Messages in this thread | | | Date | Tue, 26 Nov 2013 16:10:24 -0800 | From | Guenter Roeck <> | Subject | Re: [PATCH] watchdog: s3c2410_wdt: Handle rounding a little better for timeout |
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On 11/26/2013 01:34 PM, Doug Anderson wrote: > Guenter, > > On Tue, Nov 26, 2013 at 10:48 AM, Guenter Roeck <linux@roeck-us.net> wrote: >> On 11/26/2013 10:30 AM, 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. >>> >>> Signed-off-by: Doug Anderson <dianders@chromium.org> >>> --- >>> drivers/watchdog/s3c2410_wdt.c | 10 +++++----- >>> 1 file changed, 5 insertions(+), 5 deletions(-) >>> >>> diff --git a/drivers/watchdog/s3c2410_wdt.c >>> b/drivers/watchdog/s3c2410_wdt.c >>> index 7d8fd04..fe2322b 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", >>> @@ -201,20 +201,20 @@ static int s3c2410wdt_set_heartbeat(struct >>> watchdog_device *wdd, unsigned timeou >>> >>> if (count >= 0x10000) { >>> for (divisor = 1; divisor <= 0x100; divisor++) { >>> - if ((count / divisor) < 0x10000) >>> + if (DIV_ROUND_UP(count, divisor) < 0x10000) >>> break; >>> } >>> >> Since you are at it, >> divisor = DIV_ROUND_UP(count + 1, 0x10000); >> might be faster, simpler, and easier to understand than the loop. > > Way to see the forest for the trees! > > Your math ends up with a slightly different result than the old code, > though. One example is when the count is 0x1ffff. You'll end up with > a divider of 2 and I'll end up with a divider of 3. > > I think we just want: > > divisor = DIV_ROUND_UP(count, 0xffff); > > ...that produces the same result as the old loop, but am curious to > know why you chose the "count + 1" and "0x10000". >
Hi Doug,
I thought the idea was to keep (count / div) less than 0x10000, which you get by dividing through 0x10000. 0x10000 / 0x10000 = 1, though, so I added 1 to the counter. But maybe I was thinking too much ;-).
Now, 0x1ffff / 2 = 0xffff is still lower than 0x10000, which is what I thought is the requirement. Ultimately the error is small either way, so DIV_ROUND_UP(count, 0xffff) is just as good to me to avoid the loop.
Thanks, Guenter
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