Subject: Re: [alsa-devel] Propagating audio properties along the audio path

From: Marc Gonzalez <[email-masked]>
Date: Tue, 24 Sep 2019 16:26:20 +0200

On 24/09/2019 15:52, Charles Keepax wrote:

> In general the ALSA framework doesn't really allow for stream
> params to change whilst the stream is active. Doing so is
> also normally very hard for the types of hardware usually
> involved. For example changing the clocks on a running I2S bus,
> very difficult to get both ends to pick up those changes at
> exactly the correct sample. Some newer buses like soundwire
> have more support for things like this were the ends of the
> link can synchronise changes but even there that is normally
> used for adding/removing streams from the bus, not reconfiguring
> a running stream.

This jives with what "filt3r" wrote on #alsa-soc

"at one point we were just closing the stream (somehow) if we detected
a change in e.g. sample-rate, so the user-space application would fail
on snd_pcm_readi()"

	snd_pcm_stop(p_spdif->capture_stream, SNDRV_PCM_STATE_DISCONNECTED);

> In your case above I would imagine the system would probably be
> setup where the DSP handles the conversion between the params
> requested from the receiver and those requested by user-space.
> One of the intentions of DPCM was to allow the backend
> (DSP-receiver here) to have different params to the frontend
> (DSP-userspace here). Although as you note you still probably
> need to add something to propagate those changes to the DSP. What
> form does the physical link between the receiver and the DSP
> take?

The setup looks like this:

A = Some kind of audio source, typically a TV or game console
B = The arm64 SoC, equipped with some nice speakers

   HDMI
A ------> B

If we look inside B, we actually have
B1 = an eARC receiver (input = HDMI, output = I2S)
B2 = an audio DSP (input = I2S, output = speakers)

    I2S        ?
B1 -----> B2 -----> speakers

To answer your question, B1 and B2 are connected via I2S.

Regards.

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