Input: ads7846 - make transfer buffers DMA safe

req.sample needs its own cacheline otherwise accessing req.msg fetches
it in again.

Note: This effect doesn't occur if the underlying SPI driver doesn't use
DMA at all.
Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

drivers/input/touchscreen/ads7846.c

@@ -281,17 +281,25 @@ struct ser_req {
 	u8			command;
 	u8			ref_off;
 	u16			scratch;
-	__be16			sample;
 	struct spi_message	msg;
 	struct spi_transfer	xfer[6];
+	/*
+	 * DMA (thus cache coherency maintenance) requires the
+	 * transfer buffers to live in their own cache lines.
+	 */
+	__be16 sample ____cacheline_aligned;
 };
 
 struct ads7845_ser_req {
 	u8			command[3];
 	u8			pwrdown[3];
-	u8			sample[3];
 	struct spi_message	msg;
 	struct spi_transfer	xfer[2];
+	/*
+	 * DMA (thus cache coherency maintenance) requires the
+	 * transfer buffers to live in their own cache lines.
+	 */
+	u8 sample[3] ____cacheline_aligned;
 };
 
 static int ads7846_read12_ser(struct device *dev, unsigned command)