spi: Add support for preamble bytes

A SPI slave may take time to react to a request. For SPI flash devices
this time is defined as one bit time, or a whole byte for 'fast read'
mode.

If the SPI slave is another CPU, then the time it takes to react may
vary. It is convenient to allow the slave device to tag the start of
the actual reply so that the host can determine when this 'preamble'
finishes and the actual message starts.

Add a preamble flag to the available SPI flags. If supported by the
driver then it will ignore any received bytes before the preamble
on each transaction. This ensures that reliable communication with
the slave is possible.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rajeshwari Shinde <rajeshwari.s@samsung.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>

include/spi.h

@@ -37,11 +37,16 @@
 #define	SPI_LSB_FIRST	0x08			/* per-word bits-on-wire */
 #define	SPI_3WIRE	0x10			/* SI/SO signals shared */
 #define	SPI_LOOP	0x20			/* loopback mode */
+#define	SPI_SLAVE	0x40			/* slave mode */
+#define	SPI_PREAMBLE	0x80			/* Skip preamble bytes */
 
 /* SPI transfer flags */
 #define SPI_XFER_BEGIN	0x01			/* Assert CS before transfer */
 #define SPI_XFER_END	0x02			/* Deassert CS after transfer */
 
+/* Header byte that marks the start of the message */
+#define SPI_PREAMBLE_END_BYTE	0xec
+
 /*-----------------------------------------------------------------------
  * Representation of a SPI slave, i.e. what we're communicating with.
  *