Re: [PATCH] ASoC: soc-ops: Correct the max value for clamp in soc_mixer_reg_to_ctl()

[Charles Keepax <ckeepax@opensource.cirrus.com>]

On Wed, Dec 17, 2025 at 03:13:45PM +0200, Péter Ujfalusi wrote:
> On 17/12/2025 14:47, Charles Keepax wrote:
> > On Wed, Dec 17, 2025 at 02:06:23PM +0200, Peter Ujfalusi wrote:
> >>  sound/soc/soc-ops.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/sound/soc/soc-ops.c b/sound/soc/soc-ops.c
> >> index ce86978c158d..6a18c56a9746 100644
> >> --- a/sound/soc/soc-ops.c
> >> +++ b/sound/soc/soc-ops.c
> >> @@ -148,7 +148,7 @@ static int soc_mixer_reg_to_ctl(struct soc_mixer_control *mc, unsigned int reg_v
> >>  	if (mc->sign_bit)
> >>  		val = sign_extend32(val, mc->sign_bit);
> >>  
> >> -	val = clamp(val, mc->min, mc->max);
> >> +	val = clamp(val, mc->min, mc->min + max);
> > 
> > This won't work, for an SX control it is perfectly valid for
> > the value read from the register to be smaller than the minimum
> > value specified in the control.
> 
> Hrm, so an SX control returns sort of rand() and the value have no
> correlation to min or max?

lol, yes exactly :-) arn't they great

> The value can wrap at any random value to 0 and continue from 0 up to
> some value, which is the max?

Mostly correct, not any random value it wraps at the mask.

> How this is in practice for the cs42l43' Headphone Digital Volume?
> SOC_DOUBLE_SX_TLV("Headphone Digital Volume", CS42L43_HPPATHVOL,
> 	  CS42L43_AMP3_PATH_VOL_SHIFT, CS42L43_AMP4_PATH_VOL_SHIFT,
> 	  0x11B, 229, cs42l43_headphone_tlv),
> 
> min=283
> max=229
> shifts: 0 and 16
> masks are 0x1ff
> 
> if you step 229 from 283 then you reach 0x1ff, this is the max the mask
> can cover.

Not quite your maths is off by one, 229 + 283 = 512 = 0x200,
which is then &ed with the mask to get 0x0. Which on the cs42l43
headphones a value of 0x0->0dB. Stepping 1 back from that would
give you 0x1FF->-0.5dB.

> > I often think of it in terms of a 2's compliement number
> > with an implicit sign bit.
> 
> I see, but why???

Mostly because hardware people love to wind me up, I assume. But
more seriously, imagine an 4-bit signed number volume control
with 5 values:

0xE -> -2 -> -2dB
0xF -> -1 -> -1dB
0x0 ->  0 -> 0dB
0x1 ->  1 -> 1dB
0x2 ->  2 -> 2dB

Super, a very sensible control, but wait being a good hardware
engineer you realise you don't need 4 bits to represent 5 values
you can get away with 3 bits for that and save like 2 gates
resulting in an ice cream and a plaque from your manager. So
you drop the sign bit giving you:

0x6 -> -2dB
0x7 -> -1dB
0x0 -> 0dB
0x1 -> 1dB
0x2 -> 2dB

This then results in an SX control with a minimum of 0x6 and a
mask of 0x7.

Thanks,
Charles