spi: core: allow defining time that cs is deasserted as a multiple of SCK

Support setting a delay between cs assert and deassert as
a multiple of spi clock length.
Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -1111,6 +1111,7 @@ static void _spi_transfer_cs_change_delay(struct spi_message *msg,
 {
 	u32 delay = xfer->cs_change_delay;
 	u32 unit = xfer->cs_change_delay_unit;
+	u32 hz;
 
 	/* return early on "fast" mode - for everything but USECS */
 	if (!delay && unit != SPI_DELAY_UNIT_USECS)
@@ -1126,6 +1127,13 @@ static void _spi_transfer_cs_change_delay(struct spi_message *msg,
 		break;
 	case SPI_DELAY_UNIT_NSECS: /* nothing to do here */
 		break;
+	case SPI_DELAY_UNIT_SCK:
+		/* if there is no effective speed know, then approximate
+		 * by underestimating with half the requested hz
+		 */
+		hz = xfer->effective_speed_hz ?: xfer->speed_hz / 2;
+		delay *= DIV_ROUND_UP(1000000000, hz);
+		break;
 	default:
 		dev_err_once(&msg->spi->dev,
 			     "Use of unsupported delay unit %i, using default of 10us\n",

include/linux/spi/spi.h

@@ -831,6 +831,7 @@ struct spi_transfer {
 	u8		cs_change_delay_unit;
 #define SPI_DELAY_UNIT_USECS	0
 #define SPI_DELAY_UNIT_NSECS	1
+#define SPI_DELAY_UNIT_SCK	2
 	u32		speed_hz;
 	u16		word_delay;