Merge tag 'spi-cs-word' into spi-4.20

spi: Provide SPI_CS_WORD

This provides a SPI operation mode which changes chip select after every
word, used by some devices such as ADCs and DACs.

drivers/spi/spi.c

@@ -2774,8 +2774,10 @@ int spi_setup(struct spi_device *spi)
 		return -EINVAL;
 	/* help drivers fail *cleanly* when they need options
 	 * that aren't supported with their current controller
+	 * SPI_CS_WORD has a fallback software implementation,
+	 * so it is ignored here.
 	 */
-	bad_bits = spi->mode & ~spi->controller->mode_bits;
+	bad_bits = spi->mode & ~(spi->controller->mode_bits | SPI_CS_WORD);
 	ugly_bits = bad_bits &
 		    (SPI_TX_DUAL | SPI_TX_QUAD | SPI_RX_DUAL | SPI_RX_QUAD);
 	if (ugly_bits) {
@@ -2829,6 +2831,33 @@ static int __spi_validate(struct spi_device *spi, struct spi_message *message)
 	if (list_empty(&message->transfers))
 		return -EINVAL;
 
+	/* If an SPI controller does not support toggling the CS line on each
+	 * transfer (indicated by the SPI_CS_WORD flag), we can emulate it by
+	 * splitting transfers into one-word transfers and ensuring that
+	 * cs_change is set for each transfer.
+	 */
+	if ((spi->mode & SPI_CS_WORD) && !(ctlr->mode_bits & SPI_CS_WORD)) {
+		size_t maxsize;
+		int ret;
+
+		maxsize = (spi->bits_per_word + 7) / 8;
+
+		/* spi_split_transfers_maxsize() requires message->spi */
+		message->spi = spi;
+
+		ret = spi_split_transfers_maxsize(ctlr, message, maxsize,
+						  GFP_KERNEL);
+		if (ret)
+			return ret;
+
+		list_for_each_entry(xfer, &message->transfers, transfer_list) {
+			/* don't change cs_change on the last entry in the list */
+			if (list_is_last(&xfer->transfer_list, &message->transfers))
+				break;
+			xfer->cs_change = 1;
+		}
+	}
+
 	/* Half-duplex links include original MicroWire, and ones with
 	 * only one data pin like SPI_3WIRE (switches direction) or where
 	 * either MOSI or MISO is missing.  They can also be caused by

include/linux/spi/spi.h

@@ -163,6 +163,7 @@ struct spi_device {
 #define	SPI_TX_QUAD	0x200			/* transmit with 4 wires */
 #define	SPI_RX_DUAL	0x400			/* receive with 2 wires */
 #define	SPI_RX_QUAD	0x800			/* receive with 4 wires */
+#define SPI_CS_WORD	0x1000			/* toggle cs after each word */
 	int			irq;
 	void			*controller_state;
 	void			*controller_data;
@@ -177,7 +178,6 @@ struct spi_device {
 	 * the controller talks to each chip, like:
 	 *  - memory packing (12 bit samples into low bits, others zeroed)
 	 *  - priority
-	 *  - drop chipselect after each word
 	 *  - chipselect delays
 	 *  - ...
 	 */