ASoC: meson: axg-tdm-formatter: fix channel slot allocation

When the tdm lane mask is computed, the driver currently fills the 1st lane
before moving on to the next. If the stream has less channels than the
lanes can accommodate, slots will be disabled on the last lanes.

Unfortunately, the HW distribute channels in a different way. It distribute
channels in pair on each lanes before moving on the next slots.

This difference leads to problems if a device has an interface with more
than 1 lane and with more than 2 slots per lane.

For example: a playback interface with 2 lanes and 4 slots each (total 8
slots - zero based numbering)
- Playing a 8ch stream:
  - All slots activated by the driver
  - channel #2 will be played on lane #1 - slot #0 following HW placement
- Playing a 4ch stream:
  - Lane #1 disabled by the driver
  - channel #2 will be played on lane #0 - slot #2

This behaviour is obviously not desirable.

Change the way slots are activated on the TDM lanes to follow what the HW
does and make sure each channel always get mapped to the same slot/lane.

Fixes: 1a11d88f ("ASoC: meson: add tdm formatter base driver")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20230809171931.1244502-1-jbrunet@baylibre.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/meson/axg-tdm-formatter.c

@@ -30,27 +30,32 @@ int axg_tdm_formatter_set_channel_masks(struct regmap *map,
 					struct axg_tdm_stream *ts,
 					unsigned int offset)
 {
-	unsigned int val, ch = ts->channels;
-	unsigned long mask;
-	int i, j;
+	unsigned int ch = ts->channels;
+	u32 val[AXG_TDM_NUM_LANES];
+	int i, j, k;
+
+	/*
+	 * We need to mimick the slot distribution used by the HW to keep the
+	 * channel placement consistent regardless of the number of channel
+	 * in the stream. This is why the odd algorithm below is used.
+	 */
+	memset(val, 0, sizeof(*val) * AXG_TDM_NUM_LANES);
 
 	/*
 	 * Distribute the channels of the stream over the available slots
-	 * of each TDM lane
+	 * of each TDM lane. We need to go over the 32 slots ...
 	 */
-	for (i = 0; i < AXG_TDM_NUM_LANES; i++) {
-		val = 0;
-		mask = ts->mask[i];
-
-		for (j = find_first_bit(&mask, 32);
-		     (j < 32) && ch;
-		     j = find_next_bit(&mask, 32, j + 1)) {
-			val |= 1 << j;
-			ch -= 1;
+	for (i = 0; (i < 32) && ch; i += 2) {
+		/* ... of all the lanes ... */
+		for (j = 0; j < AXG_TDM_NUM_LANES; j++) {
+			/* ... then distribute the channels in pairs */
+			for (k = 0; k < 2; k++) {
+				if ((BIT(i + k) & ts->mask[j]) && ch) {
+					val[j] |= BIT(i + k);
+					ch -= 1;
+				}
+			}
 		}
-
-		regmap_write(map, offset, val);
-		offset += regmap_get_reg_stride(map);
 	}
 
 	/*
@@ -63,6 +68,11 @@ int axg_tdm_formatter_set_channel_masks(struct regmap *map,
 		return -EINVAL;
 	}
 
+	for (i = 0; i < AXG_TDM_NUM_LANES; i++) {
+		regmap_write(map, offset, val[i]);
+		offset += regmap_get_reg_stride(map);
+	}
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(axg_tdm_formatter_set_channel_masks);