[PATCH 2/3] ASoC: apple: mca: Fix SERDES reset sequence

["Martin Povišer" <povik+lin@cutebit.org>]

Fix the reset sequence of reads and writes that we invoke from within
the early trigger. It looks like there never was a SERDES_CONF_SOME_RST
bit that should be involved in the reset sequence, and its presence in
the driver code is a mistake from earlier.

Instead, the reset sequence should go as follows: We should switch the
the SERDES unit's SYNC_SEL mux to the value of 7 (so outside the range
of 1...6 representing cluster's SYNCGEN units), then raise the RST bit
in SERDES_STATUS and wait for it to clear.

Properly resetting the SERDES unit fixes frame desynchronization hazard
in case of long frames (longer than 4 used slots). The desynchronization
manifests itself by rotating the PCM channels.

Fixes: 3df5d0d97289 ("ASoC: apple: mca: Start new platform driver")
Signed-off-by: Martin Povišer <povik+lin@cutebit.org>
---
 sound/soc/apple/mca.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/sound/soc/apple/mca.c b/sound/soc/apple/mca.c
index 9cceeb259952..aea08c7b2ee8 100644
--- a/sound/soc/apple/mca.c
+++ b/sound/soc/apple/mca.c
@@ -101,7 +101,6 @@
 #define SERDES_CONF_UNK3	BIT(14)
 #define SERDES_CONF_NO_DATA_FEEDBACK	BIT(15)
 #define SERDES_CONF_SYNC_SEL	GENMASK(18, 16)
-#define SERDES_CONF_SOME_RST	BIT(19)
 #define REG_TX_SERDES_BITSTART	0x08
 #define REG_RX_SERDES_BITSTART	0x0c
 #define REG_TX_SERDES_SLOTMASK	0x0c
@@ -203,15 +202,24 @@ static void mca_fe_early_trigger(struct snd_pcm_substream *substream, int cmd,
 	case SNDRV_PCM_TRIGGER_START:
 	case SNDRV_PCM_TRIGGER_RESUME:
 	case SNDRV_PCM_TRIGGER_PAUSE_RELEASE:
+		mca_modify(cl, serdes_conf, SERDES_CONF_SYNC_SEL,
+			   FIELD_PREP(SERDES_CONF_SYNC_SEL, 0));
+		mca_modify(cl, serdes_conf, SERDES_CONF_SYNC_SEL,
+			   FIELD_PREP(SERDES_CONF_SYNC_SEL, 7));
 		mca_modify(cl, serdes_unit + REG_SERDES_STATUS,
 			   SERDES_STATUS_EN | SERDES_STATUS_RST,
 			   SERDES_STATUS_RST);
-		mca_modify(cl, serdes_conf, SERDES_CONF_SOME_RST,
-			   SERDES_CONF_SOME_RST);
-		readl_relaxed(cl->base + serdes_conf);
-		mca_modify(cl, serdes_conf, SERDES_STATUS_RST, 0);
+		/*
+		 * Experiments suggest that it takes at most ~1 us
+		 * for the bit to clear, so wait 2 us for good measure.
+		 */
+		udelay(2);
 		WARN_ON(readl_relaxed(cl->base + serdes_unit + REG_SERDES_STATUS) &
 			SERDES_STATUS_RST);
+		mca_modify(cl, serdes_conf, SERDES_CONF_SYNC_SEL,
+			   FIELD_PREP(SERDES_CONF_SYNC_SEL, 0));
+		mca_modify(cl, serdes_conf, SERDES_CONF_SYNC_SEL,
+			   FIELD_PREP(SERDES_CONF_SYNC_SEL, cl->no + 1));
 		break;
 	default:
 		break;
-- 
2.33.0