spi: spi-mem: Mark dummy transfers by setting dummy_data bit

This patch marks dummy transfer by setting dummy_data bit to 1.

Controllers supporting dummy transfer by hardware use this bit field
to skip software transfer of dummy bytes and use hardware dummy bytes
transfer.

Signed-off-by: Sowjanya Komatineni <skomatineni@nvidia.com>
Link: https://lore.kernel.org/r/1608585459-17250-6-git-send-email-skomatineni@nvidia.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-mem.c

@@ -354,6 +354,7 @@ int spi_mem_exec_op(struct spi_mem *mem, const struct spi_mem_op *op)
 		xfers[xferpos].tx_buf = tmpbuf + op->addr.nbytes + 1;
 		xfers[xferpos].len = op->dummy.nbytes;
 		xfers[xferpos].tx_nbits = op->dummy.buswidth;
+		xfers[xferpos].dummy_data = 1;
 		spi_message_add_tail(&xfers[xferpos], &msg);
 		xferpos++;
 		totalxferlen += op->dummy.nbytes;

include/linux/spi/spi.h

@@ -825,6 +825,7 @@ extern void spi_res_release(struct spi_controller *ctlr,
  *      transfer. If 0 the default (from @spi_device) is used.
  * @bits_per_word: select a bits_per_word other than the device default
  *      for this transfer. If 0 the default (from @spi_device) is used.
+ * @dummy_data: indicates transfer is dummy bytes transfer.
  * @cs_change: affects chipselect after this transfer completes
  * @cs_change_delay: delay between cs deassert and assert when
  *      @cs_change is set and @spi_transfer is not the last in @spi_message
@@ -937,6 +938,7 @@ struct spi_transfer {
 	struct sg_table tx_sg;
 	struct sg_table rx_sg;
 
+	unsigned	dummy_data:1;
 	unsigned	cs_change:1;
 	unsigned	tx_nbits:3;
 	unsigned	rx_nbits:3;