Message-ID: <e959fe6c-2beb-18fb-b9a0-c67c00edd9c0@microchip.com>
Date:   Wed, 18 Nov 2020 12:04:21 +0000
From:   <Eugen.Hristev@...rochip.com>
To:     <jacopo@...ndi.org>
CC:     <laurent.pinchart@...asonboard.com>,
        <linux-kernel@...r.kernel.org>, <linux-media@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <robh+dt@...nel.org>,
        <sakari.ailus@....fi>
Subject: Re: [PATCH v5 2/3] media: atmel: introduce microchip csi2dc driver

On 18.11.2020 13:38, Jacopo Mondi wrote:
> Hi Eugen,
> 
> On Wed, Nov 18, 2020 at 09:48:19AM +0000, Eugen.Hristev@...rochip.com wrote:
>> On 18.11.2020 11:11, Jacopo Mondi wrote:
>>> Hello Eugen,
>>>
> 
> [snip]
> 
>>>> Hello,
>>>>
>>>> First of all thank you for the review and explanations.
>>>>
>>>> I am still confused about how does the userspace know which elements are
>>>> in the pipeline and how to connect them ?
>>>
>>> Using the media-controller uAPI to inspect, explore and configure the
>>> media graph (it's mostly about enabling and disabling media links),
>>> and using the v4l2 subdev uAPI to configure formats, sizes and such on
>>> each subdevice device node associated with a media entity. Finally,
>>> using the v4l2 uAPI to stream from the top-level driver that expose a
>>> video device node.
>>
>> Okay but which application in userspace uses this userAPI ?
> 
> media-ctl which is part of v4l2-utils.
> 
> existing applications or frameworks like gstreamer have no notion of
> the platform they run on, they can't configure it, they only
> understand '/dev/video0'
> 
>> So I could use it to test my pipeline.
>>
> 
> The first step is to use media-ctl to manually setup the pipeline and
> test. Once it's setup you can usually keep using the legacy
> applications that only stream from /dev/video0.
> 
> Of course manually setting up the pipeline using a script does not
> scale, that's what the 'libcamera pipeline handler' does in facts.
> It implements the same logic that you would manually apply with
> media-ctl
> 
>>>
>>> It's really (conceptually) not different than doing the same using
>>> media-ctl and v4l2-ctl.
>>>
>>>> And if this is the case, what is the purpose of the endpoints in the DT ?
>>>>
>>>
>>> DTS describe the hardware. Usually a driver parses the firmware graph
>>> to create and model the media graph and the available connections
>>> between media entities but there's no requirement to have a 1-to-1
>>> match (as far I'm aware).
>>
>> So the top level v4l2 driver should parse the whole graph ?
>>
> 
> Userspace should :)
> 
>>>
>>> Media links on a media graph then need to be enabled opportunely
>>> depending on the desired use case, the platform topology etc
>>>
>>>> At this moment, I just use the /dev/video0, because the top v4l2 driver
>>>> configures the subdevice, and this subdevice configures it's own
>>>> subdevice and so on, until the sensor itself.
>>>>
>>>> My top v4l2 driver will not 'complete' unless a subdevice registers
>>>> itself , and if this subdevice does not provide any information
>>>> regarding its formats, the probe of the top v4l2 driver will fail,
>>>> because it will not find compatibility between it's supported input
>>>> formats and what the subdevice provides.
>>>
>>> ouch, I see.. Which driver is that ?
>>
>> The atmel image sensor controller driver.
>> All sensor drivers that we use (for example : ov5640, ov7740, ov7670)
>> register as subdevices, and the atmel-isc will 'complete' when the
>> sensor is probed and registered.
> 
> That's ok, the complete callback is called when all the async devices
> registered in a notifier are matched.
> 
>> To have the csi2dc module compatible with what we have on atmel-isc,
>> it's natural to have the csi2dc register itself as a subdevice such that
>> the atmel-isc can 'complete'.
> 
> the csi2d registering as subdev is all good (apart from the issue of
> having 2 v4l2_device, but as you've see there's a solution for that
> and it's called sub-notifiers).
> 
> The 'issue' here is that your top driver wants to match formats to initialize
> its supported format list (in isc_formats_init() from what I can see).
> 
> An MC driver should not care about that. Each piece of the pipeline
> has to be considered in isolation, and the format matching happens at
> link_validate() time for links between subdevices and usually at
> s_stream(1) time between sub-devices and video devices.
> 
> In example, format enumeration on the video device registered by the
> top v4l2 driver is conditional to the presence of the mbus_code field
> as you can read in:
> https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/vidioc-enum-fmt.html?highlight=enum_fmt#c.V4L.VIDIOC_ENUM_FMT
> 
> And it should be possible by setting mbus_code = 0 to get a list of
> all formats supported -by the hardware- not limited to the current use
> case.
> 
> Userspace knows the platform and if asked for 640x480[YUV420] will go
> and set the right format on each piece of the pipeline, after having
> enabled the relevant links, using the right media bus codes and sizes
> 
> Totally random example:
> 
>          [video0] s_fmt(640x480, PIXFMT_YUV422)
>           (pad 0)
>             |
>             v
>           (pad 1) sudev_s_fmt(1, 640x480, MBUS_YUYV8_1X16)
>          [csi2dc]       --- your subdev can crop ---
>           (pad 0) sudev_s_fmt(0, 648x496, MBUS_YUYV8_1X16)
>             |
>             V
>           (pad 0) subdev_s_fmt(0, 648x496, MBUS_YUYV_1X16)
>          [ov5640]
> 
> If your top v4l2 driver can produce RGB from a YUV stream it's
> perfectly legit then to s_fmt(video0, 640x480, PIXFMT_RGB565) and
> subdev_s_fmt(csi2dc, 1, 640x480, MBUS_YUYV8_1X16).
> 
> As you can see it all depends on what your hardware can do.
> 

Hi again,

Thank you very much for the explanations. It is different from what I 
had in mind (and how we use the subsystem right now in our drivers here 
). I will take some time to understand all this, and hopefully I can 
come with better drivers, better pipeline handling and libcamera 
platform support.

I may have questions during the development, but I think I have enough 
information to get started.

thanks again !

Eugen
>>
>>
>>>
>>> If that's an MC driver, and that seems the case, it should not bother
>>> validating the subdevice formats, but only care about listing what its
>>> capabilities are.
>>
>> if this atmel-isc does not offer format capabilities for the userspace
>> via the /dev/video0, then all applications that we use (gstreamer
>> v4l2src, ffmpeg ) fail.
>> What can we use then, if we cannot use these applications ?
>>
> 
> I think if you setup the pipeline with media-ctl correctly (ie you
> don't get any -EPIPE at s_stream) you should be able to keep using
> legacy apps, but it very much depends on how they are implemented.
> 
> Usually setting up the pipeline using media-ctl and streaming using
> v4l2-ctl or yavta is the first validation test.
> 
>>>
>>>>
>>>> So what to do in this case ? Libcamera will configure this whole
>>>> pipeline for me , if the drivers just probe ( and start/stop streaming
>>>> by selecting the respective registers) ? or how to do this whole
>>>> pipeline configuration ?
>>>>
>>>
>>> Well, libcamera offers a framework to enable specialized code (called
>>> "pipeline handler") to operate under an high-level API towards application
>>> and implement image tuning algorithms on top of that common API.
>>>
>>> The code to configure the pipeline is device specific (unless your
>>> pipeline is very simple). So it's not that libcamera just magically
>>> knows how to operate on every platforms it runs on, it needs platform
>>> specific code to do so. What you get in exchange is an high level API
>>> to offer to application developers, a framework to implement 3A and
>>> tuning algorithms, integration with a growing set of other userspace
>>> frameworks (android, gstreamer, pipewire and a legacy v4l2-compat
>>> layer)
>>
>> I tried a buildroot with libcamera and gstreamer libcamerasrc , to see
>> how it goes. Libcamera does not recognize the /dev/video0 device that we
>> have. The v4l2 compatibility layer should not perform exactly this ?
> 
> There's no pipeline handler for your platform (unless you have
> developed one)
> 
>>
>> # gst-launch-1.0 libcamerasrc ! fakesink
>> Setting pipeline to PAUSED ...
>> [0:00:28.448687200] [216]  WARN IPAManager ipa_manager.cpp:147 No IPA
>> found in '/usr/lib/libcamera'
>> [0:00:28.448890800] [216]  INFO Camera camera_manager.cpp:287 libcamera
>> v0.0.0+54328-2020.11-rc2-dirty (2020-11-17T20:58:37+02:00)
>> ERROR: from element
>> /GstPipeline:pipeline0/GstLibcameraSrc:libcamerasrc0: Could not find any
>> supported camera on this system.
>              ^
> And this is the expected outcome :)
> 
> As said, it's not that libcamera magically knows how to deal with all
> devices it is run on. We would not need libcamera at all if that was
> the case, any application would be self-configurable and that's it :)
> 

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