Date:   Fri, 29 Apr 2022 15:28:11 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Jacopo Mondi <jacopo@...ndi.org>, Eugen.Hristev@...rochip.com,
        linux-media@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Claudiu.Beznea@...rochip.com, robh+dt@...nel.org,
        Nicolas.Ferre@...rochip.com
Subject: Re: [PATCH v9 08/13] media: atmel: atmel-isc: change format
 propagation to subdev into only verification

On Fri, Apr 29, 2022 at 02:23:45PM +0200, Hans Verkuil wrote:
> On 29/04/2022 14:02, Laurent Pinchart wrote:
> > On Fri, Apr 29, 2022 at 01:17:27PM +0200, Hans Verkuil wrote:
> >> On 29/04/2022 12:20, Laurent Pinchart wrote:
> >>> On Fri, Apr 29, 2022 at 12:13:46PM +0200, Hans Verkuil wrote:
> >>>> On 29/04/2022 12:07, Laurent Pinchart wrote:
> >>>>> On Fri, Apr 29, 2022 at 11:58:48AM +0200, Jacopo Mondi wrote:
> >>>>>> On Fri, Apr 29, 2022 at 10:43:09AM +0200, Hans Verkuil wrote:
> >>>>>>> On 29/04/2022 10:28, Eugen.Hristev@...rochip.com wrote:
> >>>>>>>> On 4/29/22 11:17 AM, Hans Verkuil wrote:
> >>>>>>>>> On 10/03/2022 10:51, Eugen Hristev wrote:
> >>>>>>>>>> As a top MC video driver, the atmel-isc should not propagate the format to the
> >>>>>>>>>> subdevice, it should rather check at start_streaming() time if the subdev is properly
> >>>>>>>>>> configured with a compatible format.
> >>>>>>>>>> Removed the whole format finding logic, and reworked the format verification
> >>>>>>>>>> at start_streaming time, such that the ISC will return an error if the subdevice
> >>>>>>>>>> is not properly configured. To achieve this, media_pipeline_start
> >>>>>>>>>> is called and a link_validate callback is created to check the formats.
> >>>>>>>>>> With this being done, the module parameter 'sensor_preferred' makes no sense
> >>>>>>>>>> anymore. The ISC should not decide which format the sensor is using. The
> >>>>>>>>>> ISC should only cope with the situation and inform userspace if the streaming
> >>>>>>>>>> is possible in the current configuration.
> >>>>>>>>>> The redesign of the format propagation has also risen the question of the
> >>>>>>>>>> enumfmt callback. If enumfmt is called with an mbus_code, the enumfmt handler
> >>>>>>>>>> should only return the formats that are supported for this mbus_code.
> >>>>>>>>>> Otherwise, the enumfmt will report all the formats that the ISC could output.
> >>>>>>>>>> With this rework, the dynamic list of user formats is removed. It makes no
> >>>>>>>>>> more sense to identify at complete time which formats the sensor could emit,
> >>>>>>>>>> and add those into a separate dynamic list.
> >>>>>>>>>> The ISC will start with a simple preconfigured default format, and at
> >>>>>>>>>> link validate time, decide whether it can use the format that is configured
> >>>>>>>>>> on the sink or not.
> >>>>>>>>>>
> >>>>>>>>>> Signed-off-by: Eugen Hristev <eugen.hristev@...rochip.com>
> >>>>>>>>>> Reviewed-by: Jacopo Mondi <jacopo@...ndi.org>
> >>>>>>>>>> ---
> >>>>>>>>>> Changes in v9:
> >>>>>>>>>> - isc_link_validate now static
> >>>>>>>>>>
> >>>>>>>>>> Changes in v7:
> >>>>>>>>>> - minor typos as suggested by Jacopo
> >>>>>>>>>> - small changes, reduce some indentation, modified an index, as suggested by
> >>>>>>>>>> Jacopo
> >>>>>>>>>>
> >>>>>>>>>> Changes in v6:
> >>>>>>>>>> - reworked a bit enum_fmt as suggested by Jacopo
> >>>>>>>>>>
> >>>>>>>>>> Changes in v5:
> >>>>>>>>>> - removed user_formats dynamic list as it is now pointless
> >>>>>>>>>> - greatly simplified the enum_fmt function
> >>>>>>>>>> - removed some init code that was useless now
> >>>>>>>>>>
> >>>>>>>>>> Changes in v4:
> >>>>>>>>>> - moved validation code into link_validate and used media_pipeline_start
> >>>>>>>>>> - merged this patch with the enum_fmt patch which was previously in v3 of
> >>>>>>>>>> the series
> >>>>>>>>>>
> >>>>>>>>>> Changes in v3:
> >>>>>>>>>> - clamp to maximum resolution once the frame size from the subdev is found
> >>>>>>>>>>   drivers/media/platform/atmel/atmel-isc-base.c | 412 ++++++++----------
> >>>>>>>>>>   .../media/platform/atmel/atmel-isc-scaler.c   |   5 +
> >>>>>>>>>>   drivers/media/platform/atmel/atmel-isc.h      |  13 +-
> >>>>>>>>>>   .../media/platform/atmel/atmel-sama5d2-isc.c  |  20 +
> >>>>>>>>>>   .../media/platform/atmel/atmel-sama7g5-isc.c  |  20 +
> >>>>>>>>>>   5 files changed, 236 insertions(+), 234 deletions(-)
> >>>>>>>>>>
> >>>>>>>>>> diff --git a/drivers/media/platform/atmel/atmel-isc-base.c b/drivers/media/platform/atmel/atmel-isc-base.c
> >>>>>>>>>> index ee1dda6707a0..fe2c0af58060 100644
> >>>>>>>>>> --- a/drivers/media/platform/atmel/atmel-isc-base.c
> >>>>>>>>>> +++ b/drivers/media/platform/atmel/atmel-isc-base.c
> >>>>>>>>>> @@ -36,11 +36,6 @@ static unsigned int debug;
> >>>>>>>>>>   module_param(debug, int, 0644);
> >>>>>>>>>>   MODULE_PARM_DESC(debug, "debug level (0-2)");
> >>>>>>>>>>
> >>>>>>>>>> -static unsigned int sensor_preferred = 1;
> >>>>>>>>>> -module_param(sensor_preferred, uint, 0644);
> >>>>>>>>>> -MODULE_PARM_DESC(sensor_preferred,
> >>>>>>>>>> -              "Sensor is preferred to output the specified format (1-on 0-off), default 1");
> >>>>>>>>>> -
> >>>>>>>>>>   #define ISC_IS_FORMAT_RAW(mbus_code) \
> >>>>>>>>>>        (((mbus_code) & 0xf000) == 0x3000)
> >>>>>>>>>>
> >>>>>>>>>> @@ -337,6 +332,10 @@ static int isc_start_streaming(struct vb2_queue *vq, unsigned int count)
> >>>>>>>>>>        unsigned long flags;
> >>>>>>>>>>        int ret;
> >>>>>>>>>>
> >>>>>>>>>> +     ret = media_pipeline_start(&isc->video_dev.entity, &isc->mpipe);
> >>>>>>>>>
> >>>>>>>>> The pipeline validation is done in start_streaming, but I don't think that
> >>>>>>>>> is the best place: if STREAMON is called before buffers are queued, then
> >>>>>>>>> an invalid pipeline isn't discovered until enough buffers are queued to
> >>>>>>>>> kick off start_streaming.
> >>>>>>>>>
> >>>>>>>>> Drivers like vsp1, omap3isp and the samsung drivers all do this in streamon().
> >>>>>>>>>
> >>>>>>>>> I think that is the correct time to do this.
> >>>>>>>>
> >>>>>>>> Hello Hans,
> >>>>>>>>
> >>>>>>>> Initially (v2, v3) I had this in streamon(). The problem that I faced at
> >>>>>>>> that time was that streamoff was never called, so I could not call
> >>>>>>>> media_pipeline_stop(). Then Jacopo told me to move it to start_streaming
> >>>>>>>> (see change log for v4) , and I did not face any more problems.
> >>>>>>
> >>>>>> Yes indeed, seems I suggested to use media_pipeline_handler in a
> >>>>>> comment on your v3
> >>>>>>
> >>>>>> "at s_stream time your top driver calls media_pipeline_start()"
> >>>>>>
> >>>>>> sorry about that, I should have looked around a bit more carefully and
> >>>>>> notice most drivers do so at vb2 streamon
> >>>>>>
> >>>>>> However I don't see media_pipeline_start being called at all in v3 of
> >>>>>> the patch
> >>>>>>
> >>>>>>> It's a mess. Looking at some drivers I see that omap3isp calls media_pipeline_stop
> >>>>>>> in streamoff (so will have the same problem as you described if VIDIOC_STREAMOFF
> >>>>>>> isn't called), exynos4-is does the same, but it also checks the streaming state in
> >>>>>>> the release() fop callback, so that would fix this problem. And vimc does this
> >>>>>>> in stop_streaming.
> >>>>>>>
> >>>>>>> I'm in favor of fixing this in vb2, that framework knows exactly when this needs
> >>>>>>> to be called.
> >>>>>>
> >>>>>> Are you suggesting to have vb2 to call media_pipeline_start() or is it
> >>>>>> more complex than this ?
> >>>>>
> >>>>> I think Hans meant adding a .validate() operation to vb2.
> >>>>>
> >>>>> vb2 is already quite complex, I don't think adding more features is a
> >>>>> good idea. I'd rather have vb2 focus on buffer management only
> >>>>> (.start_streaming() and .stop_streaming() shouldn't have been in there
> >>>>> in my opinion), and handle validation in the .streamon() handler. I'd
> >>>>> expect most drivers that deal with media pipelines to do more work in
> >>>>> .streamon() anyway.
> >>>>
> >>>> I disagree with that :-)
> >>>>
> >>>> It's vb2 that keeps track of the streaming state and when what actions
> >>>> need to be taken. Drivers really shouldn't need to care about the ioctls
> >>>> themselves, and just implement the relevant vb2 callbacks. Relying on
> >>>> drivers to handle any of the streaming ioctls is asking for problems,
> >>>> as this shows: most drivers implement this wrong today.
> >>>>
> >>>> The vb2 framework knows when e.g. the pipeline needs to be started or
> >>>> stopped, and can do this at the best time, without drivers needing to
> >>>> keep track of when streamon/off/release is called. Keep that logic in
> >>>> vb2.
> >>>
> >>> Pipeline management and buffer management are two different issues.
> >>> Don't forget about devices that have multiple video nodes, part of the
> >>> same pipeline (possibly a combination of output and capture nodes, or
> >>> all of the same type). Forcing drivers to go through vb2 operations to
> >>> handle the pipeline will be messy, will result in more bloat in vb2, and
> >>> make the result more bug-prone and harder to maintain.
> >>>
> >>> If pipeline management is too complex, let's simplify it, new helpers
> >>> can make sense, but not through vb2.
> >>
> >> But it is vb2 that knows when streaming starts and stops.
> > 
> > That's right, but pipeline start (which includes validation and resource
> > reservation) needs to be performed synchronously with VIDIOC_STREAMON.
> > The streaming state managed by vb2 is not relevant,
> > media_pipeline_start() must not be delayed the same way
> > .start_streaming() is.
> 
> It will be the first thing that vb2_streamon calls. This has nothing to do
> with start_streaming: that's called when sufficient number of buffers are
> queued up to be able to start the DMA. This proposed prepare_streaming op
> will be called when VIDIOC_STREAMON is called.

Then it doesn't need vb2's knowledge of the stream state :-)

> >> The driver just
> >> needs to be informed (e.g. prepare_streaming and unprepare_streaming ops).
> >>
> >> vb2 deals with buffer management and it keeps track of the streaming state
> >> and makes the streaming state transitions. That *is* an integral part of
> >> vb2. What is missing at the moment are callbacks done at streamon time and
> >> when the streaming stops (streamoff, or close() when is_streaming is true).
> >>
> >> If you want to implement stream validation in a driver, then there are a
> >> lot of things you need to do:
> >>
> >> - override streamon, make sure you call vb2_queue_is_busy(), validate the
> >>   pipeline, then call vb2_streamon, if that fails, remember to stop the
> >>   pipeline.
> >>
> >> - override streamoff, make sure you call vb2_queue_is_busy(), stop the
> >>   pipeline and call vb2_streamoff.
> >>
> >> - in the release() function when the fh is closed, you have to check
> >>   vb2_is_streaming(), check that you are the owner of the queue, and if true,
> >>   stop the pipeline.
> > 
> > I'm not opposed to helper functions to implement that, they can bundle
> > vb2 calls and pipeline management.
> > 
> >> By moving this to vb2 ops all you need to implement are the prepare and
> >> unprepare ops.
> >>
> >> Esp. the release() implementation is tricky. I'm pretty sure that
> >> drivers/media/platform/samsung/exynos4-is/fimc-lite.c is wrong, since it
> >> should only call media_pipeline_stop() for the owner of the queue. Instead
> >> it calls it for the last user of the queue.
> >>
> >> I see that fimc_lite_streamoff() is wrong too: you can safely call
> >> VIDIOC_STREAMOFF twice: the second streamoff just returns 0 without
> >> doing anything. Instead media_pipeline_stop is called without testing if
> >> the queue is streaming.
> >>
> >> And yes, this is in part because V4L2 has quite some history and certainly
> >> API choice were made in the past that we wouldn't make today. But vb2
> >> shields you from that, and behaves much more like a proper state machine.
> >>
> >> I know you prefer to give a lot more control to driver developers, but
> >> in my experience very few developers can do things like this right. And
> >> it is really hard as a reviewer to check if all the corner cases are handled
> >> correctly in a driver. If vb2 is used, then I know things are called at the
> >> right time, and that makes my life as reviewer so much easier.
> > 
> > It's not just about giving more control to drivers, it's about
> > organizing the software layers in a way that keeps them maintainable,
> > with layered abstractions and not midlayers.
> > 
> > We are extensively reworking the media pipeline management as part of
> > the stream series, and there will be more work on top of that that will
> > make even more fundamental changes. I would like to at least postpone
> > any work on vb2 until then, to be able to evaluate the impact.
> 
> I'll make an RFC patch for vb2 so you have a better idea of what it does.

As long as we don't merge it before I get the chance to send the media
pipeline management rework, I'm all for RFCs :-)

> >> There may still be a few drivers that really need to do this manually, and
> >> that's OK, but a driver like the atmel-isc doesn't need that at all.

-- 
Regards,

Laurent Pinchart

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