Re: [alsa-devel] [PATCH 00/11] Add AXD Audio Processing IP driver

[Pierre-Louis Bossart]

On 10/28/14, 8:18 AM, Qais Yousef wrote:
> On 10/28/2014 11:55 AM, Clemens Ladisch wrote:
> > Qais Yousef wrote:
> > > AXD Audio Processing IP performs audio decoding, encoding, mixing,
> > > equalisation,
> > > synchronisation and playback.
> > What exactly do you mean with "synchronisation" and "playback"?
>
> Synchronisation refers to accurate audio playout relative to a master
> clock source including compensation of drift between the master clock
> source and the playout clock of the audio hardware. Hence allowing
> synchronised audio playout across multiple independent devices.
>
> Playback simple refers to the fact that AXD is capable of managing audio
> playout hardware like I2S and SPDIF interfaces.
>
>
> > > It doesn't fit in alsa subsystem but I Cced them to confirm.
> > ... because those two words sound like something that a sound card
> > could do.
>
> The problem mainly stems from the fact that we take a variety of
> compressed audio as input and we could perform audio encoding. The
> problem with the compressed audio is that the range of decoders and
> configuration supported in alsa is limited and there's no support for
> taking raw pcm and producing compressed output. I'm not an expert on
> alsa but when I looked it looked like there's more infra structure
> required.
>
> The following not supported points from
> Documentation/sound/alsa/compress_offload.txt affect us:
>
> - Volume control/routing is not handled by this API. Devices exposing a
>    compressed data interface will be considered as regular ALSA devices;
>    volume changes and routing information will be provided with regular
>    ALSA kcontrols.
>
> - Embedded audio effects. Such effects should be enabled in the same
>    manner, no matter if the input was PCM or compressed.
>
> - Encoding/decoding acceleration is not supported as mentioned
>    above. It is possible to route the output of a decoder to a capture
>    stream, or even implement transcoding capabilities. This routing
>    would be enabled with ALSA kcontrols.

Volume control, effects and routing aren't supported directly with the 
snd_pcm streaming interface either. we didn't add these features either 
in the compressed API since they could be enabled through existing means 
(kcontrols and routing).

> > > I added it to drivers/char as this is the best location I could think
> > > of at the
> > > moment.
> > drivers/misc?
>
> Yeah could do if there's consensus.
>
> Thanks,
> Qais
>
> > Regards,
> > Clemens
>
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