[PATCH] watchdog: s3c2410_wdt: Handle rounding a little better for timeout

[Doug Anderson]

Guenter,

On Tue, Nov 26, 2013 at 10:48 AM, Guenter Roeck <linux at 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 at 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".

Thanks!

-Doug