ASoC: fsl_ssi: Fix channel slipping in Playback at startup

Previously, SCR.SSIEN and SCR.TE were enabled at once if no capture
stream was also running.
This may not give a chance for the DMA to write the first sample in
TX FIFO before the streaming starts on the PCM bus, inserting void
samples first.
Those void samples are then responsible for slipping the channels.

Signed-off-by: Arnaud Mouiche <arnaud.mouiche@invoxia.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Caleb Crome <caleb@crome.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/fsl/fsl_ssi.c

@@ -507,8 +507,40 @@ static void fsl_ssi_config(struct fsl_ssi_private *ssi_private, bool enable,
 
 config_done:
 	/* Enabling of subunits is done after configuration */
-	if (enable)
+	if (enable) {
+		if (ssi_private->use_dma && (vals->scr & CCSR_SSI_SCR_TE)) {
+			/*
+			 * Be sure the Tx FIFO is filled when TE is set.
+			 * Otherwise, there are some chances to start the
+			 * playback with some void samples inserted first,
+			 * generating a channel slip.
+			 *
+			 * First, SSIEN must be set, to let the FIFO be filled.
+			 *
+			 * Notes:
+			 * - Limit this fix to the DMA case until FIQ cases can
+			 *   be tested.
+			 * - Limit the length of the busy loop to not lock the
+			 *   system too long, even if 1-2 loops are sufficient
+			 *   in general.
+			 */
+			int i;
+			int max_loop = 100;
+			regmap_update_bits(regs, CCSR_SSI_SCR,
+					CCSR_SSI_SCR_SSIEN, CCSR_SSI_SCR_SSIEN);
+			for (i = 0; i < max_loop; i++) {
+				u32 sfcsr;
+				regmap_read(regs, CCSR_SSI_SFCSR, &sfcsr);
+				if (CCSR_SSI_SFCSR_TFCNT0(sfcsr))
+					break;
+			}
+			if (i == max_loop) {
+				dev_err(ssi_private->dev,
+					"Timeout waiting TX FIFO filling\n");
+			}
+		}
 		regmap_update_bits(regs, CCSR_SSI_SCR, vals->scr, vals->scr);
+	}
 }