[alsa-devel] Mixer control for signed gain value

Patrick Lai plai at codeaurora.org
Tue Jun 28 18:21:31 CEST 2011

On 6/23/2011 3:37 AM, Mark Brown wrote:
> On Wed, Jun 22, 2011 at 11:32:19PM -0700, Patrick Lai wrote:
>> The CODEC I am working with takes byte-long gain value in two's
>> complement form.
>> In soc-core.c, the closest functions I see are
>>   snd_soc_info_volsw_s8
>> snd_soc_get_volsw_s8
>> snd_soc_set_volsw_s8
>> These functions are coded to work with two integer values. For my
>> purpose, I suppose that I can always set 2nd integer input as zero.
>> However, would I still be able to set volume like
>> amixer sset "Codec Gain" 100 as percentage. If yes, how will amixer
>> know to put upper 8 bits of 16 bit gain in 2nd integer value element?
> Could you clarify what the control you're dealing with is?  You say
> above that your gains are byte long but here you're talking about them
> actually being 16 bit.  If they're 16 bit you probably need to write a
> new control type.

Yes, CODEC in question takes byte-long signed gain value and would like
to reuse snd_soc_xxx_volsw_xx mixer callback functions in soc-core.c.
The only functions I see handle signed value are
snd_soc_info_volsw_s8, snd_soc_get_volsw_s8, snd_soc_get_volsw_s8
Unless I am mistaken, these functions deal with two signed byte-long
values which get concatenated into 16-bit value. Perhaps, original
purpose is to set left and right 8-bit gain values at the same time on
a single codec register which takes 16-bit value.
So, my question is whether I should reuse volsw_s8 mixer control
functions and have mixer application ignore integer.value[1] to suit my
purpose or redefine new set of mixer control functions i.e
snd_soc_set_volsw_single_s8. Otherwise, is there existing alternative
to handle set/put byte-long signed value?


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