Message-ID: <aff5572e-d391-7a96-e760-01e5813963ba@xs4all.nl>
Date:   Fri, 29 Apr 2022 14:50:25 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Jacopo Mondi <jacopo@...ndi.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     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 29/04/2022 14:38, Jacopo Mondi wrote:
> Hi
> 
> On Fri, Apr 29, 2022 at 03:28:11PM +0300, Laurent Pinchart wrote:
>> 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 :-)
>>
> 
> To unblock Eugen is it fine if he moves media_pipeline_start() at
> streamon() time for now ? It will require overriding
> v4l2_ioctl.vidioc_streamon which might be a bit of additional work
> (but probably easier to replace once a proper solution lands)
> 
> Otherwise, should the series go in as it is now ?

I had a comment about patch 6, so I want a new series anyway. All the
patches with just fixes (everything but 4 and 8) can go in once I have
them.

I think - all things considered - it is currently best to start and stop the
pipeline in start/stop_streaming: that at least avoids all the complications
with streamon/off/close() that is really hard to get right without (IMHO)
my proposed vb2 changes.

With that I will accept the series.

Regards,

	Hans

> 
> Thanks
>   j
> 
>>>>> 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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