On 11/26/2013 01:34 PM, Doug Anderson wrote:
Guenter,
On Tue, Nov 26, 2013 at 10:48 AM, Guenter Roeck <li...@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 <diand...@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".
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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