[PATCH v15 00/16] Add audio support in v4l2 framework

[Jaroslav Kysela]

On 15. 05. 24 11:17, Hans Verkuil wrote:
> Hi Jaroslav,
> 
> On 5/13/24 13:56, Jaroslav Kysela wrote:
>> On 09. 05. 24 13:13, Jaroslav Kysela wrote:
>>> On 09. 05. 24 12:44, Shengjiu Wang wrote:
>>>>>> mem2mem is just like the decoder in the compress pipeline. which is
>>>>>> one of the components in the pipeline.
>>>>>
>>>>> I was thinking of loopback with endpoints using compress streams,
>>>>> without physical endpoint, something like:
>>>>>
>>>>> compress playback (to feed data from userspace) -> DSP (processing) ->
>>>>> compress capture (send data back to userspace)
>>>>>
>>>>> Unless I'm missing something, you should be able to process data as fast
>>>>> as you can feed it and consume it in such case.
>>>>>
>>>>
>>>> Actually in the beginning I tried this,  but it did not work well.
>>>> ALSA needs time control for playback and capture, playback and capture
>>>> needs to synchronize.  Usually the playback and capture pipeline is
>>>> independent in ALSA design,  but in this case, the playback and capture
>>>> should synchronize, they are not independent.
>>>
>>> The core compress API core no strict timing constraints. You can eventually0
>>> have two half-duplex compress devices, if you like to have really independent
>>> mechanism. If something is missing in API, you can extend this API (like to
>>> inform the user space that it's a producer/consumer processing without any
>>> relation to the real time). I like this idea.
>>
>> I was thinking more about this. If I am right, the mentioned use in gstreamer
>> is supposed to run the conversion (DSP) job in "one shot" (can be handled
>> using one system call like blocking ioctl).  The goal is just to offload the
>> CPU work to the DSP (co-processor). If there are no requirements for the
>> queuing, we can implement this ioctl in the compress ALSA API easily using the
>> data management through the dma-buf API. We can eventually define a new
>> direction (enum snd_compr_direction) like SND_COMPRESS_CONVERT or so to allow
>> handle this new data scheme. The API may be extended later on real demand, of
>> course.
>>
>> Otherwise all pieces are already in the current ALSA compress API
>> (capabilities, params, enumeration). The realtime controls may be created
>> using ALSA control API.
> 
> So does this mean that Shengjiu should attempt to use this ALSA approach first?

I've not seen any argument to use v4l2 mem2mem buffer scheme for this data 
conversion forcefully. It looks like a simple job and ALSA APIs may be 
extended for this simple purpose.

Shengjiu, what are your requirements for gstreamer support? Would be a new 
blocking ioctl enough for the initial support in the compress ALSA API?

						Jaroslav

-- 
Jaroslav Kysela <perex at perex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.