mxc: mx1/mx2 DMA: add a possibility to create an endless DMA transfer

This is useful for audio where we do not want to setup a new scatterlist
after playing 4GB of audio data. This would cause skips in the playback.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

arch/arm/plat-mxc/dma-mx1-mx2.c

@@ -156,7 +156,8 @@ static inline int imx_dma_sg_next(int channel, struct scatterlist *sg)
 	}
 
 	now = min(imxdma->resbytes, sg->length);
-	imxdma->resbytes -= now;
+	if (imxdma->resbytes != IMX_DMA_LENGTH_LOOP)
+		imxdma->resbytes -= now;
 
 	if ((imxdma->dma_mode & DMA_MODE_MASK) == DMA_MODE_READ)
 		__raw_writel(sg->dma_address, DMA_BASE + DMA_DAR(channel));

arch/arm/plat-mxc/include/mach/dma-mx1-mx2.h

@@ -58,6 +58,14 @@ imx_dma_setup_single(int channel, dma_addr_t dma_address,
 		unsigned int dma_length, unsigned int dev_addr,
 		unsigned int dmamode);
 
+
+/*
+ * Use this flag as the dma_length argument to imx_dma_setup_sg()
+ * to create an endless running dma loop. The end of the scatterlist
+ * must be linked to the beginning for this to work.
+ */
+#define IMX_DMA_LENGTH_LOOP	((unsigned int)-1)
+
 int
 imx_dma_setup_sg(int channel, struct scatterlist *sg,
 		unsigned int sgcount, unsigned int dma_length,