ASoC: Handle read failures in codec_reg

When a device is powered down volatile registers can't be read so
attempts to display codec_reg will show error values, and obviously
it is also possible for there to be hardware errors too. Check for
errors from reads and display them more clearly when formatting
codec_reg.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>

sound/soc/soc-core.c

@@ -84,7 +84,7 @@ static int run_delayed_work(struct delayed_work *dwork)
 /* codec register dump */
 static ssize_t soc_codec_reg_show(struct snd_soc_codec *codec, char *buf)
 {
-	int i, step = 1, count = 0;
+	int ret, i, step = 1, count = 0;
 
 	if (!codec->reg_cache_size)
 		return 0;
@@ -101,12 +101,24 @@ static ssize_t soc_codec_reg_show(struct snd_soc_codec *codec, char *buf)
 		if (count >= PAGE_SIZE - 1)
 			break;
 
-		if (codec->display_register)
+		if (codec->display_register) {
 			count += codec->display_register(codec, buf + count,
 							 PAGE_SIZE - count, i);
-		else
-			count += snprintf(buf + count, PAGE_SIZE - count,
-					  "%4x", codec->read(codec, i));
+		} else {
+			/* If the read fails it's almost certainly due to
+			 * the register being volatile and the device being
+			 * powered off.
+			 */
+			ret = codec->read(codec, i);
+			if (ret >= 0)
+				count += snprintf(buf + count,
+						  PAGE_SIZE - count,
+						  "%4x", ret);
+			else
+				count += snprintf(buf + count,
+						  PAGE_SIZE - count,
+						  "<no data: %d>", ret);
+		}
 
 		if (count >= PAGE_SIZE - 1)
 			break;