Re: [PATCH 00/15] Improve DVB documentation and reduce its gap

[Soeren Moch]

On 04.09.2017 13:29, Mauro Carvalho Chehab wrote:
> Em Mon, 4 Sep 2017 02:55:15 +0200
> Soeren Moch <sm...@web.de> escreveu:
>
>> Hi Mauro,
>>
>> On 01.09.2017 11:32, Mauro Carvalho Chehab wrote:
>>> Em Fri, 1 Sep 2017 10:40:28 +0200
>>> Honza Petrouš <jpetr...@gmail.com> escreveu:
>>>  
>>>> 2017-09-01 1:46 GMT+02:00 Mauro Carvalho Chehab 
>>>> <mche...@s-opensource.com>:  
>>>>> The DVB documentation was negligected for a long time, with
>>>>> resulted on several gaps between the API description and its
>>>>> documentation.
>>>>>
>>>>> I'm doing a new reading at the documentation. As result of it,
>>>>> this series:
>>>>>
>>>>> - improves the introductory chapter, making it more generic;
>>>>> - Do some adjustments at the frontend API, using kernel-doc
>>>>>   when possible.
>>>>> - Remove unused APIs at DVB demux. I suspect that the drivers
>>>>>   implementing such APIs were either never merged upstream,
>>>>>   or the API itself  were never used or was deprecated a long
>>>>>   time ago. In any case, it doesn't make any sense to carry
>>>>>   on APIs that aren't properly documented, nor are used on the
>>>>>   upstream Kernel.
>>>>>
>>>>> With this patch series, the gap between documentation and
>>>>> code is solved for 3 DVB APIs:
>>>>>
>>>>>   - Frontend API;
>>>>>   - Demux API;
>>>>>   - Net API.
>>>>>
>>>>> There is still a gap at the CA API that I'll try to address when I
>>>>> have some time[1].
>>>>>
>>>>> [1] There's a gap also on the legacy audio, video and OSD APIs,
>>>>> but, as those are used only by a single very old deprecated
>>>>> hardware (av7110), it is probably not worth the efforts.
>>>>>
>> av7110 cards may be old, but there are still users of these cards. 
>> For instance I'm watching TV received and decoded with such card in this 
>> moment.
>> So what do you mean with "deprecated hardware"?
> Nobody is telling otherwise. What I mean by "deprecated" is that it is
> not a product that you could got to a shop and buy a new one. Its 
> production stopped a long time ago.
>
>>>> I agree that av7110 is very very old piece of hw (but it is already
>>>> in my hall of fame because of its Skystar 1 incarnation as
>>>> first implementation of DVB in Linux) and it is sad that we still
>>>> don't have at least one driver for any SoC with embedded DVB
>>>> devices.  
>>> Yeah, av7110 made history. Please notice that this series doesn't
>>> remove any API that it is used by it. All it removes are the APIs
>>> that there's no Kernel driver using.
>>>
>>> Carry on APIs without client is usually a very bad idea, as nobody
>>> could be certain about how to use it. It is even worse when such
>>> APIs are not properly documented (with is the case here).
>>>  
>>>> I understand that the main issue is that no any DVB-enabled
>>>> SoC vendor is interested in upstreaming theirs code, but I still hope
>>>> it will change in near future(*)  
>>> We have one driver for a SoC DVB hardware at:
>>> drivers/media/platform/sti/c8sectpfe/
>>>
>>> I guess it still doesn't cover the entire SoC, but this is a WiP. If I
>>> remember well, at the midia part of the SoC, they started by submitting
>>> the Remote Controller code.
>>>  
>>>> Without having full-featured DVB device in vanilla, we surely don't
>>>> get some parts of DVB API covered. I can imagine that  when
>>>> somebody comes with such full-featured device he wants to reinvent
>>>> just removed bits.  
>>> Re-adding the removed bits is easy. However, the API defined for
>>> av7110 is old and it is showing its age: it assumes a limited number
>>> of possible inputs/outputs. Modern SoC has a lot more ways link the
>>> audio/video IP blocks than what the API provides. On a modern SoC,
>>> not only DVB is supported, but also analog inputs (to support things
>>> like composite input), cameras, HDMI inputs and even analog TV.
>>> All of them interconnected to a media ISP. The current FF API can't
>>> represent such hardware.
>>>
>>> The best API to represent those pipeli

Re: [PATCH 00/15] Improve DVB documentation and reduce its gap

[Soeren Moch]

Hi Mauro,

On 01.09.2017 11:32, Mauro Carvalho Chehab wrote:
> Em Fri, 1 Sep 2017 10:40:28 +0200
> Honza Petrouš  escreveu:
>
>> 2017-09-01 1:46 GMT+02:00 Mauro Carvalho Chehab :
>>> The DVB documentation was negligected for a long time, with
>>> resulted on several gaps between the API description and its
>>> documentation.
>>>
>>> I'm doing a new reading at the documentation. As result of it,
>>> this series:
>>>
>>> - improves the introductory chapter, making it more generic;
>>> - Do some adjustments at the frontend API, using kernel-doc
>>>   when possible.
>>> - Remove unused APIs at DVB demux. I suspect that the drivers
>>>   implementing such APIs were either never merged upstream,
>>>   or the API itself  were never used or was deprecated a long
>>>   time ago. In any case, it doesn't make any sense to carry
>>>   on APIs that aren't properly documented, nor are used on the
>>>   upstream Kernel.
>>>
>>> With this patch series, the gap between documentation and
>>> code is solved for 3 DVB APIs:
>>>
>>>   - Frontend API;
>>>   - Demux API;
>>>   - Net API.
>>>
>>> There is still a gap at the CA API that I'll try to address when I
>>> have some time[1].
>>>
>>> [1] There's a gap also on the legacy audio, video and OSD APIs,
>>> but, as those are used only by a single very old deprecated
>>> hardware (av7110), it is probably not worth the efforts.
>>>  
av7110 cards may be old, but there are still users of these cards. For
instance I'm watching TV received and decoded with such card in this moment.
So what do you mean with "deprecated hardware"?
>> I agree that av7110 is very very old piece of hw (but it is already
>> in my hall of fame because of its Skystar 1 incarnation as
>> first implementation of DVB in Linux) and it is sad that we still
>> don't have at least one driver for any SoC with embedded DVB
>> devices.
> Yeah, av7110 made history. Please notice that this series doesn't
> remove any API that it is used by it. All it removes are the APIs
> that there's no Kernel driver using.
>
> Carry on APIs without client is usually a very bad idea, as nobody
> could be certain about how to use it. It is even worse when such
> APIs are not properly documented (with is the case here).
>
>> I understand that the main issue is that no any DVB-enabled
>> SoC vendor is interested in upstreaming theirs code, but I still hope
>> it will change in near future(*)
> We have one driver for a SoC DVB hardware at:
>   drivers/media/platform/sti/c8sectpfe/
>
> I guess it still doesn't cover the entire SoC, but this is a WiP. If I
> remember well, at the midia part of the SoC, they started by submitting
> the Remote Controller code.
>
>> Without having full-featured DVB device in vanilla, we surely don't
>> get some parts of DVB API covered. I can imagine that  when
>> somebody comes with such full-featured device he wants to reinvent
>> just removed bits.
> Re-adding the removed bits is easy. However, the API defined for
> av7110 is old and it is showing its age: it assumes a limited number
> of possible inputs/outputs. Modern SoC has a lot more ways link the
> audio/video IP blocks than what the API provides. On a modern SoC,
> not only DVB is supported, but also analog inputs (to support things
> like composite input), cameras, HDMI inputs and even analog TV.
> All of them interconnected to a media ISP. The current FF API can't
> represent such hardware.
>
> The best API to represent those pipelines that exist on SoC for
> multimedia is the media controller, where all IP blocks and their
> links (whatever they are) can be represented, if needed.
>
> The audio and video DVB API is also too limited: it hasn't
> evolved since when it was added. For audio, the ALSA API
> allows a way more control of the hardware; for video, the
> V4L2 API nowadays has all the bits to control video decoding
> and encoding. Both APIs provide support for audio and video
> inputs commonly found on those devices.
The real advantage of the DVB audio/video/osd API is the possibility
of frame synchronous audio/video/overlay output for high-quality
audio/video playback, maybe with picture-in-picture functionality.

Especially audio synchronization is not easy when the audio format
changes from compressed audio (e.g. AC-3) to PCM (stereo), e.g. on
HDMI output. While HDMI output hardware usually takes video frames and
audio packets (and AVI info frames for audio/video format signalization)
synchronously, V4L2 and ALSA rip these data blocks apart and push these
through different pipelines with different buffering properties. This
makes it very difficult for userspace applications. With the DVB API
the hardware takes care of the synchronisation.
> Also, nowadays, video decoding usually happens at the GPU on SoC. So, 
> in practice, a SoC FF would likely use the DRM subsystem to control the
> video codecs.
I think this is a common misunderstanding. Video is decoded on separate
hardware