ASoC: mxs-saif: Fix channel swap for 24-bit format

Playing 24-bit format file leads to channel swap on mx28 and the reason is that
the current driver performs one write/read to/from the SAIF_DATA register to
trigger the transfer.

This approach works fine for S16_LE case because SAIF_DATA is a 32-bit register
and thus is capable of storing the 16-bit left and right channels, but for the
S24_LE case it can only store one channel, so in order to not lose the FIFO sync
an extra read/write is needed.
Reported-by: Dan Winner <DWinner@tc-helicon.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Tested-by: Dan Winner <DWinner@tc-helicon.com>
Acked-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

sound/soc/mxs/mxs-saif.c

@@ -523,16 +523,24 @@ static int mxs_saif_trigger(struct snd_pcm_substream *substream, int cmd,
 
 		if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {
 			/*
-			 * write a data to saif data register to trigger
-			 * the transfer
+			 * write data to saif data register to trigger
+			 * the transfer.
+			 * For 24-bit format the 32-bit FIFO register stores
+			 * only one channel, so we need to write twice.
+			 * This is also safe for the other non 24-bit formats.
 			 */
 			__raw_writel(0, saif->base + SAIF_DATA);
+			__raw_writel(0, saif->base + SAIF_DATA);
 		} else {
 			/*
-			 * read a data from saif data register to trigger
-			 * the receive
+			 * read data from saif data register to trigger
+			 * the receive.
+			 * For 24-bit format the 32-bit FIFO register stores
+			 * only one channel, so we need to read twice.
+			 * This is also safe for the other non 24-bit formats.
 			 */
 			__raw_readl(saif->base + SAIF_DATA);
+			__raw_readl(saif->base + SAIF_DATA);
 		}
 
 		master_saif->ongoing = 1;