linux-kernel - Re: [tip:timers/core] time: Add timekeeping_inject

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Date:	Fri, 29 Apr 2011 18:12:05 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	john.stultz@...aro.org, arnd@...db.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:timers/core] time: Add timekeeping_inject_sleeptime

2011/4/29 Thomas Gleixner <tglx@...utronix.de>:
> On Fri, 29 Apr 2011, Arve Hjønnevåg wrote:
>> On Fri, Apr 29, 2011 at 10:31 AM, tip-bot for John Stultz
>> <john.stultz@...aro.org> wrote:
>> > -       set_normalized_timespec(&time,
>> > -                               newtime + delta.tv_sec,
>> > -                               (NSEC_PER_SEC >> 1) + delta.tv_nsec);
>> > -       do_settimeofday(&time);
>> > +       /* subtract kernel time between rtc_suspend to rtc_resume */
>> > +       time = timespec_sub(time, timespec_sub(newts, oldts));
>>
>> The delta you got from the rtc can be almost a second to long or
>> short. Do you do anything to prevent these errors from accumulating?
>
> By using the the magic crystal ball to avoid it or what do you have in
> mind ?
>

This is how we worked around the problem with the old code:

diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c
index 09b4437..e55828a 100644
--- a/drivers/rtc/class.c
+++ b/drivers/rtc/class.c
@@ -42,25 +42,32 @@ static void rtc_device_release(struct device *dev)
  */

 static struct timespec delta;
+static struct timespec delta_delta;
 static time_t          oldtime;

 static int rtc_suspend(struct device *dev, pm_message_t mesg)
 {
        struct rtc_device       *rtc = to_rtc_device(dev);
        struct rtc_time         tm;
-       struct timespec         ts = current_kernel_time();
+       struct timespec         ts;
+       struct timespec         new_delta;

        if (strcmp(dev_name(&rtc->dev), CONFIG_RTC_HCTOSYS_DEVICE) != 0)
                return 0;

+       getnstimeofday(&ts);
        rtc_read_time(rtc, &tm);
        rtc_tm_to_time(&tm, &oldtime);

        /* RTC precision is 1 second; adjust delta for avg 1/2 sec err */
-       set_normalized_timespec(&delta,
+       set_normalized_timespec(&new_delta,
                                ts.tv_sec - oldtime,
                                ts.tv_nsec - (NSEC_PER_SEC >> 1));

+       /* prevent 1/2 sec errors from accumulating */
+       delta_delta = timespec_sub(new_delta, delta);
+       if (delta_delta.tv_sec < -2 || delta_delta.tv_sec >= 2)
+               delta = new_delta;
        return 0;
 }

@@ -80,6 +87,8 @@ static int rtc_resume(struct device *dev)
                return 0;
        }
        rtc_tm_to_time(&tm, &newtime);
+       if (delta_delta.tv_sec < -1)
+               newtime++;
        if (newtime <= oldtime) {
                if (newtime < oldtime)
                        pr_debug("%s:  time travel!\n", dev_name(&rtc->dev));



-- 
Arve Hjønnevåg
--
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