Bug#917906: linux-image-4.17.0-1-amd64 (and onward): CONFIG_SND_HDA_POWER_SAVE_DEFAULT=1 might cause long hangs for user
Package: src:linux
Version: 4.17.0-1
Severity: normal
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Hash: SHA512
Dear kernel maintainers,
starting from `linux-image-4.17.0-1-amd64`, the debian kernel comes
with `CONFIG_SND_HDA_POWER_SAVE_DEFAULT=1` (it was set to `0` before).
I wonder if this change was intended, for two reasons.
1. According to the documentation at [1]:
``CONFIG_SND_HDA_POWER_SAVE_DEFAULT`` Kconfig options. Setting this
to 1 (the minimum value) isn't recommended because many applications
try to reopen the device frequently. 10 would be a good choice for
normal operations.
2. Moreover, grepping through the config shows that this change was
only applied to `CONFIG_SND_HDA`, not to `CONFIG_SND_AC97`.
$ grep POWER_SAVE_DEFAULT config-4.18.0-3-amd64
CONFIG_SND_AC97_POWER_SAVE_DEFAULT=0
CONFIG_SND_HDA_POWER_SAVE_DEFAULT=1
In any case, this breaks things on my laptop. In short, the function
call `snd_pcm_open()` (to open an audio device), which used to take
around 1 ms, now takes an average of 40 ms, with maximum peaks measured
to 3 seconds.
What's unfortunate is that this function might be called from within
the GTK+ main loop, synchronously, causing apps like the terminal or
a text editor to hang until the call returns. And this function is
called A LOT, due to event sounds.
I described this a bit more in details on the ALSA mailing list at [2].
The kernel documentation at [1] mentions:
Also, it often takes certain time to wake up from the
power-down to the active state. These are often hardly to fix, so
don't report extra bug reports unless you have a fix patch ;-)
Reading that, and after discovering that event sounds are implemented
synchronously (well, at least using GNOME/Wayland), I wonder if
`CONFIG_SND_HDA_POWER_SAVE_DEFAULT` shouldn't be reverted to 0.
Thanks,
Arnaud
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[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/sound/designs/powersave.rst
[2]: http://mailman.alsa-project.org/pipermail/alsa-devel/2018-December/143167.html
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