Messages in this thread | | | Date | Tue, 26 Nov 2013 17:04:09 -0800 | From | Guenter Roeck <> | Subject | Re: [PATCH v2] watchdog: s3c2410_wdt: Handle rounding a little better for timeout |
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On 11/26/2013 04:57 PM, Doug Anderson wrote: > The existing watchdog timeout worked OK but didn't deal with > rounding in an ideal way when dividing out all of its clocks. > > Specifically if you had a timeout of 32 seconds and an input clock of > 66666666, you'd end up setting a timeout of 31.9998 seconds and > reporting a timeout of 31 seconds. > > Specifically DBG printouts showed: > s3c2410wdt_set_heartbeat: count=16666656, timeout=32, freq=520833 > s3c2410wdt_set_heartbeat: timeout=32, divisor=255, count=16666656 (0000ff4f) > and the final timeout reported to the user was: > ((count / divisor) * divisor) / freq > (0xff4f * 255) / 520833 = 31 (truncated from 31.9998) > the technically "correct" value is: > (0xff4f * 255) / (66666666.0 / 128) = 31.9998 > > By using "DIV_ROUND_UP" we can be a little more correct. > s3c2410wdt_set_heartbeat: count=16666688, timeout=32, freq=520834 > s3c2410wdt_set_heartbeat: timeout=32, divisor=255, count=16666688 (0000ff50) > and the final timeout reported to the user: > (0xff50 * 255) / 520834 = 32 > the technically "correct" value is: > (0xff50 * 255) / (66666666.0 / 128) = 32.0003 > > We'll use a DIV_ROUND_UP to solve this, generally erroring on the side > of reporting shorter values to the user and setting the watchdog to > slightly longer than requested: > * Round input frequency up to assume watchdog is counting faster. > * Round divisions by divisor up to give us extra time. > > At the same time we can avoid a for loop by just doing the right math. > > Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
> --- > Changes in v2: > - Avoid a for loop as per Guenter. > > drivers/watchdog/s3c2410_wdt.c | 13 +++++-------- > 1 file changed, 5 insertions(+), 8 deletions(-) > > diff --git a/drivers/watchdog/s3c2410_wdt.c b/drivers/watchdog/s3c2410_wdt.c > index 7d8fd04..d9bcd6e 100644 > --- a/drivers/watchdog/s3c2410_wdt.c > +++ b/drivers/watchdog/s3c2410_wdt.c > @@ -188,7 +188,7 @@ static int s3c2410wdt_set_heartbeat(struct watchdog_device *wdd, unsigned timeou > if (timeout < 1) > return -EINVAL; > > - freq /= 128; > + freq = DIV_ROUND_UP(freq, 128); > count = timeout * freq; > > DBG("%s: count=%d, timeout=%d, freq=%lu\n", > @@ -200,21 +200,18 @@ static int s3c2410wdt_set_heartbeat(struct watchdog_device *wdd, unsigned timeou > */ > > if (count >= 0x10000) { > - for (divisor = 1; divisor <= 0x100; divisor++) { > - if ((count / divisor) < 0x10000) > - break; > - } > + divisor = DIV_ROUND_UP(count, 0xffff); > > - if ((count / divisor) >= 0x10000) { > + if (divisor > 0x100) { > dev_err(wdt->dev, "timeout %d too big\n", timeout); > return -EINVAL; > } > } > > DBG("%s: timeout=%d, divisor=%d, count=%d (%08x)\n", > - __func__, timeout, divisor, count, count/divisor); > + __func__, timeout, divisor, count, DIV_ROUND_UP(count, divisor)); > > - count /= divisor; > + count = DIV_ROUND_UP(count, divisor); > wdt->count = count; > > /* update the pre-scaler */ >
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