[alsa-devel] "Negative" volume settings in a kcontrol

[Mike Looijmans]

On 05/06/2013 03:22 PM, Clemens Ladisch wrote:
> Mike Looijmans wrote:
>> Some codec registers need negative values. For example,
>> sound/soc/codecs/tlv320aic32x4.c specifies this:
>>
>>   78         SOC_DOUBLE_R_TLV("PCM Playback Volume", AIC32X4_LDACVOL,
>>   79                         AIC32X4_RDACVOL, 0, 0x30, 0, tlv_step_0_5),
>>
>> It's incomplete. The actual range is from -63.5dB to +24dB in 0.5 dB steps.
>> The 8-bit value is interpreted as a signed 8-bit integer, so to get -3dB
>> the register value must be -6 or 0xFA.
>>
>> How do I explain that to Alsa?
>
> There are some ASoC helper macros that handle signed register fields, such
> as SOC_DOUBLE_S8_TLV and SOC_SINGLE_XR_SX.  If those don't do what you want,
> you have to write your own.

SOC_DOUBLE_S8_TLV seems to be a misnamed hardware-specific macro. It 
provides no way to specify the location of the left/right bits so the 
macro name is misleading.

SOC_SINGLE_XR_SX is too new for my kernel. And from what I gather, I 
cannot use it anyway for my purposes. So the solution turns out to be 
"write my own". The SOC_DOUBLE_S8_TLV macro did provide a very nice 
starting point though.

Thanks for the pointer though, it turned out to be the key to a solution.

>> (and on a side note "amixer" doesn't accept negative values either)
>
> A workaround would be to set the control value first to 0 and then to
> the desired negative value ... but this bug needs fixing.

I have no idea what you meant here.

Kind regards,
Mike.