Re: [RFC PATCH] timekeeping: Correct run-time detection of real-time clock.

[Feng Tang <feng.tang@intel.com>]

Hi Zoran,

Thanks for the patch! (This reply may be toooo late :)) 

One question just for curiosity: for the counter_32K timer, it's running
at 32K Hz and has one 32b counter. I understand it is only for suspend
time calculation use, but the wrap time for it is about
	4G/32K ~= 128K seconds ~= 35 hours
What if one suspend time is longer than that?

- Feng

On Fri, May 17, 2013 at 11:24:05AM -0700, Zoran Markovic wrote:
> Since commit <31ade30692dc9680bfc95700d794818fa3f754ac>, timekeeping_init()
> checks for presence of persistent clock by attempting to read a non-zero
> time value from real-time clock. This is an issue on platforms where
> persistent_clock (instead of a RTC) is implemented as a free-running counter
> starting from zero on each boot and running during suspend. Examples are some
> ARM platforms (e.g. PandaBoard). An attempt to read such a clock during
> timekeeping_init() may return zero value and falsely declare persistent clock
> as missing. Additionally, in the above case suspend times may be accounted
> twice (once from timekeeping_resume() and once from rtc_resume()), resulting
> in a gradual drift of system time.
> 
> This patch does a run-time correction of the issue by doing the same check
> during timekeeping_suspend().
> 
> A better long-term solution would have to return error when trying to read
> non-existing clock and zero when trying to read an uninitialized clock, but
> that would require changing all persistent_clock implementations.
> 
> This patch addresses the immediate breakage, for now.
> 
> Cc: John Stultz <john.stultz@linaro.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Feng Tang <feng.tang@intel.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Zoran Markovic <zoran.markovic@linaro.org>
> ---
>  kernel/time/timekeeping.c |    8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 98cd470..baeeb5c 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -975,6 +975,14 @@ static int timekeeping_suspend(void)
>  
>  	read_persistent_clock(&timekeeping_suspend_time);
>  
> +	/*
> +	 * On some systems the persistent_clock can not be detected at
> +	 * timekeeping_init by its return value, so if we see a valid
> +	 * value returned, update the persistent_clock_exists flag.
> +	 */
> +	if (timekeeping_suspend_time.tv_sec || timekeeping_suspend_time.tv_nsec)
> +		persistent_clock_exist = true;
> +
>  	raw_spin_lock_irqsave(&timekeeper_lock, flags);
>  	write_seqcount_begin(&timekeeper_seq);
>  	timekeeping_forward_now(tk);
> -- 
> 1.7.9.5