dmaengine: fsl-edma: fix edma4 channel enable failure on second attempt

When attempting to start DMA for the second time using
fsl_edma3_enable_request(), channel never start.

CHn_MUX must have a unique value when selecting a peripheral slot in the
channel mux configuration. The only value that may overlap is source 0.
If there is an attempt to write a mux configuration value that is already
consumed by another channel, a mux configuration of 0 (SRC = 0) will be
written.

Check CHn_MUX before writing in fsl_edma3_enable_request().

Fixes: 72f5801 ("dmaengine: fsl-edma: integrate v3 support")
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Link: https://lore.kernel.org/r/20230823182635.2618118-1-Frank.Li@nxp.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/fsl-edma-common.c

@@ -92,8 +92,14 @@ static void fsl_edma3_enable_request(struct fsl_edma_chan *fsl_chan)
 
 	edma_writel_chreg(fsl_chan, val, ch_sbr);
 
-	if (flags & FSL_EDMA_DRV_HAS_CHMUX)
-		edma_writel_chreg(fsl_chan, fsl_chan->srcid, ch_mux);
+	if (flags & FSL_EDMA_DRV_HAS_CHMUX) {
+		/*
+		 * ch_mux: With the exception of 0, attempts to write a value
+		 * already in use will be forced to 0.
+		 */
+		if (!edma_readl_chreg(fsl_chan, ch_mux))
+			edma_writel_chreg(fsl_chan, fsl_chan->srcid, ch_mux);
+	}
 
 	val = edma_readl_chreg(fsl_chan, ch_csr);
 	val |= EDMA_V3_CH_CSR_ERQ;