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>

1 files changed, 20 insertions, 9 deletions

diff --git a/drivers/spi/atmel_spi.c b/drivers/spi/atmel_spi.c
index 02c8e305b14f..e81d59d78910 100644
--- a/drivers/spi/atmel_spi.c
+++ b/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 (!as->new_1)
+                bus_hz /= 2;
+
         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);
+                /*
+                 * 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);