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

[Shengjiu Wang]

On Wed, May 15, 2024 at 6:46 PM Jaroslav Kysela <perex at perex.cz> wrote:
>
> On 15. 05. 24 12:19, Takashi Iwai wrote:
> > On Wed, 15 May 2024 11:50:52 +0200,
> > Jaroslav Kysela wrote:
> >>
> >> 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?
> >
> > If it works with compress API, it'd be great, yeah.
> > So, your idea is to open compress-offload devices for read and write,
> > then and let them convert a la batch jobs without timing control?
> >
> > For full-duplex usages, we might need some more extensions, so that
> > both read and write parameters can be synchronized.  (So far the
> > compress stream is a unidirectional, and the runtime buffer for a
> > single stream.)
> >
> > And the buffer management is based on the fixed size fragments.  I
> > hope this doesn't matter much for the intended operation?
>
> It's a question, if the standard I/O is really required for this case. My
> quick idea was to just implement a new "direction" for this job supporting
> only one ioctl for the data processing which will execute the job in "one
> shot" at the moment. The I/O may be handled through dma-buf API (which seems
> to be standard nowadays for this purpose and allows future chaining).
>
> So something like:
>
> struct dsp_job {
>     int source_fd;     /* dma-buf FD with source data - for dma_buf_get() */
>     int target_fd;     /* dma-buf FD for target data - for dma_buf_get() */
>     ... maybe some extra data size members here ...
>     ... maybe some special parameters here ...
> };
>
> #define SNDRV_COMPRESS_DSPJOB _IOWR('C', 0x60, struct dsp_job)
>
> This ioctl will be blocking (thus synced). My question is, if it's feasible
> for gstreamer or not. For this particular case, if the rate conversion is
> implemented in software, it will block the gstreamer data processing, too.
>

Thanks.

I have several questions:
1.  Compress API alway binds to a sound card.  Can we avoid that?
     For ASRC, it is just one component,

2.  Compress API doesn't seem to support mmap().  Is this a problem
     for sending and getting data to/from the driver?

3. How does the user get output data from ASRC after each conversion?
   it should happen every period.

best regards
Shengjiu Wang.