Message-ID: <801aaaaaa77f28c6fc8f1bcb46b4410f663ec97e.camel@ndufresne.ca>
Date:   Fri, 21 Jul 2023 12:22:05 -0400
From:   Nicolas Dufresne <nicolas@...fresne.ca>
To:     Hsia-Jun Li <Randy.Li@...aptics.com>,
        Tomasz Figa <tfiga@...omium.org>
Cc:     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,
        Sebastian Fricke <sebastian.fricke@...labora.com>
Subject: Re: [PATCH 1/2] [RESEND] media: v4l2-mem2mem: allow device run
 without buf

Le vendredi 21 juillet 2023 à 16:56 +0800, Hsia-Jun Li a écrit :
> 
> On 7/17/23 22:00, Nicolas Dufresne wrote:
> > CAUTION: Email originated externally, do not click links or open attachments unless you recognize the sender and know the content is safe.
> > 
> > 
> > Le mercredi 12 juillet 2023 à 09:31 +0000, Tomasz Figa a écrit :
> > > 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://urldefense.proofpoint.com/v2/url?u=https-3A__gitlab.collabora.com_chipsnmedia_kernel_-2D_commit_ac59eafd5076c4deb3bfe1fb85b3b776586ef3eb&d=DwIFaQ&c=7dfBJ8cXbWjhc0BhImu8wVIoUFmBzj1s88r8EGyM0UY&r=P4xb2_7biqBxD4LGGPrSV6j-jf3C3xlR7PXU-mLTeZE&m=9eWwqueFnh1yZHTW11j-syNVQvema7iBzNQeX1GKUQwXZ9pm6V4HDL_R2tIYKoOw&s=Ez5AyEYFIAJmC_k00IPO_ImzVdLZjr_veRq1bN4RSNg&e=
> > > > https://urldefense.proofpoint.com/v2/url?u=https-3A__gitlab.collabora.com_chipsnmedia_kernel_-2D_commit_5de4fbe0abb20b8e8d862b654f93e3efeb1ef251&d=DwIFaQ&c=7dfBJ8cXbWjhc0BhImu8wVIoUFmBzj1s88r8EGyM0UY&r=P4xb2_7biqBxD4LGGPrSV6j-jf3C3xlR7PXU-mLTeZE&m=9eWwqueFnh1yZHTW11j-syNVQvema7iBzNQeX1GKUQwXZ9pm6V4HDL_R2tIYKoOw&s=tM81gjNe-bTjpjmidZ1sAhiodMh6npcVJNOhMCi1mPo&e=
> > > > 
> > > 
> > > 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.
> > 
> > Its a bit late to try and bring that argument. It should have been raised couple
> > of years ago (before I even started helping with these CODEC). Now that all the
> > newly written stately decoders uses this framework, it is logical to keep
> > reducing the boiler plate for these too. In my opinion, the job_ready()
> > callback, should have been a lot more flexible from the start. And allowing
> > driver to make it more powerful does not really add that much complexity.
> > 
> > Speaking of complexity, driving the output manually (outside of the job
> > workqueue) during sequence initialization is a way more complex and risky then
> > this. Finally, sticking with 1:1 pattern means encoder, detilers, image
> > enhancement reducing framerate, etc. would all be unwelcome to use this. Which
> > in short, means no one should even use this.
> > 
> I think those things are m2m, but it would be hard to present in current 
> m2m framework:
> 1. N:1 compositor(It may be implemented as a loop running 2:1 compositor).

Correct, only SRC/DST/BG type of blitters can be supported for compositing,
which is quite limiting. Currently there is no way to make an N:1 M2M, as M2M
instances are implemented at the video node layer, and not at the MC layer. This
is a entirely new subject and API design space to tackle (same goes for 1:N,
like multi scalers, svc decoders etc.).

> 2. AV1 film gain

For AV1/HEVC film grain, it is handle similar to inline converters and scalers.
The driver secretly allocate the reference frames, and post process into the
user visible buffers. It breaks some assumption made by most protected memory
setup though, as not all allocation is user driven, meaning the decoder needs to
know if its secure or not. Secure memory is a also another API design space to
tackle.

> 3. HDR with dynamic meta data to SDR

True, but easy to design around the stateless model. I'm not worried for these.

> 
> The video things fix for m2m model could be just a little less complex 
> than ISP or camera pipeline. The only difference is just ISP won't have 
> multiple contexts running at the same time.

I thought that having the kernel schedule ISP reprocessing jobs (which requires
instances) would be nice. But this can only be solved after we have solved the
N:N use cases of m2m (with multiple instances).

> If we could design a model for the video encoder I think we could solve 
> the most of problems.
> A video encoder would have:
> 1. input graphics buffer
> 2. reconstruction graphics buffer
> 3. motion vector cache buffer(optional)
> 4. coded bitstream output
> 5. encoding statistic report
> > > 
> > > 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.
> > 
> > The bend is already there, of course I'd be happy to help with any new
> > framework. Specially on modern stateless, were there is a need to do better
> > scheduling.
> I didn't know the schedule problem about stateless codec, are they 
> supposed to be in the job queue when the buffers that DPB requests are 
> own by the driver and its registers are prepared except the trigger bit?

On RK3588 at least, decoder scheduling is going to be complex. There is an even
number of cores, but when you need to decode 8K, you have to pair two cores
(there is a specific set of cores that are to be paired with). We need a decent
scheduling logic to ensure we don't starve 8K decoding session when there is
multiple smaller resolution session on-going.

On MTK, the entropy decoding (LAT) and the reconstruction (CORE) is split. MTK
vcodec is using multiple workqueues to move jobs around, which is clearly
expensive. Also, the life time of a job is not exactly easy to manage.

On RPi HEVC (not upstream yet, but being worked on), the entropy decoding and
reconstruction is done one the same core, but remains 2 concurrent operation.
Does not impose a complex scheduling issue, but it raised the need for a way to
fully utilize such HW.

This is just some examples of complexity for which the current framework is not
that helpful (even though, its not impossible either).

>   Just ping me if you have some effort starting, I don't currently
> > have a budget or bandwidth to write new drivers or port existing drivers them on
> > a newly written framework.
> > 
> > Nicolas
> > 
> > 
> > [...]
> 

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