[PATCH v2 1/2] spi: Add optional number of additional clock cycles to be generated

[Christophe Leroy <christophe.leroy@xxxxxxxxxx>]

Some components require a few clock cycles with chipselect off before
or/and after the data transfer done with CS on.

Typically IDT 801034 QUAD PCM CODEC datasheet states "Note *: CCLK
should have one cycle before CS goes low, and two cycles after
CS goes high".

The cycles "before" are implicitely provided by all previous activity
on the SPI bus. But the cycles "after" must be provided in order to
achieve the SPI transfer.

In order to use that kind of component, implement a new option for
SPI slaves in order to implement trailing clock of a given number of
bits with ChipSelect off at the end of the transfer.

Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
---
 .../devicetree/bindings/spi/spi-peripheral-props.yaml      | 5 +++++
 drivers/spi/spi.c                                          | 7 +++++--
 include/linux/spi/spi.h                                    | 1 +
 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/Documentation/devicetree/bindings/spi/spi-peripheral-props.yaml b/Documentation/devicetree/bindings/spi/spi-peripheral-props.yaml
index 5dd209206e88..2fd5a5084dbe 100644
--- a/Documentation/devicetree/bindings/spi/spi-peripheral-props.yaml
+++ b/Documentation/devicetree/bindings/spi/spi-peripheral-props.yaml
@@ -82,6 +82,11 @@ properties:
     description:
       Delay, in microseconds, after a write transfer.
 
+  spi-trailing-bits:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      Number of clock cycles with chipselect OFF after transfers.
+
 # The controller specific properties go here.
 allOf:
   - $ref: cdns,qspi-nor-peripheral-props.yaml#
diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 4599b121d744..2b204fd20337 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -2159,6 +2159,9 @@ static int of_spi_parse_dt(struct spi_controller *ctlr, struct spi_device *spi,
 		}
 	}
 
+	if (!of_property_read_u32(nc, "spi-trailing-bits", &value))
+		spi->trailing_bits = value;
+
 	if (spi_controller_is_slave(ctlr)) {
 		if (!of_node_name_eq(nc, "slave")) {
 			dev_err(&ctlr->dev, "%pOF is not called 'slave'\n",
@@ -3538,14 +3541,14 @@ int spi_setup(struct spi_device *spi)
 
 	trace_spi_setup(spi, status);
 
-	dev_dbg(&spi->dev, "setup mode %lu, %s%s%s%s%u bits/w, %u Hz max --> %d\n",
+	dev_dbg(&spi->dev, "setup mode %lu, %s%s%s%s%u bits/w, %u Hz max, %d trailing bits --> %d\n",
 			spi->mode & SPI_MODE_X_MASK,
 			(spi->mode & SPI_CS_HIGH) ? "cs_high, " : "",
 			(spi->mode & SPI_LSB_FIRST) ? "lsb, " : "",
 			(spi->mode & SPI_3WIRE) ? "3wire, " : "",
 			(spi->mode & SPI_LOOP) ? "loopback, " : "",
 			spi->bits_per_word, spi->max_speed_hz,
-			status);
+			spi->trailing_bits, status);
 
 	return status;
 }
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 7ab3fed7b804..3f1e3d788f08 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -166,6 +166,7 @@ struct spi_device {
 	u32			max_speed_hz;
 	u8			chip_select;
 	u8			bits_per_word;
+	u8			trailing_bits;
 	bool			rt;
 #define SPI_NO_TX	BIT(31)		/* no transmit wire */
 #define SPI_NO_RX	BIT(30)		/* no receive wire */
-- 
2.34.1