linux-kernel - Re: [PATCH 1/2] [RESEND] media: v4l2-mem2mem: allow device run without buf

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Message-ID: <CAAFQd5DiB-i+YBt9E8Rh5ngJ9KVCjdjXStV4s-f-m-YdqeCOEQ@mail.gmail.com>
Date:   Thu, 13 Jul 2023 12:13:32 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Sebastian Fricke <sebastian.fricke@...labora.com>
Cc:     Nicolas Dufresne <nicolas@...fresne.ca>,
        Hsia-Jun Li <randy.li@...aptics.com>,
        linux-media@...r.kernel.org, ayaka@...lik.info,
        hans.verkuil@...co.com, mchehab@...nel.org,
        laurent.pinchart@...asonboard.com, hiroh@...omium.org,
        hverkuil@...all.nl, linux-kernel@...r.kernel.org,
        mcasas@...omium.org, Steve Cho <stevecho@...omium.org>,
        Fritz Koenig <frkoenig@...omium.org>,
        "wenst@...omium.org" <wenst@...omium.org>,
        "nhebert@...omium.org" <nhebert@...omium.org>
Subject: Re: [PATCH 1/2] [RESEND] media: v4l2-mem2mem: allow device run
 without buf

On Wed, Jul 12, 2023 at 6:44 PM Sebastian Fricke
<sebastian.fricke@...labora.com> wrote:
>
> Hey Tomasz,
>
> On 12.07.2023 09:31, Tomasz Figa wrote:
> >On Fri, Jul 07, 2023 at 03:14:23PM -0400, Nicolas Dufresne wrote:
> >> Hi Randy,
> >>
> >> Le mardi 04 juillet 2023 à 12:00 +0800, Hsia-Jun Li a écrit :
> >> > From: Randy Li <ayaka@...lik.info>
> >> >
> >> > For the decoder supports Dynamic Resolution Change,
> >> > we don't need to allocate any CAPTURE or graphics buffer
> >> > for them at inital CAPTURE setup step.
> >> >
> >> > We need to make the device run or we can't get those
> >> > metadata.
> >> >
> >> > Signed-off-by: Randy Li <ayaka@...lik.info>
> >> > ---
> >> >  drivers/media/v4l2-core/v4l2-mem2mem.c | 5 +++--
> >> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >> >
> >> > diff --git a/drivers/media/v4l2-core/v4l2-mem2mem.c b/drivers/media/v4l2-core/v4l2-mem2mem.c
> >> > index 0cc30397fbad..c771aba42015 100644
> >> > --- a/drivers/media/v4l2-core/v4l2-mem2mem.c
> >> > +++ b/drivers/media/v4l2-core/v4l2-mem2mem.c
> >> > @@ -301,8 +301,9 @@ static void __v4l2_m2m_try_queue(struct v4l2_m2m_dev *m2m_dev,
> >> >
> >> >    dprintk("Trying to schedule a job for m2m_ctx: %p\n", m2m_ctx);
> >> >
> >> > -  if (!m2m_ctx->out_q_ctx.q.streaming
> >> > -      || !m2m_ctx->cap_q_ctx.q.streaming) {
> >> > +  if (!(m2m_ctx->out_q_ctx.q.streaming || m2m_ctx->out_q_ctx.buffered)
> >> > +      || !(m2m_ctx->cap_q_ctx.q.streaming
> >> > +           || m2m_ctx->cap_q_ctx.buffered)) {
> >>
> >> I have a two atches with similar goals in my wave5 tree. It will be easier to
> >> upstream with an actual user, though, I'm probably a month or two away from
> >> submitting this driver again.
> >>
> >> https://gitlab.collabora.com/chipsnmedia/kernel/-/commit/ac59eafd5076c4deb3bfe1fb85b3b776586ef3eb
> >> https://gitlab.collabora.com/chipsnmedia/kernel/-/commit/5de4fbe0abb20b8e8d862b654f93e3efeb1ef251
> >>
> >
> >While I'm not going to NAK this series or those 2 patches if you send
> >them, I'm not really convinced that adding more and more complexity to
> >the mem2mem helpers is a good idea, especially since all of those seem
> >to be only needed by stateful video decoders.
> >
> >The mem2mem framework started as a set of helpers to eliminate boiler
> >plate from simple drivers that always get 1 CAPTURE and 1 OUTPUT buffer,
> >run 1 processing job on them and then return both of the to the userspace
> >and I think it should stay like this.
> >
> >I think we're strongly in need of a stateful video decoder framework that
> >would actually address the exact problems that those have rather than
> >bending something that wasn't designed with them in mind to work around the
> >differences.
>
> Thanks for the feedback.
>
> I have recently discussed how we could approach creating a framework for
> the codecs side, with Hans Verkuil and Nicolas Dufresne.

That's great to hear, thanks. :)

>
> The first step we would have to do is come up with a list of
> requirements for that framework and expected future needs, maybe we can
> start a public discussion on the mailing list to generate a list like
> that.

Makes sense. Let me CC some ChromeOS folks working on video codec
drivers these days.

> But all in all this endeavor will probably require quite a bit of time
> and effort, do you think we could modify M2M a bit for our use case and
> then when we are in the process of creating the new framework, we could
> maybe think about simplifying the M2M framework again?

Sure, as I said, I'm not NAKing this series.

>
> >
> >Best regards,
> >Tomasz
>
> Greetings,
> Sebastian
>
> >
> >> Sebastien and I authored this without giving it much thought, but we believe
> >> this massively simplify our handling of DRC (dynamic resolution change).
> >>
> >> The main difference, is that we added ignore_streaming to the ctx, so that
> >> drivers can opt-in the mode of operation. Thinking it would avoid any potential
> >> side effects in drivers that aren't prepared to that. We didn't want to tied it
> >> up to buffered, this is open to discussion of course, we do use buffered on both
> >> queues and use a slightly more advance job_ready function, that take into
> >> account our driver state.
> >>
> >> In short, Sebastien and I agree this small change is the right direction, we
> >> simply have a different implementation. I can send it as RFC if one believe its
> >> would be useful now (even without a user).
> >>
> >> >            dprintk("Streaming needs to be on for both queues\n");
> >> >            return;
> >> >    }
> >>