Re: [RFC] dB range compound

[James Courtier-Dutton <James@superbug.co.uk>]

Takashi Iwai wrote:
> Hi,
> 
> I found that some hardwares require the combination of dB scale
> ranges.  For example, es1938 has volume controls with two curves,
> e.g. the volume step in the value 0-8 is 3 dB while it's 1.5 dB in the
> values 8-15.
> 
> The patch below introduces a new TLV type, DB_RANGE, to implement
> the compound dB scales.  It can contain one or more child TLVs,
> together with min and max values for each.
> 
> The first patch is for the kernel and the second for alsa-lib.
> 
> Any comments?
> 
> 
> Takashi
> 
+static int do_convert_to_dB(unsigned int *tlv, long rangemin, long
rangemax,
+			    long volume, long *db_gain)
+{
+	switch (tlv[0]) {
+	case SND_CTL_TLVT_DB_RANGE: {
+		unsigned int pos, len;
+		len = tlv[1] / sizeof(int);
+		pos = 2;

Be careful with this len limit in the while loop. I suggest adding a
further comparison to limit the loop to a constant size:
e.g.
while (pos + 4 <= len && (pos + 4 < MAX_BUFFER_SIZE) ) {
There are similar cases elsewhere in the patch.


+		while (pos + 4 <= len) {
+			rangemin = (int)tlv[pos];
+			rangemax = (int)tlv[pos + 1];
+			if (volume >= rangemin && volume <= rangemax)
+				return do_convert_to_dB(tlv + pos + 2,
+							rangemin, rangemax,
+							volume, db_gain);
+			pos += tlv[pos + 3] + 4;
+		}
 		return -EINVAL;


Apart from that, It looks OK to me.

With regard to this control actually being to separate register in the
chip, I think it might be nice to implement two separate controls
between kernel and alsa-lib, but add some TLV tag to link the two
controls together so that they appear as one. This will save code size
in the kernel.
For example, with the EMU DSPs one control can do 0 dB to -inf gain, and
another can to 0 dB to X gain. I was thinking of implementing these in
the emu10k1 module, with special code to link the two at the kernel
module level, but it might save code duplication if we let alsa-lib do
this task.
We might be able to do a similar link between mic gain and mic boost
switches.

James

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