On 12/18/18 5:32 AM, Dimitris Papavasiliou wrote:
On 12/17/18 8:08 PM, Pierre-Louis Bossart wrote:
The machine driver should use clk_set_rate() and not directly handle
regmap or codec stuff. If it does, or if the clock framework isn't
relevant here then we can simplify all this as suggested in
https://patchwork.kernel.org/patch/10444387/. What I was trying to do
with the github update is to keep the clock framework, tie it closer
with the codec parts with a state variable that prevents wild changes
without going back to a 'safe' idle state (similar idea as PulseAudio
clock changes, which can only happen when the PCM is not opened and
used).
I have documented, in a previous message, various approaches I tried
to prevent the pop, which were mostly based on the assumption that the
clock source was changed too "abruptly", or wasn't allowed to settle.
In the end, the only state, where the clocks can be switched without a
pop seemed to be when the chip is suspended.
It would of course be great if this could be achieved in a natural
manner, by waiting for the card to be suspended and only switching
clocks in such a state, but I don't see how this can be achieved
robustly. The power management behavior is outside of the control of
the machine driver, so it can only refuse to switch sample rate when
it's not suspended. Perhaps this will work in practice, but I fear it
would break applications depending on less restrained control of the
hardware.
My point was that the machine driver can track DAPM events and put the
device in a 'safe' state on SND_SOC_DAPM_EVENT_OFF so that the rate can
be changed when playback restarts. I don't see what prevents us from
using the same config as during suspend? It's pretty common to play with
clocks with these events, and calling clk_disable_unprepare() could lead
to whatever configuration sequence is needed for pop-free restart on
startup.
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