dmaengine: omap-dma: move barrier to omap_dma_start_desc()

We don't need to issue a barrier for every segment of a DMA transfer;
doing this just once per descriptor will do.
Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

drivers/dma/omap-dma.c

@@ -195,7 +195,6 @@ static void omap_dma_start(struct omap_chan *c, struct omap_desc *d)
 
 	val = c->plat->dma_read(CCR, c->dma_ch);
 	val |= CCR_ENABLE;
-	mb();
 	c->plat->dma_write(val, CCR, c->dma_ch);
 }
 
@@ -301,6 +300,13 @@ static void omap_dma_start_desc(struct omap_chan *c)
 	c->desc = d = to_omap_dma_desc(&vd->tx);
 	c->sgidx = 0;
 
+	/*
+	 * This provides the necessary barrier to ensure data held in
+	 * DMA coherent memory is visible to the DMA engine prior to
+	 * the transfer starting.
+	 */
+	mb();
+
 	c->plat->dma_write(d->ccr, CCR, c->dma_ch);
 	if (dma_omap1())
 		c->plat->dma_write(d->ccr >> 16, CCR2, c->dma_ch);