Bug 94763

Summary:	ALSA module: tsched=0 causes skipping on nVidia HDMI output under QEMU-KVM
Product:	PulseAudio	Reporter:	Daniel Edgecumbe <dan>
Component:	pavucontrol	Assignee:	pulseaudio-bugs
Status:	RESOLVED MOVED	QA Contact:	pulseaudio-bugs
Severity:	normal
Priority:	medium	CC:	lennart
Version:	unspecified
Hardware:	x86-64 (AMD64)
OS:	Linux (All)
Whiteboard:
i915 platform:		i915 features:

Description Daniel Edgecumbe 2016-03-31 02:15:21 UTC

bool pa_alsa_may_tsched [modules/alsa/alsa-util.c] disables tsched if PA determines that it is running within a VM.

This causes awful skipping audio on my system.
Removing the check enables the use of tsched and PA functions without a hitch.

Details
-------
Guest VM running under QEMU-KVM.
Arch Linux current 4.4.5-1-ARCH as of 2016/03/31
pulseaudio-git
commit 0f48b7c82378c32194b625d1fd886eee7f1e5928
Date:   Sun Mar 13 23:56:54 2016 +0200
NVIDIA proprietary driver on nVidia GeForce 960GTX - using nVidia HDMI audio.

git blame appears to stem from 2009 - has support for tsched improved since then?

53b046d5 src/modules/alsa/alsa-util.c (Lennart Poettering   2009-11-05 03:23:08 +0100 1482)         /* We cannot depend on being woken up when we ask for in a VM,
53b046d5 src/modules/alsa/alsa-util.c (Lennart Poettering   2009-11-05 03:23:08 +0100 1483)          * so let's fallback to classic IO based playback then. */

I would go ahead and submit a pull request to disable this check but I'm unsure of the ramifications across different virtualization solutions.

Comment 1 Raymond 2016-03-31 03:06:40 UTC

the patch was intended for the emulated intel8x0 sound card inside VM

do you mean nvidia hdmi using pci pass through ?

since only emulated sb16, intel8x0 and hda-intel

Comment 2 Raymond 2016-03-31 03:08:27 UTC

which backend audio using by your host ?

http://git.kernel.org/cgit/virt/kvm/qemu-kvm.git/tree/audio

http://git.kernel.org/cgit/virt/kvm/qemu-kvm.git/tree/hw

Comment 3 Daniel Edgecumbe 2016-03-31 03:50:39 UTC

Yes, that's correct, I am using the nVidia HDMI audio directly using pci passthrough.

Does enabling tsched break support for the emulated cards? I can test this when I have time.

Comment 4 Raymond 2016-03-31 07:17:35 UTC

do it really work since hdmi require graphic driver to request EDID from your HDMI monitor/receiver ?

can you post the output of alsa-info.sh inside VM and pactl list sinks

Comment 5 Raymond 2016-03-31 07:22:21 UTC

http://mailman.alsa-project.org/pipermail/alsa-devel/2014-September/081501.html

you can run pcm_avail.c with the emulated sound card intel8x0, hda-intel and your nvidia HDMI pci passthrough

Comment 6 GitLab Migration User 2018-07-30 09:30:34 UTC

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