[alsa-devel] Disable DAPM for click avoidance ?

[Ricard Wanderlof]

Hi Mark,

Picking up an old thread here, I'm working on a VMID based codec with 
associated driver from AKM, where the VMID and audio outputs are only 
switched on when the devices is actually outputting audio data (in the 
playback case), causing loud clicks when a stream is started and stopped.

On Wed, 22 Nov 2017, Mark Brown wrote:

> On Wed, Nov 22, 2017 at 01:33:37PM +0100, Ricard Wanderlof wrote:
> 
> > I'm having an issue with a codec where the normal DAPM procedure of 
> > disabling most of the codec when there is no stream ongoing results in 
> > large clicks when a playback stream starts and stops, because the output 
> > jumps from 0V when the output is disabled to over a volt when the output 
> > is enabled. 
> 
> > There appears to be a 'slow stop/start' feature in the codec, however, it 
> > takes about one second to ramp up and down the output voltage, and that is 
> > too long a delay when starting a stream.
> 
> The CODEC driver is broken, it shouldn't be powering off on idle if it
> takes a second to ramp, it should be leaving VMID up while the system is
> running - this is what the _STANDBY bias level is all about.  Look at
> what older CODEC drivers like the wm8731 do.

I was tracing through what actually is called in the codec driver when a 
stream is started, and from what I can see ALSA sequences through the 
STANDBY, PREPARE and ON states when a stream is started, with no other 
calls to set_bias_level prior to that. So it would seem that the STANDBY 
level is reached during the process of starting the stream, in other 
words, VMID and the outputs would have to be enabled already in the OFF 
state.

In our system, the power consumed by the codec is negligible compared to 
the rest of the system at all times, so it's not a problem to leave 
basically the whole codec up, but I am trying understand what is the 
Right Thing to do here.

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30