kw_spi: fix clock prescaler computation

The computation was not correct with low clock values: setting a 1MHz
clock would result in an overlap that would then configure a 25Mhz
clock.

This patch implements a correct computation method according to the
kirkwood functionnal spec. table 600 (Serial Memory Interface
Configuration Register).

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Holger Brunck <holger.brunck@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>
Acked-by: Prafulla Wadaskar <Prafulla@marvell.com>

Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>

arch/arm/include/asm/arch-kirkwood/spi.h

@@ -49,6 +49,7 @@ struct kwspi_registers {
 #define MISO_MPP11	(1 << 2)
 
 #define KWSPI_CLKPRESCL_MASK	0x1f
+#define KWSPI_CLKPRESCL_MIN	0x12
 #define KWSPI_CSN_ACT		1 /* Activates serial memory interface */
 #define KWSPI_SMEMRDY		(1 << 1) /* SerMem Data xfer ready */
 #define KWSPI_IRQUNMASK		1 /* unmask SPI interrupt */

drivers/spi/kirkwood_spi.c

@@ -56,8 +56,9 @@ struct spi_slave *spi_setup_slave(unsigned int bus, unsigned int cs,
 	writel(~KWSPI_CSN_ACT | KWSPI_SMEMRDY, &spireg->ctrl);
 
 	/* calculate spi clock prescaller using max_hz */
-	data = ((CONFIG_SYS_TCLK / 2) / max_hz) & KWSPI_CLKPRESCL_MASK;
-	data |= 0x10;
+	data = ((CONFIG_SYS_TCLK / 2) / max_hz) + 0x10;
+	data = data < KWSPI_CLKPRESCL_MIN ? KWSPI_CLKPRESCL_MIN : data;
+	data = data > KWSPI_CLKPRESCL_MASK ? KWSPI_CLKPRESCL_MASK : data;
 
 	/* program spi clock prescaller using max_hz */
 	writel(KWSPI_ADRLEN_3BYTE | data, &spireg->cfg);