spi: spi_mpc83xx clockrate fixes

This updates the SPI clock rate calculations for the spi_mpc83xx driver.
Some boundary conditions were wrong, and in several cases divide-by-16
wasn't always needed

Signed-off-by: Chen Gong <g.chen@freescale.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/spi/spi_mpc83xx.c

@@ -266,21 +266,24 @@ int mpc83xx_spi_setup_transfer(struct spi_device *spi, struct spi_transfer *t)
 
 	cs->hw_mode |= SPMODE_LEN(bits_per_word);
 
-	if ((mpc83xx_spi->spibrg / hz) >= 64) {
-		pm = mpc83xx_spi->spibrg / (hz * 64) - 1;
-		if (pm > 0x0f) {
-			dev_err(&spi->dev, "Requested speed is too "
-				"low: %d Hz. Will use %d Hz instead.\n",
-				hz, mpc83xx_spi->spibrg / 1024);
-			pm = 0x0f;
+	if ((mpc83xx_spi->spibrg / hz) > 64) {
+		pm = mpc83xx_spi->spibrg / (hz * 64);
+		if (pm > 16) {
+			cs->hw_mode |= SPMODE_DIV16;
+			pm /= 16;
+			if (pm > 16) {
+				dev_err(&spi->dev, "Requested speed is too "
+					"low: %d Hz. Will use %d Hz instead.\n",
+					hz, mpc83xx_spi->spibrg / 1024);
+				pm = 16;
+			}
 		}
-		cs->hw_mode |= SPMODE_PM(pm) | SPMODE_DIV16;
-	} else {
+	} else
 		pm = mpc83xx_spi->spibrg / (hz * 4);
-		if (pm)
-			pm--;
-		cs->hw_mode |= SPMODE_PM(pm);
-	}
+	if (pm)
+		pm--;
+
+	cs->hw_mode |= SPMODE_PM(pm);
 	regval =  mpc83xx_spi_read_reg(&mpc83xx_spi->base->mode);
 	if (cs->hw_mode != regval) {
 		unsigned long flags;