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

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Message-ID: <3cdb2041-59d4-4d43-ac4d-39d7f9640cef@linux.intel.com>
Date: Wed, 14 Aug 2024 11:40:18 +0200
From: Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To: Shengjiu Wang <shengjiu.wang@...il.com>
Cc: Jaroslav Kysela <perex@...ex.cz>, Shengjiu Wang <shengjiu.wang@....com>,
 vkoul@...nel.org, tiwai@...e.com, alsa-devel@...a-project.org,
 linux-sound@...r.kernel.org, linux-kernel@...r.kernel.org,
 Xiubo.Lee@...il.com, festevam@...il.com, nicoleotsuka@...il.com,
 lgirdwood@...il.com, broonie@...nel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH 1/6] ALSA: compress: add Sample Rate Converter codec
 support

> 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.

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.

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