Sound/ARM/i.MX/SSI: fix SDMA starvation

[Juergen Beisert]

In cases when capturing is already running and someone enables also
playback, the SDMA unit of the i.MX SoC does not see an edge on its TX DMA
request line. This is due to the TX FIFOs are empty, and this request line is
active all the time, when the CCSR_SSI_SIER_TDMAE bit is set.
In this case also the TX FIFO underrun interrupt will flood the system forever.

To ensure the first edge happens after enabling the TX side of the SSI unit and
to trigger the SDMA unit successfully enable the CCSR_SSI_SIER_TDMAE on
demand only. To be sure, this patch does the same for the CCSR_SSI_SIER_RDMAE
(RX side) bit.

Signed-off-by: Juergen Beisert <jbe at pengutronix.de>

---
 sound/soc/fsl/fsl_ssi.c |   24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

Index: linux.git/sound/soc/fsl/fsl_ssi.c
===================================================================
--- linux.git.orig/sound/soc/fsl/fsl_ssi.c
+++ linux.git/sound/soc/fsl/fsl_ssi.c
@@ -86,10 +86,9 @@ static inline void write_ssi_mask(u32 __
 #endif
 
 /* SIER bitflag of interrupts to enable */
-#define SIER_FLAGS (CCSR_SSI_SIER_TFRC_EN | CCSR_SSI_SIER_TDMAE | \
-		    CCSR_SSI_SIER_TIE | CCSR_SSI_SIER_TUE0_EN | \
-		    CCSR_SSI_SIER_TUE1_EN | CCSR_SSI_SIER_RFRC_EN | \
-		    CCSR_SSI_SIER_RDMAE | CCSR_SSI_SIER_RIE | \
+#define SIER_FLAGS (CCSR_SSI_SIER_TFRC_EN | CCSR_SSI_SIER_TIE | \
+		    CCSR_SSI_SIER_TUE0_EN | CCSR_SSI_SIER_TUE1_EN | \
+		    CCSR_SSI_SIER_RFRC_EN | CCSR_SSI_SIER_RIE | \
 		    CCSR_SSI_SIER_ROE0_EN | CCSR_SSI_SIER_ROE1_EN)
 
 /**
@@ -489,6 +488,9 @@ static int fsl_ssi_hw_params(struct snd_
  *
  * The DMA channel is in external master start and pause mode, which
  * means the SSI completely controls the flow of data.
+ *
+ * To use the i.MX's SDMA unit it is important to create an edge on the
+ * corresponding request line. Otherwise the request will be ignored!
  */
 static int fsl_ssi_trigger(struct snd_pcm_substream *substream, int cmd,
 			   struct snd_soc_dai *dai)
@@ -500,20 +502,26 @@ static int fsl_ssi_trigger(struct snd_pc
 	switch (cmd) {
 	case SNDRV_PCM_TRIGGER_START:
 	case SNDRV_PCM_TRIGGER_PAUSE_RELEASE:
-		if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
+		if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {
 			write_ssi_mask(&ssi->scr, 0,
 				CCSR_SSI_SCR_SSIEN | CCSR_SSI_SCR_TE);
-		else
+			write_ssi_mask(&ssi->sier, 0, CCSR_SSI_SIER_TDMAE);
+		} else {
 			write_ssi_mask(&ssi->scr, 0,
 				CCSR_SSI_SCR_SSIEN | CCSR_SSI_SCR_RE);
+			write_ssi_mask(&ssi->sier, 0, CCSR_SSI_SIER_RDMAE);
+		}
 		break;
 
 	case SNDRV_PCM_TRIGGER_STOP:
 	case SNDRV_PCM_TRIGGER_PAUSE_PUSH:
-		if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
+		if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {
 			write_ssi_mask(&ssi->scr, CCSR_SSI_SCR_TE, 0);
-		else
+			write_ssi_mask(&ssi->sier, CCSR_SSI_SIER_TDMAE, 0);
+		} else {
 			write_ssi_mask(&ssi->scr, CCSR_SSI_SCR_RE, 0);
+			write_ssi_mask(&ssi->sier, CCSR_SSI_SIER_RDMAE, 0);
+		}
 		break;
 
 	default:

-- 
Pengutronix e.K.                              | Juergen Beisert             |
Linux Solutions for Science and Industry      | http://www.pengutronix.de/  |