linux-kernel - Re: DP/HDMI Jack and Pipewire

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Message-ID: <874ir4ribm.wl-tiwai@suse.de>
Date: Sat, 08 Nov 2025 10:21:01 +0100
From: Takashi Iwai <tiwai@...e.de>
To: Srinivas Kandagatla <srinivas.kandagatla@....qualcomm.com>
Cc: Mark Brown <broonie@...nel.org>,
	Takashi Iwai <tiwai@...e.com>,
	linux-sound <linux-sound@...r.kernel.org>,
	wtaymans@...hat.com,
	arun@...mptotic.io,
	Jaroslav Kysela <perex@...ex.cz>,
	lkml <linux-kernel@...r.kernel.org>,
	Krzysztof Kozlowski <krzk@...nel.org>,
	Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
Subject: Re: DP/HDMI Jack and Pipewire

On Fri, 07 Nov 2025 11:15:50 +0100,
Srinivas Kandagatla wrote:
> 
> Hi Everyone,
> 
> On Qualcomm platforms we have an issue enabling Display port on a full
> Distro setup with pipewire and wireplumber in place.
> 
> The issue is that Display port Audio IP on Qualcomm SoC is powered off
> if there is no Display connected. It make sense to keep it in this low
> power mode when there is no use. And the DP IP is not expecting any data
> in this state and any attempt to configure or send data would result in
> error from DSP.
> 
> However, we create PCM devices and jacks for all audio sinks and
> sources, including DisplayPort DAI links. When PipeWire starts up
> without any awareness of jack state it probes all available PCM devices,
> including the DisplayPort ones. Since no display is connected, the
> prepare callback for DP fails, leading PipeWire to mark the sound card
> as unusable. Consequently, it abandons entire sound card, including
> other valid audio sinks. I have also started discussing this issue with
> pipewire [1]
> 
> 
> What is the expected pcm device behavior when DP jack is not connected?
> 
> Two possibilities:
> 
> 1. Consume the data even when the Display is not connected. I see that
> in Intel case it sinks the data somewhere and gives the user an
> experience that the data is getting consumed.
> 
> 2. Throw an error to user if they attempt to configure or send the data
> to this disconnected pcm.
> 
> 
> Also what userspace ABI to for such usecase?
> 
> This is blocking end-to-end DP audio enablement on Qualcomm SoCs.
> 
> Not sure if this is right approach or will work but somehow we back the
> pcm devices with disconnected jack state to use dummy pcm ops instead of
> actual pcm ops?
> This should at least keep the pipewire happy. Is this the right approach?
> 
> 
> [1]: https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/4938
> 
> Thanks,
> Srini

I believe this has been a long-standing problem even for HD-audio.
The HD-audio HDMI driver does open and process without actual pin
setup for allowing the probe by PA/PW with the assumption of some
basic PCM parameters.  It was introduced in the commit
42b2987079eca0238b576c08af1144ed5bd52188
    ALSA: hda - hdmi playback without monitor in dynamic pcm bind mode

So I find using a dummy ops would make sense (assuming it's actually
enough to convince PA/PW).


thanks,

Takashi