[PATCH v5 2/3] spi: Add SPI_NO_TX/RX support

[Alexandru Ardelean <alexandru.ardelean@xxxxxxxxxx>]

From: Dragos Bogdan <dragos.bogdan@xxxxxxxxxx>

Transmit/receive only is a valid SPI mode. For example, the MOSI/TX line
might be missing from an ADC while for a DAC the MISO/RX line may be
optional. This patch adds these two new modes: SPI_NO_TX and
SPI_NO_RX. This way, the drivers will be able to identify if any of
these two lines is missing and to adjust the transfers accordingly.

Signed-off-by: Dragos Bogdan <dragos.bogdan@xxxxxxxxxx>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@xxxxxxxxxx>
---

Changelog v4 -> v5:
* https://lore.kernel.org/linux-spi/20201203140531.74470-2-alexandru.ardelean@xxxxxxxxxx/
* using static_assert() vs BUILD_BUG_ON()
* added comments for SPI_MODE_KERNEL_MASK & SPI_MODE_USER_MASK
* using shorter form defining SPI_MODE_KERNEL_MASK & SPI_MODE_USER_MASK

 drivers/spi/spi.c            | 26 +++++++++++++++++++++-----
 include/linux/spi/spi.h      | 17 +++++++++++++++++
 include/uapi/linux/spi/spi.h | 10 ++++++++++
 3 files changed, 48 insertions(+), 5 deletions(-)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 51d7c004fbab..0b69934698c3 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -1941,6 +1941,9 @@ static int of_spi_parse_dt(struct spi_controller *ctlr, struct spi_device *spi,
 	/* Device DUAL/QUAD mode */
 	if (!of_property_read_u32(nc, "spi-tx-bus-width", &value)) {
 		switch (value) {
+		case 0:
+			spi->mode |= SPI_NO_TX;
+			break;
 		case 1:
 			break;
 		case 2:
@@ -1962,6 +1965,9 @@ static int of_spi_parse_dt(struct spi_controller *ctlr, struct spi_device *spi,
 
 	if (!of_property_read_u32(nc, "spi-rx-bus-width", &value)) {
 		switch (value) {
+		case 0:
+			spi->mode |= SPI_NO_RX;
+			break;
 		case 1:
 			break;
 		case 2:
@@ -3329,12 +3335,17 @@ int spi_setup(struct spi_device *spi)
 	unsigned	bad_bits, ugly_bits;
 	int		status;
 
-	/* check mode to prevent that DUAL and QUAD set at the same time
+	/*
+	 * check mode to prevent that any two of DUAL, QUAD and NO_MOSI/MISO
+	 * are set at the same time
 	 */
-	if (((spi->mode & SPI_TX_DUAL) && (spi->mode & SPI_TX_QUAD)) ||
-		((spi->mode & SPI_RX_DUAL) && (spi->mode & SPI_RX_QUAD))) {
+	if ((hweight_long(spi->mode &
+		(SPI_TX_DUAL | SPI_TX_QUAD | SPI_NO_TX)) > 1) ||
+	    (hweight_long(spi->mode &
+		(SPI_RX_DUAL | SPI_RX_QUAD | SPI_NO_RX)) > 1)) {
 		dev_err(&spi->dev,
-		"setup: can not select dual and quad at the same time\n");
+		"setup: can not select any two of dual, quad and no-rx/tx "
+		"at the same time\n");
 		return -EINVAL;
 	}
 	/* if it is SPI_3WIRE mode, DUAL and QUAD should be forbidden
@@ -3348,7 +3359,8 @@ int spi_setup(struct spi_device *spi)
 	 * SPI_CS_WORD has a fallback software implementation,
 	 * so it is ignored here.
 	 */
-	bad_bits = spi->mode & ~(spi->controller->mode_bits | SPI_CS_WORD);
+	bad_bits = spi->mode & ~(spi->controller->mode_bits | SPI_CS_WORD |
+				 SPI_NO_TX | SPI_NO_RX);
 	/* nothing prevents from working with active-high CS in case if it
 	 * is driven by GPIO.
 	 */
@@ -3610,6 +3622,8 @@ static int __spi_validate(struct spi_device *spi, struct spi_message *message)
 		 * 2. check tx/rx_nbits match the mode in spi_device
 		 */
 		if (xfer->tx_buf) {
+			if (spi->mode & SPI_NO_TX)
+				return -EINVAL;
 			if (xfer->tx_nbits != SPI_NBITS_SINGLE &&
 				xfer->tx_nbits != SPI_NBITS_DUAL &&
 				xfer->tx_nbits != SPI_NBITS_QUAD)
@@ -3623,6 +3637,8 @@ static int __spi_validate(struct spi_device *spi, struct spi_message *message)
 		}
 		/* check transfer rx_nbits */
 		if (xfer->rx_buf) {
+			if (spi->mode & SPI_NO_RX)
+				return -EINVAL;
 			if (xfer->rx_nbits != SPI_NBITS_SINGLE &&
 				xfer->rx_nbits != SPI_NBITS_DUAL &&
 				xfer->rx_nbits != SPI_NBITS_QUAD)
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index a08c3f37e202..9bfdfaf286eb 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -6,6 +6,7 @@
 #ifndef __LINUX_SPI_H
 #define __LINUX_SPI_H
 
+#include <linux/bits.h>
 #include <linux/device.h>
 #include <linux/mod_devicetable.h>
 #include <linux/slab.h>
@@ -166,6 +167,18 @@ struct spi_device {
 	u8			chip_select;
 	u8			bits_per_word;
 	bool			rt;
+#define SPI_NO_TX	BIT(31)		/* no transmit wire */
+#define SPI_NO_RX	BIT(30)		/* no receive wire */
+	/*
+	 * All bits defined above should be covered by SPI_MODE_KERNEL_MASK.
+	 * The SPI_MODE_KERNEL_MASK has the SPI_MODE_USER_MASK counterpart,
+	 * which is defined in 'include/uapi/linux/spi/spi.h'.
+	 * The bits defined here are from bit 31 downwards, while in
+	 * SPI_MODE_USER_MASK are from 0 upwards.
+	 * These bits must not overlap. A static assert check should make sure of that.
+	 * If adding extra bits, make sure to decrease the bit index below as well.
+	 */
+#define SPI_MODE_KERNEL_MASK	(~(BIT(30) - 1))
 	u32			mode;
 	int			irq;
 	void			*controller_state;
@@ -189,6 +202,10 @@ struct spi_device {
 	 */
 };
 
+/* Make sure that SPI_MODE_KERNEL_MASK & SPI_MODE_USER_MASK don't overlap */
+static_assert((SPI_MODE_KERNEL_MASK & SPI_MODE_USER_MASK) == 0,
+	      "SPI_MODE_USER_MASK & SPI_MODE_KERNEL_MASK must not overlap");
+
 static inline struct spi_device *to_spi_device(struct device *dev)
 {
 	return dev ? container_of(dev, struct spi_device, dev) : NULL;
diff --git a/include/uapi/linux/spi/spi.h b/include/uapi/linux/spi/spi.h
index 703b586f35df..236a85f08ded 100644
--- a/include/uapi/linux/spi/spi.h
+++ b/include/uapi/linux/spi/spi.h
@@ -28,4 +28,14 @@
 #define	SPI_RX_OCTAL		_BITUL(14)	/* receive with 8 wires */
 #define	SPI_3WIRE_HIZ		_BITUL(15)	/* high impedance turnaround */
 
+/*
+ * All the bits defined above should be covered by SPI_MODE_USER_MASK.
+ * The SPI_MODE_USER_MASK has the SPI_MODE_KERNEL_MASK counterpart in
+ * 'include/linux/spi/spi.h'. The bits defined here are from bit 0 upwards
+ * while in SPI_MODE_KERNEL_MASK they are from the other end downwards.
+ * 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)
+
 #endif /* _UAPI_SPI_H */
-- 
2.17.1