vc4.ko brings unusable&unstable ALSA sinks

[Dave Stevenson]

On Thu, 1 Apr 2021 at 15:45, Diederik de Haas <didi.debian at cknow.org> wrote:
>
> Hi Dave/ML,
>
> Thank you for this detailed description, much appreciated :)
>
> On woensdag 31 maart 2021 14:25:04 CEST Dave Stevenson wrote:
> > ALSA will pack PCM for you into the right format if you provide an
> > appropriate conf file. The version shipped with Raspberry Pi OS will
> > do this, and is also available from LibreElec[1]. I suspect it hasn't
> > been upstreamed as yet, so Debian is unlikely to have it.
> > [1] https://github.com/LibreELEC/LibreELEC.tv/blob/master/projects/RPi/
> > filesystem/usr/share/alsa/cards/vc4-hdmi.conf
>
> I have already been experimenting with that file:
> root at rpi-mpd:/usr/share/alsa/cards# ls -l vc4*
> -rw-r--r-- 1 root root 865 Mar 19 22:44 vc4-hdmi.conf
> -rw-r--r-- 1 root root 965 Mar 19 00:27 vc4-hdmi.conf.old
> -rw-r--r-- 1 root root 820 Oct 30 09:58 vc4-hdmi.conf.org
>
> As you can see I used some brilliant names to differentiate them ;P
> When looking through them, I found a clue that was intended to help:
> root at rpi-mpd:/usr/share/alsa/cards# grep -i libreelec vc4-hdmi.conf
> # This is copied from LibreELEC for RPi 2/3
>
> I've now also created a 'vc4-hdmi.conf.libreelec' file with the contents you
> linked and that file is 1292 bytes.
> So my comment turned out to be insufficient as it doesn't contain the actual
> source. The .org file is from Debian's libasound2-data package and the .old file
> is likely from the RPi forum thread about kernel 5.10.
> I'll (soon) try the .libreelec file (renamed/linked ofc).
>
> > Until recently most of the interfacing for HDMI has been through the
> > VideoCore VPU (aka the firmware). That has included providing an audio
> > interface for HDMI and analogue audio via snd_bcm2835.
> > ...
> > Do not use enable_hdmi=1 if using the vc4 DRM/KMS driver.
>
> I've seen that with the RPF kernel DT overlays are 'often' used, but I don't
> know if it does or should work the same with upstream/Debian's kernel.
> While interested in it in the general sense, I'll restrict it to DRM/KMS (as
> far as I understand it).
> With the RPF kernel, I've seen these (config.txt) lines:
> dtoverlay=vc4-fkms-v3d
> dtoverlay=vc4-kms-v3d
>
> My question then is: should I use one of these dtoverlays and if so, which?

Mainline doesn't build dtb files with the appropriate symbols for
overlays to work, therefore they generally can't be applied.

It should have thew display pipeline (ie vc4.ko) enabled by default
via the status = "okay"; lines in
https://github.com/torvalds/linux/blob/master/arch/arm/boot/dts/bcm2711-rpi-4-b.dts#L182
for hvs, pixel valves, hdmi, ddc, and a few other blocks.

  Dave