SPI: add dma_alignment field to spi_master

Some SPI controllers have restrictions on DMAable buffers alignemt.
Currently if the buffer supplied by protocol driver is not properly
aligned, the controller silently performs transfer in PIO mode.  Addition
of dma_alignment field to spi_master allows protocol drivers to perform
proper alignment.

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Cc: Bryan Wu <bryan.wu@analog.com>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/spi/spi.h

@@ -204,6 +204,7 @@ static inline void spi_unregister_driver(struct spi_driver *sdrv)
  *	SPI slaves, and are numbered from zero to num_chipselects.
  *	each slave has a chipselect signal, but it's common that not
  *	every chipselect is connected to a slave.
+ * @dma_alignment: SPI controller constraint on DMA buffers alignment.
  * @setup: updates the device mode and clocking records used by a
  *	device's SPI controller; protocol code may call this.  This
  *	must fail if an unrecognized or unsupported mode is requested.
@@ -239,6 +240,11 @@ struct spi_master {
 	 */
 	u16			num_chipselect;
 
+	/* some SPI controllers pose alignment requirements on DMAable
+	 * buffers; let protocol drivers know about these requirements.
+	 */
+	u16			dma_alignment;
+
 	/* setup mode and clock, etc (spi driver may call many times) */
 	int			(*setup)(struct spi_device *spi);