[PATCH v3 5/9] spi: spi-mem: Allow masters to transfer dummy cycles directly by hardware

[Sowjanya Komatineni <skomatineni@xxxxxxxxxx>]

This patch adds a flag SPI_MASTER_USES_HW_DUMMY_CYCLES for the controllers
that support transfer of dummy cycles by the hardware directly.

For controller with this flag set, spi-mem driver will skip dummy bytes
transfer in the spi message.

Controller drivers can get the number of dummy cycles from spi_message.

Signed-off-by: Sowjanya Komatineni <skomatineni@xxxxxxxxxx>
---
 drivers/spi/spi-mem.c   | 18 +++++++++++-------
 include/linux/spi/spi.h |  8 ++++++++
 2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/drivers/spi/spi-mem.c b/drivers/spi/spi-mem.c
index f3a3f19..38a523b 100644
--- a/drivers/spi/spi-mem.c
+++ b/drivers/spi/spi-mem.c
@@ -350,13 +350,17 @@ int spi_mem_exec_op(struct spi_mem *mem, const struct spi_mem_op *op)
 	}
 
 	if (op->dummy.nbytes) {
-		memset(tmpbuf + op->addr.nbytes + 1, 0xff, op->dummy.nbytes);
-		xfers[xferpos].tx_buf = tmpbuf + op->addr.nbytes + 1;
-		xfers[xferpos].len = op->dummy.nbytes;
-		xfers[xferpos].tx_nbits = op->dummy.buswidth;
-		spi_message_add_tail(&xfers[xferpos], &msg);
-		xferpos++;
-		totalxferlen += op->dummy.nbytes;
+		if (ctlr->flags & SPI_MASTER_USES_HW_DUMMY_CYCLES) {
+			msg.dummy_cycles = (op->dummy.nbytes * 8) / op->dummy.buswidth;
+		} else {
+			memset(tmpbuf + op->addr.nbytes + 1, 0xff, op->dummy.nbytes);
+			xfers[xferpos].tx_buf = tmpbuf + op->addr.nbytes + 1;
+			xfers[xferpos].len = op->dummy.nbytes;
+			xfers[xferpos].tx_nbits = op->dummy.buswidth;
+			spi_message_add_tail(&xfers[xferpos], &msg);
+			xferpos++;
+			totalxferlen += op->dummy.nbytes;
+		}
 	}
 
 	if (op->data.nbytes) {
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index aa09fdc..2024149 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -512,6 +512,8 @@ struct spi_controller {
 
 #define SPI_MASTER_GPIO_SS		BIT(5)	/* GPIO CS must select slave */
 
+#define SPI_MASTER_USES_HW_DUMMY_CYCLES	BIT(6)	/* HW dummy bytes transfer */
+
 	/* flag indicating this is an SPI slave controller */
 	bool			slave;
 
@@ -1022,6 +1024,12 @@ struct spi_message {
 	unsigned		actual_length;
 	int			status;
 
+	/*
+	 * dummy cycles in the message transfer. This is used by the controller
+	 * drivers supports transfer of dummy cycles directly by the hardware.
+	 */
+	u8			dummy_cycles;
+
 	/* for optional use by whatever driver currently owns the
 	 * spi_message ...  between calls to spi_async and then later
 	 * complete(), that's the spi_controller controller driver.
-- 
2.7.4