linux-kernel - Re: ktime_set() does not check for nanoseconds

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Date:	Mon, 1 Sep 2008 12:57:29 +0100
From:	"Matt Fleming" <mattjfleming@...glemail.com>
To:	"Thomas Gleixner" <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: ktime_set() does not check for nanoseconds > one second

2008/9/1 Thomas Gleixner <tglx@...utronix.de>:
> On Mon, 1 Sep 2008, Matt Fleming wrote:
>>
>> is it intentional that ktime_set() does not check whether the
>> nanoseconds argument is greater than the number of nanoseconds in a
>> second? I've run into a problem where a value of 1600000000
>> nanoseconds was passed as an argument to ktime_set() and the return
>> value was then used in a ktime_add() call, which returned an incorrect
>> result. Should the caller of ktime_set() make this check or is it
>> possible to move this logic in to the function itself?
>
> Yeah, a check for this in ktime_set() might make sense. Currently it's
> up to the programmer to provide sane values. :)
>
> Thanks,
>
>        tglx
>

How about the attached patch?

Matt

diff --git a/include/linux/ktime.h b/include/linux/ktime.h
index ce59832..7c0ad35 100644
--- a/include/linux/ktime.h
+++ b/include/linux/ktime.h
@@ -150,7 +150,19 @@ static inline ktime_t timeval_to_ktime(struct timeval tv)
 /* Set a ktime_t variable to a value in sec/nsec representation: */
 static inline ktime_t ktime_set(const long secs, const unsigned long nsecs)
 {
-       return (ktime_t) { .tv = { .sec = secs, .nsec = nsecs } };
+       ktime_t res = { .tv = { .sec = secs, .nsec = nsecs }};;
+
+       /*
+        * performance trick: the (u32) -NSEC gives 0x00000000Fxxxxxxx
+        * so we subtract NSEC_PER_SEC and add 1 to the upper 32 bit.
+        *
+        * it's equivalent to:
+        *   tv.nsec -= NSEC_PER_SEC
+        *   tv.sec ++;
+        */
+       if (nsecs >= NSEC_PER_SEC)
+               res.tv64 += (u32)-NSEC_PER_SEC;
+       return res;
 }

 /**

View attachment "ktime-check-for-seconds-in-nanoseconds.patch" of type "text/x-diff" (802 bytes)