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watchdog: s3c2410_wdt: Handle rounding a little better for timeout 

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>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
This commit is contained in:
Doug Anderson 2013-11-26 16:57:19 -08:00 committed by Wim Van Sebroeck
parent 0f1dd98d89
commit 178624403c
1 changed files with 5 additions and 8 deletions
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13
drivers/watchdog/s3c2410_wdt.c
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@ -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 */