On Mon, Mar 29 2021 at 07:26, Richard Cochran wrote:
> On Mon, Mar 29, 2021 at 11:16:48AM +0200, Miroslav Lichvar wrote:
>> There are at least two issues with handling a zero offset as a special
>> value. One is that zero could potentially be a valid value in distant
>> future.
>
> I not losing sleep over that, but
>
>> The other is that the kernel updates the offset when a leap
>> second is inserted/deleted even if the original offset is zero, so
>> checking for zero (in the kernel or an application) works only until
>> the first leap second after boot.
>
> oh, I didn't think of that. I hate leap seconds. Good thing Earth is
> picking up the pace again!
>
>> The kernel would need to set a flag that the offset was set. Returning
>> an error in clock_gettime() until the offset is set sounds reasonable
>> to me, but I have no idea how many of the existing applications it
>> would break.
>
> I think it wiser to provide another way, sysfs or something else.
I think adjtimex is the right place and not yet another random file
somewhere. Something like the below.
Thanks,
tglx
---
include/uapi/linux/timex.h | 7 +++++--
kernel/time/ntp.c | 4 +++-
2 files changed, 8 insertions(+), 3 deletions(-)
--- a/include/uapi/linux/timex.h
+++ b/include/uapi/linux/timex.h
@@ -188,9 +188,12 @@ struct __kernel_timex {
#define STA_MODE 0x4000 /* mode (0 = PLL, 1 = FLL) (ro) */
#define STA_CLK 0x8000 /* clock source (0 = A, 1 = B) (ro) */
+#define STA_TAISET 0x10000 /* TAI offset was set via adjtimex (ro) */
+
/* read-only bits */
-#define STA_RONLY (STA_PPSSIGNAL | STA_PPSJITTER | STA_PPSWANDER | \
- STA_PPSERROR | STA_CLOCKERR | STA_NANO | STA_MODE | STA_CLK)
+#define STA_RONLY (STA_PPSSIGNAL | STA_PPSJITTER | STA_PPSWANDER | \
+ STA_PPSERROR | STA_CLOCKERR | STA_NANO | STA_MODE | \
+ STA_CLK | STA_TAISET)
/*
* Clock states (time_state)
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -741,8 +741,10 @@ static inline void process_adjtimex_mode
}
if (txc->modes & ADJ_TAI &&
- txc->constant >= 0 && txc->constant <= MAX_TAI_OFFSET)
+ txc->constant >= 0 && txc->constant <= MAX_TAI_OFFSET) {
*time_tai = txc->constant;
+ time_status |= STA_TAISET;
+ }
if (txc->modes & ADJ_OFFSET)
ntp_update_offset(txc->offset);
