SPI: add 3wire mode flag

Add a new spi->mode bit: SPI_3WIRE, for chips where the SI and SO signals
are shared (and which are thus only half duplex). Update the LM70 driver
to require support for that hardware mode from the controller.

Signed-off-by: David Brownell <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/hwmon/lm70.c

···
 	struct lm70 *p_lm70;
 	int status;
 
+	/* signaling is SPI_MODE_0 on a 3-wire link (shared SI/SO) */
+	if ((spi->mode & (SPI_CPOL|SPI_CPHA)) || !(spi->mode & SPI_3WIRE))
+		return -EINVAL;
+
 	p_lm70 = kzalloc(sizeof *p_lm70, GFP_KERNEL);
 	if (!p_lm70)
 		return -ENOMEM;

include/linux/spi/spi.h

···
 #define	SPI_MODE_3	(SPI_CPOL|SPI_CPHA)
 #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 */
 	u8			bits_per_word;
 	int			irq;
 	void			*controller_state;