[U-Boot-Users] [PATCH v2] Round the serial port clock divisor value returned by calc_divisor()

Jerry Van Baren
2008-07-15T02:44:03Z

This formula is better at avoiding integer overflow.

Signed-off-by: Gerald Van Baren <vanbaren@cideas.com>
---

This is my latest entry in the baud rate rounding dual. Since it doesn't
multiply the master BRG clock but instead adds the baud rate scaled by
1/2 the clock multiplier, it should not overflow (for a master clock
right at the edge of overflowing itself, it still will overflow, but
that is pretty unlikely).

This compiles OK on the mpc7448hpc2. I have only tested it on a
calculator. I have NOT tested it on real hardware.

drivers/serial/serial.c | 16 +++++-----------
1 files changed, 5 insertions(+), 11 deletions(-)

diff --git a/drivers/serial/serial.c b/drivers/serial/serial.c
index 4ccaee2..7f43540 100644
--- a/drivers/serial/serial.c
+++ b/drivers/serial/serial.c
@@ -124,7 +124,6 @@ static NS16550_t serial_ports[4] = {

static int calc_divisor (NS16550_t port)
{
- uint32_t clk_divisor;

#ifdef CONFIG_OMAP1510
/* If can't cleanly clock 115200 set div to 1 */
@@ -147,17 +146,12 @@ static int calc_divisor (NS16550_t port)
#define MODE_X_DIV 16
#endif

- /* Compute divisor value. Normally, we should simply return:
- * CFG_NS16550_CLK) / MODE_X_DIV / gd->baudrate
- * but we need to round that value by adding 0.5 (2/4).
- * Rounding is especially important at high baud rates.
+ /*
+ * Compute divisor value, rounding it properly. Rounding is
+ * especially important at high baud rates.
*/
- clk_divisor = (((4 * CFG_NS16550_CLK) /
- (MODE_X_DIV * gd->baudrate)) + 2) / 4;
-
- debug("NS16550 clock divisor = %d\n", clk_divisor);
-
- return clk_divisor;
+ return (CFG_NS16550_CLK + (gd->baudrate * (MODE_X_DIV / 2))) /
+ (MODE_X_DIV * gd->baudrate);
}

#if !defined(CONFIG_SERIAL_MULTI)
--
1.5.6