dmaengine: Add support for per channel coherency handling

If the DMA device supports per channel coherency configuration (a channel
can be configured to have coherent or not coherent view) then a single
device (the DMA controller's device) can not be used for dma_api for all
channels as channels can have different coherency.

Introduce custom_dma_mapping flag for the dma_chan and a new helper to get
the device pointer to be used for dma_api for the given channel.

Client drivers should be updated to be able to support per channel
coherency by:

- dma_map_single(chan->device->dev, ptr, size, DMA_TO_DEVICE);
+ struct device *dma_dev = dmaengine_get_dma_device(chan);
+
+ dma_map_single(dma_dev, ptr, size, DMA_TO_DEVICE);

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20201208090440.31792-9-peter.ujfalusi@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

include/linux/dmaengine.h

@@ -357,11 +357,14 @@ struct dma_chan {
  * @chan: driver channel device
  * @device: sysfs device
  * @dev_id: parent dma_device dev_id
+ * @chan_dma_dev: The channel is using custom/different dma-mapping
+ * compared to the parent dma_device
  */
 struct dma_chan_dev {
 	struct dma_chan *chan;
 	struct device device;
 	int dev_id;
+	bool chan_dma_dev;
 };
 
 /**
@@ -1618,4 +1621,13 @@ dmaengine_get_direction_text(enum dma_transfer_direction dir)
 		return "invalid";
 	}
 }
+
+static inline struct device *dmaengine_get_dma_device(struct dma_chan *chan)
+{
+	if (chan->dev->chan_dma_dev)
+		return &chan->dev->device;
+
+	return chan->device->dev;
+}
+
 #endif /* DMAENGINE_H */