[RFC PATCH 1/6] ALSA: compress: add Sample Rate Converter codec support

[Jaroslav Kysela]

On 08. 08. 24 13:27, Pierre-Louis Bossart wrote:
> 
> 
> On 8/8/24 11:17, Shengjiu Wang wrote:
>> On Tue, Aug 6, 2024 at 7:25 PM Pierre-Louis Bossart
>> <pierre-louis.bossart at linux.intel.com> wrote:
>>>
>>>
>>>
>>> On 8/6/24 12:26, Shengjiu Wang wrote:
>>>> Add Sample Rate Converter(SRC) codec support, define the output
>>>> format and rate for SRC.
>>>>
>>>> Signed-off-by: Shengjiu Wang <shengjiu.wang at nxp.com>
>>>> ---
>>>>   include/uapi/sound/compress_offload.h | 2 ++
>>>>   include/uapi/sound/compress_params.h  | 9 ++++++++-
>>>>   2 files changed, 10 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/include/uapi/sound/compress_offload.h b/include/uapi/sound/compress_offload.h
>>>> index 98772b0cbcb7..8b2b72f94e26 100644
>>>> --- a/include/uapi/sound/compress_offload.h
>>>> +++ b/include/uapi/sound/compress_offload.h
>>>> @@ -112,10 +112,12 @@ struct snd_compr_codec_caps {
>>>>    * end of the track
>>>>    * @SNDRV_COMPRESS_ENCODER_DELAY: no of samples inserted by the encoder at the
>>>>    * beginning of the track
>>>> + * @SNDRV_COMPRESS_SRC_RATIO_MOD: Resampling Ratio Modifier for sample rate converter
>>>>    */
>>>>   enum sndrv_compress_encoder {
>>>>        SNDRV_COMPRESS_ENCODER_PADDING = 1,
>>>>        SNDRV_COMPRESS_ENCODER_DELAY = 2,
>>>> +     SNDRV_COMPRESS_SRC_RATIO_MOD = 3,
>>>>   };
>>>
>>> this sounds wrong to me. The sample rate converter is not an "encoder",
>>> and the properties for padding/delay are totally specific to an encoder
>>> function.
>>
>> There is only decoder and encoder definition for compress,  I know
>> it is difficult to add SRC to encoder or decoder classification,
>> SRC is a Post Processing.  I hope you can have a recommandation
> 
> I don't. I think we're blurring layers in a really odd way.
> 
> The main reason why the compress API was added is to remove the
> byte-to-time conversions. But that's clearly not useful for a
> post-processing of PCM data, where the bitrate is constant. It really
> feels like we're adding this memory-to-memory API to the compress
> framework because we don't have anything else available, not because it
> makes sense to do so.

It makes sense to offload decoder/encoder tasks as batch processing for 
standard compress stream and return back result (PCM stream or encoded stream) 
to user space. So it makes sense to use the compress interface (parameters 
handshake) for it. Let's talk about the proper SRC extension.

For SRC and dynamic rate modification. I would just create an ALSA control for 
this. We are already using the "PCM Rate Shift 100000" control in the 
sound/drivers/aloop.c for this purpose (something like pitch in MIDI) for 
example. There is no requirement to add this function through metadata ioctls. 
As bonus, this control can be monitored with multiple tasks.

> Then there's the issue of parameters, we chose to only add parameters
> for standard encoders/decoders. Post-processing is highly specific and
> the parameter definitions varies from one implementation to another -
> and usually parameters are handled in an opaque way with binary
> controls. This is best handled with a UUID that needs to be known only
> to applications and low-level firmware/hardware, the kernel code should
> not have to be modified for each and every processing and to add new
> parameters. It just does not scale and it's unmaintainable.
> 
> At the very least if you really want to use this compress API, extend it
> to use a non-descript "UUID-defined" type and an opaque set of
> parameters with this UUID passed in a header.

We don't need to use UUID-defined scheme for simple (A)SRC implementation. As 
I noted, the specific runtime controls may use existing ALSA control API.

					Jaroslav


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