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

[Shengjiu Wang <shengjiu.wang@xxxxxxxxx>]

On Wed, Aug 14, 2024 at 5:40 PM Pierre-Louis Bossart
<pierre-louis.bossart@xxxxxxxxxxxxxxx> wrote:
>
>
> > Yes, to go further, I think we can use SND_AUDIOCODEC_PCM, then
> > the SRC type will be dropped.
>
> sounds good.
>
> > But my understanding of the control means the .set_metadata() API, right?
> > As I said, the output rate, output format, and ratio modifier are applied to
> > the instances of ASRC,  which is the snd_compr_stream in driver.
> > so only the .set_metadata() API can be used for these purposes.
>
> Humm, this is more controversial.
>
> The term 'metadata' really referred to known information present in
> headers or additional ID3 tags and not in the compressed file itself.
> The .set_metadata was assumed to be called ONCE before decoding.
>
> But here you have a need to update the ratio modifier on a regular basis
> to compensate for the drift. This isn't what this specific callback was
> designed for. We could change and allow this callback to be used
> multiple times, but then this could create problems for existing
> implementations which cannot deal with modified metadata on the fly.

.set_metadata can be called multi times now, no need to change currently.

>
> And then there's the problem of defining a 'key' for the metadata. the
> definition of the key is a u32, so there's plenty of space for different
> implementations, but a collision is possible. We'd need an agreement on
> how to allocate keys to different solutions without changing the header
> file for every implementation.

Can we define a private space for each case?   For example the key larger
than 0x80000000 is private, each driver can define it by themself?

>
> It sounds like we'd need a 'runtime params' callback - unless there's a
> better trick to tie the control and compress layers?
>