might be a
workaround ?

For the fact, here we queue the headers (or first frame):

https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/blob/master/sys/v4l2/gstv4l2videodec.c#L624

Then few line below this helper does G_FMT internally:

https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/blob/master/sys/v4l2/gstv4l2videodec.c#L634
https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/blob/master/sys/v4l2/gstv4l2object.c#L3907

And just plainly fails if G_FMT returns an error of any type. This was
how Kamil designed it initially for MFC driver. There was no other
alternative back then (no EAGAIN yet either).

Nicolas

p.s. it's still in my todo's to implement source change event as I
believe it is a better mechanism (specially if you header happened to
be corrupted, then the driver can consume the stream until it finds a
sync). So these sleep or normally wait exist all over to support this
legacy thing. It is unfortunate, the question is do you want to break
userspace now ? Without having first placed a patch that would maybe
warn or something for a while ?