ASoC: SDCA: Populate regmap cache for readable Controls

It is not uncommon for an SDCA Control to have no specified default
value in the DisCo. Non-volatile registers with no defaults will not be
present in the cache until they are accessed. However, if the first
operation user-space performs is a read whilst the device is runtime
suspended this read will fail.

To avoid such problems we should populate values from the hardware into
the cache for all non-volatile readable registers with no defaults.
Update the defaults handling to do this cache population since it is
iterating over the Controls and happens at a time the hardware is
always powered up.

Tested-by: Bard Liao <[e-mail]>
Reviewed-by: Maciej Strozek <[e-mail]>
Reviewed-by: Peter Ujfalusi <[e-mail]>
Tested-by: Richard Fitzgerald <[e-mail]>
Signed-off-by: Charles Keepax <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Reviewed-by: Vinod Koul <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

sound/soc/sdca/sdca_regmap.c

···
 	if (control->mode == SDCA_ACCESS_MODE_DC)
 		return 0;
 
-	if (!control->has_default && !control->has_fixed)
+	if (control->layers & SDCA_ACCESS_LAYER_DEVICE)
 		return 0;
 
 	i = 0;
 	for_each_set_bit(cn, (unsigned long *)&control->cn_list,
 			 BITS_PER_TYPE(control->cn_list)) {
-		unsigned int reg;
+		unsigned int reg, val;
 
 		reg = SDW_SDCA_CTL(function->desc->adr, entity->id, control->sel, cn);
 
-		ret = regmap_write(regmap, reg, control->values[i]);
-		if (ret) {
-			dev_err(dev, "Failed to write default %#x: %d\n",
-				reg, ret);
-			return ret;
-		}
+		if (control->has_default || control->has_fixed) {
+			ret = regmap_write(regmap, reg, control->values[i]);
+			if (ret) {
+				dev_err(dev, "Failed to write default %#x: %d\n",
+					reg, ret);
+				return ret;
+			}
 
-		i++;
+			i++;
+		} else if (!control->is_volatile) {
+			ret = regmap_read(regmap, reg, &val);
+			if (ret) {
+				dev_err(dev, "Failed to read initial %#x: %d\n",
+					reg, ret);
+				return ret;
+			}
+		}
 	}
 
 	return 0;
···
  *
  * This function will write out to the hardware all the DisCo default and
  * fixed value controls. This will cause them to be populated into the cache,
- * and subsequent handling can be done through a cache sync.
+ * and subsequent handling can be done through a cache sync. It will also
+ * read any non-volatile registers that don't have defaults/fixed values to
+ * populate those into the cache, this ensures they are available for reads
+ * even when the device is runtime suspended.
  *
  * Return: Returns zero on success, and a negative error code on failure.
  */