linux-kernel - [PATCH v1 1/2] spi: Add new mode to generate additional clock cycles

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Date:   Sun, 27 Feb 2022 11:00:34 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Mark Brown <broonie@...nel.org>, Rob Herring <robh+dt@...nel.org>,
        Pratyush Yadav <p.yadav@...com>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: [PATCH v1 1/2] spi: Add new mode to generate additional clock cycles

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 a trailing clock of a few bits
with ChipSelect off at the end of the transfer.

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 .../devicetree/bindings/spi/spi-peripheral-props.yaml        | 4 ++++
 drivers/spi/spi.c                                            | 5 ++++-
 include/uapi/linux/spi/spi.h                                 | 3 ++-
 3 files changed, 10 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..4e4fc357d667 100644
--- a/Documentation/devicetree/bindings/spi/spi-peripheral-props.yaml
+++ b/Documentation/devicetree/bindings/spi/spi-peripheral-props.yaml
@@ -82,6 +82,10 @@ properties:
     description:
       Delay, in microseconds, after a write transfer.
 
+  spi-trailing-clock:
+    description:
+      Add a few clock cycles (minimum 2) 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..1b943e112751 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -2109,6 +2109,8 @@ static int of_spi_parse_dt(struct spi_controller *ctlr, struct spi_device *spi,
 		spi->mode |= SPI_LSB_FIRST;
 	if (of_property_read_bool(nc, "spi-cs-high"))
 		spi->mode |= SPI_CS_HIGH;
+	if (of_property_read_bool(nc, "spi-trailing-clock"))
+		spi->mode |= SPI_TRAILING;
 
 	/* Device DUAL/QUAD mode */
 	if (!of_property_read_u32(nc, "spi-tx-bus-width", &value)) {
@@ -3538,12 +3540,13 @@ 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%s%u bits/w, %u Hz max --> %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->mode & SPI_TRAILING) ? "trailing clock, " : "",
 			spi->bits_per_word, spi->max_speed_hz,
 			status);
 
diff --git a/include/uapi/linux/spi/spi.h b/include/uapi/linux/spi/spi.h
index 236a85f08ded..0933a619a154 100644
--- a/include/uapi/linux/spi/spi.h
+++ b/include/uapi/linux/spi/spi.h
@@ -27,6 +27,7 @@
 #define	SPI_TX_OCTAL		_BITUL(13)	/* transmit with 8 wires */
 #define	SPI_RX_OCTAL		_BITUL(14)	/* receive with 8 wires */
 #define	SPI_3WIRE_HIZ		_BITUL(15)	/* high impedance turnaround */
+#define	SPI_TRAILING		_BITUL(16)	/* trailing clock needed */
 
 /*
  * All the bits defined above should be covered by SPI_MODE_USER_MASK.
@@ -36,6 +37,6 @@
  * These bits must not overlap. A static assert check should make sure of that.
  * If adding extra bits, make sure to increase the bit index below as well.
  */
-#define SPI_MODE_USER_MASK	(_BITUL(16) - 1)
+#define SPI_MODE_USER_MASK	(_BITUL(17) - 1)
 
 #endif /* _UAPI_SPI_H */
-- 
2.34.1