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spi_s3c24xx: fix clock rate calculation 

Currently the clock rate calculation may round as pleased, which means
that it is possible that we will round down and end up with a faster clock
rate than intended.

Change the calculation to use DIV_ROUND_UP() to ensure that we end up with
a clock rate either the same as or lower than the user requested one.

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Ben Dooks 2009-08-18 14:11:16 -07:00 committed by Linus Torvalds
parent b2503a9408
commit b897878454
1 changed files with 7 additions and 10 deletions
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17
drivers/spi/spi_s3c24xx.c
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@ -111,6 +111,7 @@ static int s3c24xx_spi_setupxfer(struct spi_device *spi,
unsigned int bpw;
unsigned int hz;
unsigned int div;
unsigned long clk;
bpw = t ? t->bits_per_word : spi->bits_per_word;
hz = t ? t->speed_hz : spi->max_speed_hz;
@ -120,20 +121,16 @@ static int s3c24xx_spi_setupxfer(struct spi_device *spi,
return -EINVAL;
}
div = clk_get_rate(hw->clk) / hz;
/* is clk = pclk / (2 * (pre+1)), or is it
* clk = (pclk * 2) / ( pre + 1) */
div /= 2;
if (div > 0)
div -= 1;
clk = clk_get_rate(hw->clk);
div = DIV_ROUND_UP(clk, hz * 2) - 1;
if (div > 255)
div = 255;
dev_dbg(&spi->dev, "setting pre-scaler to %d (hz %d)\n", div, hz);
dev_dbg(&spi->dev, "setting pre-scaler to %d (wanted %d, got %ld)\n",
div, hz, clk / (2 * (div + 1)));
writeb(div, hw->regs + S3C2410_SPPRE);
spin_lock(&hw->bitbang.lock);