SPI loopback mode definition

Loopback mode is supported by various controllers.  This mode can be
useful for testing, especially in conjunction with spidev driver.
Signed-off-by: NAnton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/linux/spi/spi.h

@@ -77,6 +77,7 @@ struct spi_device {
 #define	SPI_CS_HIGH	0x04			/* chipselect active high? */
 #define	SPI_LSB_FIRST	0x08			/* per-word bits-on-wire */
 #define	SPI_3WIRE	0x10			/* SI/SO signals shared */
+#define	SPI_LOOP	0x20			/* loopback mode */
 	u8			bits_per_word;
 	int			irq;
 	void			*controller_state;