[PATCH v2] watchdog: s3c2410_wdt: Handle rounding a little better for timeout

[Doug Anderson]

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 at chromium.org>
---
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 */
-- 
1.8.4.1