spi: core: allow defining time that cs is deasserted

For some SPI devices that support speed_hz > 1MHz the default 10 us delay
when cs_change = 1 is typically way to long and may result in poor spi bus
utilization.

This patch makes it possible to control the delay at micro or nano second
resolution on a per spi_transfer basis. It even allows an "as fast as
possible" mode with:
    xfer.cs_change_delay_unit = SPI_DELAY_UNIT_NSECS;
    xfer.cs_change_delay = 0;

The delay code is shared between delay_usecs and cs_change_delay for
consistency and reuse, so in the future this change_delay_unit could also
apply to delay_usec as well.

Note that on slower SOCs/CPU actually reaching ns deasserts on cs is not
realistic as the gpio overhead alone (without any delays added ) may
already leave cs deasserted for more than 1us - at least on a raspberry pi.
But at the very least this way we can keep it as short as possible.
Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -1090,6 +1090,52 @@ static int spi_transfer_wait(struct spi_controller *ctlr,
 	return 0;
 }
 
+static void _spi_transfer_delay_ns(u32 ns)
+{
+	if (!ns)
+		return;
+	if (ns <= 1000) {
+		ndelay(ns);
+	} else {
+		u32 us = DIV_ROUND_UP(ns, 1000);
+
+		if (us <= 10)
+			udelay(us);
+		else
+			usleep_range(us, us + DIV_ROUND_UP(us, 10));
+	}
+}
+
+static void _spi_transfer_cs_change_delay(struct spi_message *msg,
+					  struct spi_transfer *xfer)
+{
+	u32 delay = xfer->cs_change_delay;
+	u32 unit = xfer->cs_change_delay_unit;
+
+	/* return early on "fast" mode - for everything but USECS */
+	if (!delay && unit != SPI_DELAY_UNIT_USECS)
+		return;
+
+	switch (unit) {
+	case SPI_DELAY_UNIT_USECS:
+		/* for compatibility use default of 10us */
+		if (!delay)
+			delay = 10000;
+		else
+			delay *= 1000;
+		break;
+	case SPI_DELAY_UNIT_NSECS: /* nothing to do here */
+		break;
+	default:
+		dev_err_once(&msg->spi->dev,
+			     "Use of unsupported delay unit %i, using default of 10us\n",
+			     xfer->cs_change_delay_unit);
+		delay = 10000;
+	}
+	/* now sleep for the requested amount of time */
+	_spi_transfer_delay_ns(delay);
+}
+
 /*
  * spi_transfer_one_message - Default implementation of transfer_one_message()
  *
@@ -1148,14 +1194,8 @@ static int spi_transfer_one_message(struct spi_controller *ctlr,
 		if (msg->status != -EINPROGRESS)
 			goto out;
 
-		if (xfer->delay_usecs) {
-			u16 us = xfer->delay_usecs;
-
-			if (us <= 10)
-				udelay(us);
-			else
-				usleep_range(us, us + DIV_ROUND_UP(us, 10));
-		}
+		if (xfer->delay_usecs)
+			_spi_transfer_delay_ns(xfer->delay_usecs * 1000);
 
 		if (xfer->cs_change) {
 			if (list_is_last(&xfer->transfer_list,
@@ -1163,7 +1203,7 @@ static int spi_transfer_one_message(struct spi_controller *ctlr,
 				keep_cs = true;
 			} else {
 				spi_set_cs(msg->spi, false);
-				udelay(10);
+				_spi_transfer_cs_change_delay(msg, xfer);
 				spi_set_cs(msg->spi, true);
 			}
 		}
@@ -3757,4 +3797,3 @@ static int __init spi_init(void)
  * include needing to have boardinfo data structures be much more public.
  */
 postcore_initcall(spi_init);
-

include/linux/spi/spi.h

@@ -735,6 +735,9 @@ extern void spi_res_release(struct spi_controller *ctlr,
  * @bits_per_word: select a bits_per_word other than the device default
  *      for this transfer. If 0 the default (from @spi_device) is used.
  * @cs_change: affects chipselect after this transfer completes
+ * @cs_change_delay: delay between cs deassert and assert when
+ *      @cs_change is set and @spi_transfer is not the last in @spi_message
+ * @cs_change_delay_unit: unit of cs_change_delay
  * @delay_usecs: microseconds to delay after this transfer before
  *	(optionally) changing the chipselect status, then starting
  *	the next transfer or completing this @spi_message.
@@ -824,6 +827,10 @@ struct spi_transfer {
 	u8		bits_per_word;
 	u8		word_delay_usecs;
 	u16		delay_usecs;
+	u16		cs_change_delay;
+	u8		cs_change_delay_unit;
+#define SPI_DELAY_UNIT_USECS	0
+#define SPI_DELAY_UNIT_NSECS	1
 	u32		speed_hz;
 	u16		word_delay;