[U-Boot] [PATCH] kw_spi: fix clock prescaler computation

[Valentin Longchamp]

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 629 (Serial Memory Interface
Configuration Register).

Signed-off-by: Valentin Longchamp <valentin.longchamp at keymile.com>
cc: Holger Brunck <holger.brunck at keymile.com>
cc: Prafulla Wadaskar <prafulla at marvell.com>
---
 arch/arm/include/asm/arch-kirkwood/spi.h |    1 +
 drivers/spi/kirkwood_spi.c               |    5 +++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/arch-kirkwood/spi.h b/arch/arm/include/asm/arch-kirkwood/spi.h
index 1d5043f..5a38dc5 100644
--- a/arch/arm/include/asm/arch-kirkwood/spi.h
+++ b/arch/arm/include/asm/arch-kirkwood/spi.h
@@ -38,6 +38,7 @@ struct kwspi_registers {
 };
 
 #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 */
diff --git a/drivers/spi/kirkwood_spi.c b/drivers/spi/kirkwood_spi.c
index ee14669..9b3f6a4 100644
--- a/drivers/spi/kirkwood_spi.c
+++ b/drivers/spi/kirkwood_spi.c
@@ -57,8 +57,9 @@ 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);
-- 
1.7.1