Message-ID: <27bcd2cf21688b1120fcc75c38981f8585e8f9bd.camel@collabora.com>
Date:   Fri, 22 Sep 2023 16:20:24 -0400
From:   Nicolas Dufresne <nicolas.dufresne@...labora.com>
To:     Hans Verkuil <hverkuil@...all.nl>,
        Sebastian Fricke <sebastian.fricke@...labora.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Nas Chung <nas.chung@...psnmedia.com>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Jackson Lee <jackson.lee@...psnmedia.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        NXP Linux Team <linux-imx@....com>,
        Conor Dooley <conor+dt@...nel.org>,
        Pengutronix Kernel Team <kernel@...gutronix.de>
Cc:     devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Robert Beckett <bob.beckett@...labora.com>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...labora.com, Tomasz Figa <tfiga@...omium.org>
Subject: Re: [PATCH v12 1/7] media: v4l2: Add ignore_streaming flag

Le vendredi 22 septembre 2023 à 10:28 +0200, Hans Verkuil a écrit :
> On 21/09/2023 20:39, Nicolas Dufresne wrote:
> > Le mercredi 20 septembre 2023 à 16:49 +0200, Hans Verkuil a écrit :
> > > On 20/09/2023 16:08, Nicolas Dufresne wrote:
> > > > cc Tomasz Figa
> > > > 
> > > > Le mercredi 20 septembre 2023 à 14:59 +0200, Hans Verkuil a écrit :
> > > > > On 15/09/2023 23:11, Sebastian Fricke wrote:
> > > > > > Add a new flag to the `struct v4l2_m2m_dev` to toggle whether a queue
> > > > > > must be streaming in order to allow queuing jobs to the ready queue.
> > > > > > Currently, both queues (CAPTURE & OUTPUT) must be streaming in order to
> > > > > > allow adding new jobs. This behavior limits the usability of M2M for
> > > > > > some drivers, as these have to be able, to perform analysis of the
> > > > > 
> > > > > able, to -> able to
> > > > > 
> > > > > > sequence to ensure, that userspace prepares the CAPTURE queue correctly.
> > > > > 
> > > > > ensure, that -> ensure that
> > > > > 
> > > > > > 
> > > > > > Signed-off-by: Sebastian Fricke <sebastian.fricke@...labora.com>
> > > > > > Signed-off-by: Nicolas Dufresne <nicolas.dufresne@...labora.com>
> > > > > > ---
> > > > > >  include/media/v4l2-mem2mem.h | 17 +++++++++++++++++
> > > > > >  1 file changed, 17 insertions(+)
> > > > > > 
> > > > > > diff --git a/include/media/v4l2-mem2mem.h b/include/media/v4l2-mem2mem.h
> > > > > > index d6c8eb2b5201..97a48e61e358 100644
> > > > > > --- a/include/media/v4l2-mem2mem.h
> > > > > > +++ b/include/media/v4l2-mem2mem.h
> > > > > > @@ -57,6 +57,16 @@ struct v4l2_m2m_dev;
> > > > > >   * @rdy_spinlock: spin lock to protect the struct usage
> > > > > >   * @num_rdy:	number of buffers ready to be processed
> > > > > >   * @buffered:	is the queue buffered?
> > > > > > + * @ignore_streaming: Dictates whether the queue must be streaming for a job to
> > > > > > + *		      be queued.
> > > > > > + *		      This is useful, for example, when the driver requires to
> > > > > > + *		      initialize the sequence with a firmware, where only a
> > > > > > + *		      queued OUTPUT queue buffer and STREAMON on the OUTPUT
> > > > > > + *		      queue is required to perform the anlysis of the bitstream
> > > > > > + *		      header.
> > > > > > + *		      This means the driver is responsible for implementing the
> > > > > > + *		      job_ready callback correctly to make sure that requirements
> > > > > > + *		      for actual decoding are met.
> > > > > 
> > > > > This is a bad description and field name.
> > > > 
> > > > I wonder what's your opinion about the buffered one then :-D
> > > 
> > > Even worse :-)
> > > 
> > > I still don't really understand what that does. Patches welcome.
> > > 
> > > > 
> > > > > 
> > > > > Basically what this field does is that, if true, the streaming state of the
> > > > > capture queue is ignored. So just call it that: ignore_cap_streaming.
> > > > > 
> > > > > And explain that, if true, job_ready() will be called even if the capture
> > > > > queue is not streaming, and that that can be used to allow hardware to
> > > > > analyze the bitstream header that arrives on the OUTPUT queue.
> > > > 
> > > > Ack.
> > > > 
> > > > > 
> > > > > Also, doesn't this field belong to struct v4l2_m2m_ctx? It makes no sense
> > > > > for the output queue, this is really a configuration for the m2m context as
> > > > > a whole.
> > > > 
> > > > Unless we come up with a completely new type of M2M that can behave like a gap
> > > > filler (like a video rate m2m), it indeed makes no sense for output. I'm just
> > > > illustrating that this is true "now" but someone can come up with valid
> > > > expectation. So I agree with you, we can move it up in the hierarchy.
> > > > 
> > > > Recently over IRC and other threads, Tomasz raised a concern that CODECs where
> > > > introducing too much complexity into M2M. And I believe buffered (which is
> > > > barely documented) and this mechanism was being pointed.
> > > > 
> > > > My take on that is that adding boolean configuration is what introduce
> > > > complexity, and we can fix it by doing less in the m2m. After this discussion, I
> > > > came with the idea that we should remove buffered and ignore_streaming. For
> > > > drivers that don't implement job_ready, this logic would be moved inside the
> > > > default implementation. We can then add a helper to check the common conditions.
> > > > 
> > > > The alternative suggested by Tomasz, was to layer two ops. We'd have a
> > > > device_ready() ops and its default implementation would include the check we
> > > > have and would call job_ready(). Personally, I'd rather remove then add, but I
> > > > understadt the reasoning and would be fine committing to that instead.
> > > > 
> > > > I'd like your feedback on this proposal. If this is something we want, I'll do
> > > > this prior to V13, otherwise we will address your comments and fix the added
> > > > mechanism. I think though that we agree that for decoders, this is nice addition
> > > > to not have to trigger work manually from vb2 ops.
> > > 
> > > It comes down to a matter of taste, I guess. I personally think that using bools
> > > to tweak the behavior of a framework does not necessarily increase complexity,
> > > provided it is clearly documented what it does and why it is needed.
> > > 
> > > I think an ignore_cap_streaming bool is pretty straightforward and has minimal
> > > impact in the code. As long as there are good comments.
> > 
> > So for wave5 we will opt for this and apply your suggested changes. And I may
> > come back later on the subject.
> > 
> > > 
> > > The 'buffered' flag is were this clearly failed completely, since I couldn't figure
> > > out what it is supposed to do. But that is not because it makes the code more
> > > complex, it is just because of shoddy documentation and naming.
> > > 
> > > Quite often implementing tweaks like that are quite easy in a framework, since
> > > you have all the information readily available. In a driver it can quickly become
> > > messy.
> > 
> > In this case, "buffered" is used to disable the checks for having at least one
> > buffer in the ready queues. In most cases, if you don't have at least 1 pending
> > capture and 1 pending output buffer, there is no point in calling device_run.
> 
> So it is really similar to ignore_cap_streaming: that relaxes the streaming test,
> and 'buffered' relaxes the 'must have at least one capture and output buffer ready'
> test.
> 
> So this should be renamed to: allow_empty_queues
> 
> Although I would prefer to split this into two bools: allow_empty_capture_queue and
> allow_empty_output_queue. It is more flexible that way and I actually think it is
> easier to understand.

Its on the queue ctx, so it does not have to be typed. It would have to be typed
if moved to m2m ctx.

> 
> I see also see in the v4l2-mem2mem.c source that the debug messages are very poorly
> worded:
> 	src = v4l2_m2m_next_src_buf(m2m_ctx);
> 
>         if (!src && !m2m_ctx->out_q_ctx.buffered) {
>                 dprintk("No input buffers available\n");
>                 goto job_unlock;
>         }
> 
> This should be either "source buffers" or "output buffers", but definitely not
> "input buffers".
> 
> Ditto for the dst part.

Indeed, I'll store this node somewhere for future work on the framework, this is
not strictly related to wave5 anymore.

> 
> > 
> > In reality, drivers will add use case specific checks in their job_ready()
> > implementation. For decoders, the cases I can think of are:
> > 
> > - On capture if you haven't parsed the stream header
> > - On capture if the driver removes them from ready queue as a way to track which
> > one are considered free and may be used at any time by the firmware
> > - On output queue, if you need device_run() to be called to complete the drain
> > the reorder queue
> > 
> > Yet, you want this check after stream headers are parsed, or whenever a new
> > bitstream decode operation is to be queued in the firmware. So this check gets
> > re-implemented, but dynamically, in all decoders.
> > 
> > Deinterlacers may needs this too with some algorithms (the one that introduce
> > delays at least). Its not clear to me why it was called buffered,
> > ignore_rdy_queue might have been an option, though I'm not fully confident. Note
> > that M2M can be confusing, since whenever you ask for last something, its always
> > relative to the ready queue, and may not make a lot of sense in the context it
> > is used.
> > 
> > > 
> > > For codec support there are a number of issues that increase complexity:
> > > implementing support for the LAST flag and events, and supporting buffers
> > > that can be held. Especially since driver implementations tend to vary.
> > > 
> > > I've been experimenting with some cleanups and changes in v4l2-mem2mem.c
> > > (https://git.linuxtv.org/hverkuil/media_tree.git/log/?h=enc-dec-cmd), mainly
> > > surrounding the handling of the LAST flag. Note: this is failing the compliance
> > > tests, I haven't had the time to pursue this further.
> > > 
> > > I'm not sure whether the best approach is to move things out of the m2m framework,
> > > or move things into the m2m framework, or add a more codec-specific layer on top
> > > of the m2m framework, or a combination of all of these.
> > > 
> > > It is something that needs experimentation, just see what works.
> > 
> > I can see you have omitted mark_stopped() calles when refactoring, which makes
> > these patches change the behaviour. Could be related.
> 
> Could be. I hope to be able to spend a bit of time on this today.
> 
> > 
> > This is no longer strictly related to this patch, but I think cmd_stop()
> > implementation (even after your changes) are miss-fit for driver that speaks to
> > firmware. As the firmware is being made aware of the free buffers, you can't
> > just cherry-pick from the capture queue, you have to synchronise your state with
> > the firmware while draining. The helper should be split in two parts I suppose,
> > but cutting the line isn't easy.
> > 
> > Thread safe usage of the numerous boolean implicated in the draining state is
> > also difficult. There is no other option then introduce a mutex or spinlock (if
> > the state is needed in job_ready() implementation) to make this thread safe and
> > reliable.
> 
> Regards,
> 
> 	Hans
> 
> > 
> > > 
> > > But for this specific flag: I think it is fine to put that in the m2m framework,
> > > just document and name it well.
> > 
> > Ack.
> > 
> > > 
> > > Regards,
> > > 
> > > 	Hans
> > > 
> > > > 
> > > > regards,
> > > > Nicolas
> > > > 
> > > > > 
> > > > > >   *
> > > > > >   * Queue for buffers ready to be processed as soon as this
> > > > > >   * instance receives access to the device.
> > > > > > @@ -69,6 +79,7 @@ struct v4l2_m2m_queue_ctx {
> > > > > >  	spinlock_t		rdy_spinlock;
> > > > > >  	u8			num_rdy;
> > > > > >  	bool			buffered;
> > > > > > +	bool			ignore_streaming;
> > > > > >  };
> > > > > >  
> > > > > >  /**
> > > > > > @@ -564,6 +575,12 @@ static inline void v4l2_m2m_set_dst_buffered(struct v4l2_m2m_ctx *m2m_ctx,
> > > > > >  	m2m_ctx->cap_q_ctx.buffered = buffered;
> > > > > >  }
> > > > > >  
> > > > > > +static inline void v4l2_m2m_set_dst_ignore_streaming(struct v4l2_m2m_ctx *m2m_ctx,
> > > > > > +						     bool ignore_streaming)
> > > > > > +{
> > > > > > +	m2m_ctx->cap_q_ctx.ignore_streaming = ignore_streaming;
> > > > > > +}
> > > > > > +
> > > > > 
> > > > > I think this is overkill, esp. when the field is moved to m2m_ctx. Just clearly
> > > > > document that drivers can set this.
> > > > > 
> > > > > Regards,
> > > > > 
> > > > > 	Hans
> > > > > 
> > > > > >  /**
> > > > > >   * v4l2_m2m_ctx_release() - release m2m context
> > > > > >   *
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

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