Message-ID: <540a488a-5e3e-6031-b358-a02448b4d52a@xs4all.nl>
Date:   Fri, 29 Apr 2022 14:23:45 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
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 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.

> 
>> 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.

Regards,

	Hans

> 
>> 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.
> 

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