linux-kernel - Re: [PATCH v2 0/4] ASoC: qcom: display port changes

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Date: Mon, 29 Apr 2024 17:00:41 +0200
From: Johan Hovold <johan@...nel.org>
To: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
Cc: broonie@...nel.org, perex@...ex.cz, tiwai@...e.com, lgirdwood@...il.com,
	alsa-devel@...a-project.org, linux-kernel@...r.kernel.org,
	Bjorn Andersson <quic_bjorande@...cinc.com>
Subject: Re: [PATCH v2 0/4] ASoC: qcom: display port changes

On Tue, Apr 23, 2024 at 04:59:56PM +0100, Srinivas Kandagatla wrote:
> On 23/04/2024 15:58, Johan Hovold wrote:

> >> It is absolutely possible to run all the streams in parallel from the
> >> Audio hardware and DSP point of view.
> >>
> >> One thing to note is, On Qualcomm DP IP, we can not read/write registers
> >> if the DP port is not connected, which means that we can not send data
> >> in such cases.
> >>
> >> This makes it challenging to work with sound-servers like pipewire or
> >> pulseaudio as they tend to send silence data at very early stages in the
> >> full system boot up, ignoring state of the Jack events.

> > This bit sounds like it can and should be worked around by the driver to
> > avoid hard-coding policy which would prevent use cases such as the ones
> > mentioned above.

> This is not simple as you say. We have to fit these into a proper DPCM.
> Either we have a dummy Backend connected for each of these pcm 
> sub-devices when DP port is not connected and then switch back to DP 
> when its connected.

I don't know how best to implement it, but we shouldn't necessarily let
that determine the user experience.

> Or somehow find a way to not let the pipewire talk to devices which are 
> not connected.

Yes, perhaps it requires a change in user space.

But it seems the kernel should be able to fake whatever probing user
space currently does to determine if the there is a DP jack (even when
there is nothing connected).

Johan