[alsa-devel] ADAU1761 default register value problem

[Ricard Wanderlof]

On Tue, 26 Apr 2016, Lars-Peter Clausen wrote:

> On 04/21/2016 04:01 PM, Ricard Wanderlof wrote:
> > 
> > ...
> > The problem I have is that the driver assumes that the CODEC has been 
> > initialized with the default register settings when the system boots. 
> > However, if the system has not been power cycled but just rebooted, this 
> > is not necessarily the case, as the CODEC has no external reset pin but 
> > derives its reset signal internally from the power level.
> > ...

Sorry I haven't been able to respond earlier.

> I've been thinking about this for the last few days trying to come up with a
> good solution, but it is really difficult.
> 
> The issue you are describing is simply an oversight from my side.
> 
> One solution would be do drop the register default values and let regmap
> fill the register cache on demand when a read is done the first time for a
> register. The problem with this approach is that we leave the device in an
> unknown state when we probe the driver, but we really want to have it in a
> known state of lowest power.
> 
> The alternative is to write out the register defaults when the device is
> probed, sort of a poor mans software reset, this brings us into a known
> state. This approach has the downside that there is the overhead of having
> to write all registers.
> 
> I don't like either, but given the limitations of the hardware we'll have to
> choose one and in that case I'd slightly lean towards the later solution has
> it gives us consistent and predictable behavior and the overhead of writing
> the registers should be manageable.

Either way, there will be a fair amount of communication with the codec, 
either as a result of setting the default values, or as a result of 
reading the existing ones in order to determine what would be necessary to 
write.

I would think that having the codec in a controlled known (low power) 
state would tip the balance in favor of the second solution.

One question is though should one write out _all_ registers, including 
those not actually used by the driver? I'm thinking of an 
upgrade/downgrade situation, where a previously running kernel version has 
set up a register which the currently running version doesn't need to 
touch.

/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