atmel_spi: clean up baud rate divisor calculation

Make the baud rate divisor calculation code a bit more readable and add a
few comments.

Also fix wrong debug information being displayed when !new_1 and
max_speed_hz == 0.

[david-b@pacbell.net: fix it]
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Cc: "Janesh Ramakrishnan" <jramakrishnan@neuropace.com>
Acked-by David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/spi/atmel_spi.c

@@ -497,7 +497,7 @@ static int atmel_spi_setup(struct spi_device *spi)
 	struct atmel_spi	*as;
 	u32			scbr, csr;
 	unsigned int		bits = spi->bits_per_word;
-	unsigned long		bus_hz, sck_hz;
+	unsigned long		bus_hz;
 	unsigned int		npcs_pin;
 	int			ret;
 
@@ -536,14 +536,25 @@ static int atmel_spi_setup(struct spi_device *spi)
 		return -EINVAL;
 	}
 
-	/* speed zero convention is used by some upper layers */
+	/*
+	 * Pre-new_1 chips start out at half the peripheral
+	 * bus speed.
+	 */
 	bus_hz = clk_get_rate(as->clk);
-	if (spi->max_speed_hz) {
-		/* assume div32/fdiv/mbz == 0 */
 	if (!as->new_1)
 		bus_hz /= 2;
-		scbr = ((bus_hz + spi->max_speed_hz - 1)
-			/ spi->max_speed_hz);
+
+	if (spi->max_speed_hz) {
+		/*
+		 * Calculate the lowest divider that satisfies the
+		 * constraint, assuming div32/fdiv/mbz == 0.
+		 */
+		scbr = DIV_ROUND_UP(bus_hz, spi->max_speed_hz);
+
+		/*
+		 * If the resulting divider doesn't fit into the
+		 * register bitfield, we can't satisfy the constraint.
+		 */
 		if (scbr >= (1 << SPI_SCBR_SIZE)) {
 			dev_dbg(&spi->dev,
 				"setup: %d Hz too slow, scbr %u; min %ld Hz\n",
@@ -551,8 +562,8 @@ static int atmel_spi_setup(struct spi_device *spi)
 			return -EINVAL;
 		}
 	} else
+		/* speed zero means "as slow as possible" */
 		scbr = 0xff;
-	sck_hz = bus_hz / scbr;
 
 	csr = SPI_BF(SCBR, scbr) | SPI_BF(BITS, bits - 8);
 	if (spi->mode & SPI_CPOL)
@@ -589,7 +600,7 @@ static int atmel_spi_setup(struct spi_device *spi)
 
 	dev_dbg(&spi->dev,
 		"setup: %lu Hz bpw %u mode 0x%x -> csr%d %08x\n",
-		sck_hz, bits, spi->mode, spi->chip_select, csr);
+		bus_hz / scbr, bits, spi->mode, spi->chip_select, csr);
 
 	spi_writel(as, CSR0 + 4 * spi->chip_select, csr);