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SubjectRe: [PATCH] fix adjtimex() freq return error
Hi,

On Tue, 19 Aug 2008, Andrew Morton wrote:

> This fix caused Roman's
> ntp-fix-adj_offset_ss_read-bug-and-do_adjtimex-cleanup.patch (which I
> have queued for when Thomas wakes up) to toss a reject, but fixing that
> seemed pretty simple. I think. The scripts/kconfig/conf segfaults are
> being a bit of a PITA at present.

Below is a better patch on top of the above patch, note that this not a
bug fix and thus not an issue for stable.

bye, Roman


[PATCH] improve adjtimex frequency rounding

Change PPM_SCALE_INV_SHIFT so that it doesn't throw away any input bits
(19 is the amount of the factor 2 in PPM_SCALE), the output frequency
can then be calculated back to its input value, as the inverse divide
produce a slightly larger value, which is then correctly rounded by the
final shift.

Signed-off-by: Roman Zippel <zippel@linux-m68k.org>

---
include/linux/timex.h | 2 +-
kernel/time/ntp.c | 5 ++---
2 files changed, 3 insertions(+), 4 deletions(-)

Index: linux-2.6-mm/include/linux/timex.h
===================================================================
--- linux-2.6-mm.orig/include/linux/timex.h
+++ linux-2.6-mm/include/linux/timex.h
@@ -82,7 +82,7 @@
*/
#define SHIFT_USEC 16 /* frequency offset scale (shift) */
#define PPM_SCALE (NSEC_PER_USEC << (NTP_SCALE_SHIFT - SHIFT_USEC))
-#define PPM_SCALE_INV_SHIFT 20
+#define PPM_SCALE_INV_SHIFT 19
#define PPM_SCALE_INV ((1ll << (PPM_SCALE_INV_SHIFT + NTP_SCALE_SHIFT)) / \
PPM_SCALE + 1)

Index: linux-2.6-mm/kernel/time/ntp.c
===================================================================
--- linux-2.6-mm.orig/kernel/time/ntp.c
+++ linux-2.6-mm/kernel/time/ntp.c
@@ -402,9 +402,8 @@ int do_adjtimex(struct timex *txc)
if (!(time_status & STA_NANO))
txc->offset /= NSEC_PER_USEC;
}
- txc->freq = shift_right((s32)(time_freq >> PPM_SCALE_INV_SHIFT) *
- (s64)PPM_SCALE_INV,
- NTP_SCALE_SHIFT);
+ txc->freq = shift_right((time_freq >> PPM_SCALE_INV_SHIFT) *
+ (s64)PPM_SCALE_INV, NTP_SCALE_SHIFT);
txc->maxerror = time_maxerror;
txc->esterror = time_esterror;
txc->status = time_status;

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