fsl_dspi: Introduce DT bindings for CS-SCK and SCK-CS delays

Communication with some SPI slaves just won't cut it if these delays
(before the beginning, and after the end of a transfer) are not added to
the Chip Select signal.

These are a straight copy from Linux:
Documentation/devicetree/bindings/spi/spi-fsl-dspi.txt
drivers/spi/spi-fsl-dspi.c

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
[Rebased]
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>

doc/device-tree-bindings/spi/spi-mcf-dspi.txt

@@ -13,6 +13,10 @@ Optional properties:
 - ctar-params: CTAR0 to 7 register configuration, as an array
   of 8 integer fields for each register, where each register
   is defined as: <fmsz, pcssck, pasc, pdt, cssck, asc, dt, br>.
+- fsl,spi-cs-sck-delay: a delay in nanoseconds between activating chip
+  select and the start of clock signal, at the start of a transfer.
+- fsl,spi-sck-cs-delay: a delay in nanoseconds between stopping the clock
+  signal and deactivating chip select, at the end of a transfer.
 
 Example:
 

drivers/spi/fsl_dspi.c

@@ -9,6 +9,7 @@
  * Haikun Wang (B53464@freescale.com)
  */
 
+#include <linux/math64.h>
 #include <common.h>
 #include <dm.h>
 #include <errno.h>
@@ -25,6 +26,9 @@
 #include <linux/bitops.h>
 #include <linux/delay.h>
 
+/* linux/include/time.h */
+#define NSEC_PER_SEC	1000000000L
+
 DECLARE_GLOBAL_DATA_PTR;
 
 /* fsl_dspi_platdata flags */
@@ -379,6 +383,40 @@ static int fsl_dspi_hz_to_spi_baud(int *pbr, int *br,
 	return -EINVAL;
 }
 
+static void ns_delay_scale(unsigned char *psc, unsigned char *sc, int delay_ns,
+			   unsigned long clkrate)
+{
+	int scale_needed, scale, minscale = INT_MAX;
+	int pscale_tbl[4] = {1, 3, 5, 7};
+	u32 remainder;
+	int i, j;
+
+	scale_needed = div_u64_rem((u64)delay_ns * clkrate, NSEC_PER_SEC,
+				   &remainder);
+	if (remainder)
+		scale_needed++;
+
+	for (i = 0; i < ARRAY_SIZE(pscale_tbl); i++)
+		for (j = 0; j <= DSPI_CTAR_SCALE_BITS; j++) {
+			scale = pscale_tbl[i] * (2 << j);
+			if (scale >= scale_needed) {
+				if (scale < minscale) {
+					minscale = scale;
+					*psc = i;
+					*sc = j;
+				}
+				break;
+			}
+		}
+
+	if (minscale == INT_MAX) {
+		pr_warn("Cannot find correct scale values for %dns delay at clkrate %ld, using max prescaler value",
+			delay_ns, clkrate);
+		*psc = ARRAY_SIZE(pscale_tbl) - 1;
+		*sc = DSPI_CTAR_SCALE_BITS;
+	}
+}
+
 static int fsl_dspi_cfg_speed(struct fsl_dspi_priv *priv, uint speed)
 {
 	int ret;
@@ -412,6 +450,9 @@ static int fsl_dspi_child_pre_probe(struct udevice *dev)
 {
 	struct dm_spi_slave_platdata *slave_plat = dev_get_parent_platdata(dev);
 	struct fsl_dspi_priv *priv = dev_get_priv(dev->parent);
+	u32 cs_sck_delay = 0, sck_cs_delay = 0;
+	unsigned char pcssck = 0, cssck = 0;
+	unsigned char pasc = 0, asc = 0;
 
 	if (slave_plat->cs >= priv->num_chipselect) {
 		debug("DSPI invalid chipselect number %d(max %d)!\n",
@@ -419,7 +460,18 @@ static int fsl_dspi_child_pre_probe(struct udevice *dev)
 		return -EINVAL;
 	}
 
-	priv->ctar_val[slave_plat->cs] = DSPI_CTAR_DEFAULT_VALUE;
+	ofnode_read_u32(dev->node, "fsl,spi-cs-sck-delay", &cs_sck_delay);
+	ofnode_read_u32(dev->node, "fsl,spi-sck-cs-delay", &sck_cs_delay);
+
+	/* Set PCS to SCK delay scale values */
+	ns_delay_scale(&pcssck, &cssck, cs_sck_delay, priv->bus_clk);
+
+	/* Set After SCK delay scale values */
+	ns_delay_scale(&pasc, &asc, sck_cs_delay, priv->bus_clk);
+
+	priv->ctar_val[slave_plat->cs] = DSPI_CTAR_DEFAULT_VALUE |
+					 DSPI_CTAR_PCSSCK(pcssck) |
+					 DSPI_CTAR_PASC(pasc);
 
 	debug("DSPI pre_probe slave device on CS %u, max_hz %u, mode 0x%x.\n",
 	      slave_plat->cs, slave_plat->max_hz, slave_plat->mode);

include/fsl_dspi.h

@@ -94,6 +94,7 @@ struct dspi {
 #define DSPI_CTAR_ASC(x)		(((x) & 0x0F) << 8)
 #define DSPI_CTAR_DT(x)			(((x) & 0x0F) << 4)
 #define DSPI_CTAR_BR(x)			((x) & 0x0F)
+#define DSPI_CTAR_SCALE_BITS		0xf
 
 /* Status */
 #define DSPI_SR_TCF			0x80000000