[U-Boot] [PATCH] spi: soft_spi: Support NULL din/dout buffers

[Andrew Ruder <andrew.ruder@elecsyscorp.com>]

This mirrors the conventions used in other SPI drivers (kirkwood,
davinci, atmel, et al) where the din/dout buffer can be NULL when the
received/transmitted data isn't important.  This reduces the need for
allocating additional buffers when write-only/read-only functionality is
needed.

In the din == NULL case, the received data is simply not stored.  In the
dout == NULL case, zeroes are transmitted.

Signed-off-by: Andrew Ruder <andrew.ruder@elecsyscorp.com>
---
 drivers/spi/soft_spi.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/drivers/spi/soft_spi.c b/drivers/spi/soft_spi.c
index 5d22351..5fdd091 100644
--- a/drivers/spi/soft_spi.c
+++ b/drivers/spi/soft_spi.c
@@ -137,9 +137,15 @@ int  spi_xfer(struct spi_slave *slave, unsigned int bitlen,
 		 * Check if it is time to work on a new byte.
 		 */
 		if((j % 8) == 0) {
-			tmpdout = *txd++;
+			if (txd) {
+				tmpdout = *txd++;
+			} else {
+				tmpdout = 0;
+			}
 			if(j != 0) {
-				*rxd++ = tmpdin;
+				if (rxd) {
+					*rxd++ = tmpdin;
+				}
 			}
 			tmpdin  = 0;
 		}
@@ -164,9 +170,11 @@ int  spi_xfer(struct spi_slave *slave, unsigned int bitlen,
 	 * bits over to left-justify them.  Then store the last byte
 	 * read in.
 	 */
-	if((bitlen % 8) != 0)
-		tmpdin <<= 8 - (bitlen % 8);
-	*rxd++ = tmpdin;
+	if (rxd) {
+		if((bitlen % 8) != 0)
+			tmpdin <<= 8 - (bitlen % 8);
+		*rxd++ = tmpdin;
+	}
 
 	if (flags & SPI_XFER_END)
 		spi_cs_deactivate(slave);
-- 
1.8.4.rc3