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>

1 files changed, 5 insertions, 8 deletions

diff --git a/drivers/watchdog/s3c2410_wdt.c b/drivers/watchdog/s3c2410_wdt.c
index c1ada330e125..8beaa1799df6 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 */