spi/davinci: Fix clock prescale factor computation

Computation of the clock prescaler value returned bogus results if
the requested SPI clock was impossible to set. It now sets either
the maximum or minimum clock frequency, as appropriate.
Signed-off-by: Thomas Koeller <thomas.koeller@baslerweb.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/davinci_spi.c

@@ -301,7 +301,7 @@ static int davinci_spi_setup_transfer(struct spi_device *spi,
 	struct davinci_spi *davinci_spi;
 	struct davinci_spi_platform_data *pdata;
 	u8 bits_per_word = 0;
-	u32 hz = 0, prescale;
+	u32 hz = 0, prescale = 0, clkspeed;
 
 	davinci_spi = spi_master_get_devdata(spi->master);
 	pdata = davinci_spi->pdata;
@@ -338,10 +338,16 @@ static int davinci_spi_setup_transfer(struct spi_device *spi,
 	set_fmt_bits(davinci_spi->base, bits_per_word & 0x1f,
 			spi->chip_select);
 
-	prescale = ((clk_get_rate(davinci_spi->clk) / hz) - 1) & 0xff;
+	clkspeed = clk_get_rate(davinci_spi->clk);
+	if (hz > clkspeed / 2)
+		prescale = 1 << 8;
+	if (hz < clkspeed / 256)
+		prescale = 255 << 8;
+	if (!prescale)
+		prescale = ((clkspeed / hz - 1) << 8) & 0x0000ff00;
 
 	clear_fmt_bits(davinci_spi->base, 0x0000ff00, spi->chip_select);
-	set_fmt_bits(davinci_spi->base, prescale << 8, spi->chip_select);
+	set_fmt_bits(davinci_spi->base, prescale, spi->chip_select);
 
 	return 0;
 }