dmatest: properly handle duplicate DMA channels

Update the the dmatest driver so that it handles duplicate DMA channels
properly.

When a DMA client is notified of an available DMA channel, it must check if it
has already allocated resources for that channel.  If so, it should return
DMA_DUP.  This can happen, for example, if a DMA driver calls
dma_async_device_register() more than once.
Acked-by: NHaavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: NTimur Tabi <timur@freescale.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

drivers/dma/dmatest.c

@@ -325,6 +325,11 @@ static enum dma_state_client dmatest_add_channel(struct dma_chan *chan)
 	struct dmatest_thread	*thread;
 	unsigned int		i;
 
+	/* Have we already been told about this channel? */
+	list_for_each_entry(dtc, &dmatest_channels, node)
+		if (dtc->chan == chan)
+			return DMA_DUP;
+
 	dtc = kmalloc(sizeof(struct dmatest_chan), GFP_ATOMIC);
 	if (!dtc) {
 		pr_warning("dmatest: No memory for %s\n", chan->dev.bus_id);