[alsa-devel] [PATCH] ASoC: nau8825: Add Class G function options

[Ben Zhang]

On Wed, Dec 14, 2016 at 6:57 AM, Mark Brown <broonie at kernel.org> wrote:
> On Mon, Dec 12, 2016 at 10:12:06PM -0800, Ben Zhang wrote:
>> Add a DAPM_MUX for userspace to control the Class G function:
>> "Switching": Use +-1.8V or +-0.9V supply, based on signal amplitude.
>> "1.8V": Always use +-1.8V.
>
> This doesn't affect audio routing so why is it being done with a DAPM
> mux?

When user wants the switching voltages, the Class G register bit needs
to be toggled at a specific time in the widget power up/down sequences
on the playback route. It helps reduce pop and is consistent with
existing behavior. DAPM_MUX achieves this by taking DAPM_PGA_S in and
out of the playback route based on user selection. If the register bit
is directly controlled by a separate kcontrol in
snd_soc_codec_driver.controls unrelated to widgets, the bit will be
set immediately by user even when there is no active playback. Then
when playback starts, Class G is already on before the DACs/charge
pumps/output drivers/pulldowns are enabled. The existing/desired
behavior is to turn on Class G last and turn it off first.

John has sent me a patch which uses a separate SOC_ENUM_EXT kcontrol
in snd_soc_codec_driver.controls to set a flag, then has DAPM_PGA_S PM
event callbacks selectively setting the register bit based on the
flag. But it feels cleaner to implement this with a DAPM_MUX: The
DAPM_PGA_S widget power status correctly reflects the actual register
bit. User can experiment with the setting during playback.

John, to make SND_SOC_NOPM work with DAPM_MUX on older kernels, commit
236aaa686358 ("ASoC: dapm: Consolidate MUXs and virtual MUXs") can be
backported, which landed in v3.15. chromeos-3.14 has backported it
already. Another solution is to backport my patch by using
SND_SOC_DAPM_VIRT_MUX on older kernels.