Re: [alsa-devel] [PATCH 2/2] [RFC] ALSA: hda: hdmi - preserve non-MST PCM routing for Intel platforms

From: Kai Vehmanen <kai.vehmanen@linux.intel.com>
To: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Cc: tiwai@suse.de, alsa-devel@alsa-project.org,
	Nikhil Mahale <nmahale@nvidia.com>
Subject: Re: [alsa-devel] [PATCH 2/2] [RFC] ALSA: hda: hdmi - preserve non-MST PCM routing for Intel platforms
Date: Fri, 29 Nov 2019 16:47:11 +0200 (EET)	[thread overview]
Message-ID: <alpine.DEB.2.21.1911291638430.16459@zeliteleevi> (raw)
In-Reply-To: <20191129143756.23941-2-kai.vehmanen@linux.intel.com>

Hi Takashi, Nikil and others,

On Fri, 29 Nov 2019, Kai Vehmanen wrote:
> This difference leads to some subtle differences in hdmi_find_pcm_slot()
> with regards to how non-MST monitors are assigned to PCMs.
> This patch restores the original behaviour on Intel platforms while
> keeping the new allocation policy on other platforms.

hmm, there seems a couple of more issues. The first patch is a clear bug 
that leads to segfault with SOF+patch_hdmi on some platforms (pipe B used
for single monitor HDMI case -> dev_id=1 -> non-existant pcm selected
and eventual kernel oops).

This second patch is however trickier. Nikhil your patch changed the 
default allocation a bit, so the routing might be difference also with 
snd-hda-intel (i.e. not SOF) for existing platforms and this may surprise 
users.

Digging deeper, we seem to have a slight semantics difference in how
intel_pin_eld_notify() and generic_acomp_pin_eld_notify() handle
the third pipe/dev_id parameter.

Any thoughts how to solve? I first I thought making separate functions
for hdmi_find_pcm_slot() and allow platforms to define an alternative
implementation, but in this RFC patch I opted for a simpler quirk in the 
function. This is becoming fairly messy I must say -- the amount of 
code commentary needed is a good indication some simplifaction would
be in order.

PS I did not have time to fully test the RFC patch, so this is just
   for discussion now...

Br, Kai
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