Re: [rtc-linux] Re: [PATCH 4/9 RESEND] RFC: timekeeping: introduceflag systime_was_set

From: Alexander Holler
Date: Fri Jun 14 2013 - 14:06:35 EST

Am 14.06.2013 19:41, schrieb John Stultz:
On 06/14/2013 09:52 AM, Alexander Holler wrote:
In order to let an RTC set the time at boot without the problem that a
second RTC overwrites it, the flag systime_was_set is introduced.

systime_was_set will be true, if a persistent clock sets the time at
or if do_settimeofday() is called (e.g. by the RTC subsystem or

Signed-off-by: Alexander Holler <holler@xxxxxxxxxxxxx>
include/linux/time.h | 6 ++++++
kernel/time/timekeeping.c | 10 +++++++++-
2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/include/linux/time.h b/include/linux/time.h
index d5d229b..888280f 100644
--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -129,6 +129,12 @@ extern int update_persistent_clock(struct
timespec now);
void timekeeping_init(void);
extern int timekeeping_suspended;
+ * Will be true if the system time was set at least once by
+ * a persistent clock, RTC or userspace.
+ */
+extern bool systime_was_set;

Probably should make this static to timekeeping.c and create an accessor
function so you don't have to export locking rules on this.

unsigned long get_seconds(void);
struct timespec current_kernel_time(void);
struct timespec __current_kernel_time(void); /* does not take
xtime_lock */
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index baeeb5c..07d8531 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -37,6 +37,9 @@ int __read_mostly timekeeping_suspended;
/* Flag for if there is a persistent clock on this platform */
bool __read_mostly persistent_clock_exist = false;
+/* Flag for if the system time was set at least once */
+bool __read_mostly systime_was_set;
Probably should also move this to be part of the timekeeper structure
(since it will be protected by the timekeeper lock.

I wanted to avoid locks for this silly flag at all. It is only set once at boot (and resume) and set to 0 at suspend. And I don't see any possible race condition which could make a lock necessary. Therefor I've decided to not use a lock or atomic_* in order to skip any delay in setting the time.

Of course, I might be wrong and there might be a use case where multiple things do set the system time concurrently and nothing else did set system time before, but I found that extremly unlikely.

static inline void tk_normalize_xtime(struct timekeeper *tk)
while (tk->xtime_nsec >= ((u64)NSEC_PER_SEC << tk->shift)) {
@@ -498,6 +501,9 @@ int do_settimeofday(const struct timespec *tv)
raw_spin_lock_irqsave(&timekeeper_lock, flags);
+ systime_was_set = true;
xt = tk_xtime(tk);

Might also want to add the flag to inject_offset as well, since that
could be used to set the time.

I wasn't sure about that because I had only a quick look at inject_offset() and had the impression it's only able to inject a relative small offset (so not usable at boot). And, as written sometimes before, I haven't had a deep look at suspend/resume, which might be the only place where it is really used to set the clock when
systime_was_set is false.


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