Re: [PATCH] watchdog: s3c2410_wdt: Handle rounding a little betterfor timeout

[Guenter Roeck]

On 11/26/2013 01:34 PM, Doug Anderson wrote:
> Guenter,
>
> On Tue, Nov 26, 2013 at 10:48 AM, Guenter Roeck <linux@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > > ---
> > >    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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