spi: soft_spi: Support NULL din/dout buffers

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 <[email protected]>

---
 drivers/spi/soft_spi.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

drivers/spi/soft_spi.c +8 -4

index 5d22351..5fdd091 100644
@@ -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);