[alsa-devel] [PATCH - alsa-lib] tlv: Check dB range only within the control's volume range

[Peter Ujfalusi]

On Wednesday 19 May 2010 09:55:40 ext Jaroslav Kysela wrote:
> On Wed, 19 May 2010, Peter Ujfalusi wrote:
> > The DB_RANGE need to be used on some HW, since the gain on
> > volume control is not continuous, and has to be divided into
> > several sub DB_SCALE ranges.
> > ASoC has a feature to override the HW default volume range,
> > and in this case when the volume range is less than the
> > HW maximum we do not need to go through the whole DB_RANGE,
> > but we need to stop where the kcontrol's maximum tell us.
> > 
> > Signed-off-by: Peter Ujfalusi <peter.ujfalusi at nokia.com>
> 
> Unfortunately, it does not look like a clean way in my eyes. If the driver
> pushes some limits to the control (volume) range, it should do it also
> for TLVs.

My thinking was that the snd_tlv_get_dB_range SNDRV_CTL_TLVT_DB_RANGE case 
should obey the control's range provided by the driver, similarly how the 
SNDRV_CTL_TLVT_DB_SCALE is using the rangemax, rangemin.
In case of SNDRV_CTL_TLVT_DB_RANGE, we should not go beyond the control's 
rangemax.

In this way both of the following volume control is going to be handled 
correctly:

1.
static DECLARE_TLV_DB_SCALE(dac_digivol_tlv, -6350, 50, 0);

static const struct snd_kcontrol_new dac33_snd_controls[] = {
	SOC_DOUBLE_R_TLV("DAC Digital Playback Volume",
		DAC33_LDAC_DIG_VOL_CTRL, DAC33_RDAC_DIG_VOL_CTRL,
		0, 0x7f, 1, dac_digivol_tlv),
};

Original range (127): -63.5 .. 0 dB
Limited range  (100): -63.5 .. -13.5 dB

2.
static const unsigned int tpa6140_tlv[] = {
	TLV_DB_RANGE_HEAD(3),
	0, 8, TLV_DB_SCALE_ITEM(-5900, 400, 0),
	9, 16, TLV_DB_SCALE_ITEM(-2500, 200, 0),
	17, 31, TLV_DB_SCALE_ITEM(-1000, 100, 0),
};

static const struct snd_kcontrol_new tpa6140a2_controls[] = {
	SOC_SINGLE_EXT_TLV("TPA6140A2 Headphone Playback Volume",
		       TPA6130A2_REG_VOL_MUTE, 1, 0x1f, 0,
		       tpa6130a2_get_volsw, tpa6130a2_put_volsw,
		       tpa6140_tlv),
};

Original range (31): -59 .. 4 dB
Limited range  (21): -59 .. -6 dB
Limited range  (16): -59 .. -11 dB

In case of DB_SCALE in theory the maximum gain depends on the maximum volume 
possible on the control.
When the gain is not linear, it has to divided into sub DB_SCALE sections to get 
accurate mapping.
Now, if the kernel code for the limiting has to do more than changing the 
maximum volume, than it means that it _has_ to rewrite the TLV table in a clever 
way, or provide a new one which fits with the limited volume.
Rewriting the TLV table taking into account all possible cases is not that safe, 
and I'm sure there could be still some corners, where it would fail.

I think the patch provides uniform, and error prone way of handling this 
situation, while not breaking any existing parts.

-- 
Péter