linux-kernel - Re: [rtc-linux] Re: [PATCH 4/9 RESEND] RFC: timekeeping: introduce flag systime_was

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Message-ID: <51BB5B65.3000400@ahsoftware.de>
Date:	Fri, 14 Jun 2013 20:05:25 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	rtc-linux@...glegroups.com
CC:	John Stultz <john.stultz@...aro.org>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alessandro Zummo <a.zummo@...ertech.it>
Subject: Re: [rtc-linux] Re: [PATCH 4/9 RESEND] RFC: timekeeping: introduce
 flag systime_was_set

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
>> boot,
>> or if do_settimeofday() is called (e.g. by the RTC subsystem or
>> userspace).
>>
>> Signed-off-by: Alexander Holler <holler@...oftware.de>
>> ---
>>   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);
>>       write_seqcount_begin(&timekeeper_seq);
>> +    systime_was_set = true;
>> +
>> +
>>       timekeeping_forward_now(tk);
>>       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.

Regards,

Alexander
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