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From: Jason Gunthorpe <jgg@ziepe.ca>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Alexandre Belloni <alexandre.belloni@bootlin.com>,
	Miroslav Lichvar <mlichvar@redhat.com>,
	John Stultz <john.stultz@linaro.org>,
	Prarit Bhargava <prarit@redhat.com>,
	Alessandro Zummo <a.zummo@towertech.it>,
	linux-rtc@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>
Subject: Re: [patch 3/8] rtc: cmos: Make rtc_cmos sync offset correct
Date: Mon, 7 Dec 2020 16:50:20 -0400	[thread overview]
Message-ID: <20201207205020.GI5487@ziepe.ca> (raw)
In-Reply-To: <20201206220541.830517160@linutronix.de>

On Sun, Dec 06, 2020 at 10:46:16PM +0100, Thomas Gleixner wrote:
> The offset for rtc_cmos must be -500ms to work correctly with the current
> implementation of rtc_set_ntp_time() due to the following:
> 
>   tsched       twrite(t2.tv_sec - 1) 	 t2 (seconds increment)
> 
> twrite - tsched is the transport time for the write to hit the device,
> which is negligible for this chip because it's accessed directly.
> 
> t2 - twrite = 500ms according to the datasheet.
> 
> But rtc_set_ntp_time() calculation of tsched is:
> 
>     tsched = t2 - 1sec - (t2 - twrite)
> 
> The default for the sync offset is 500ms which means that the write happens
> at t2 - 1.5 seconds which is obviously off by a second for this device.
> 
> Make the offset -500ms so it works correct.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
>  drivers/rtc/rtc-cmos.c |    3 +++
>  1 file changed, 3 insertions(+)

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Matches what sync_cmos_clock() does at least.

My recollection is this change was not supposed to change anything, so
either I got things mixed up and this is wrong:

drivers/rtc/class.c:    rtc->set_offset_nsec =  NSEC_PER_SEC / 2;

Or it faithfully preserved some nonsense from before..

Jason

  reply	other threads:[~2020-12-07 20:51 UTC|newest]

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-06 21:46 [patch 0/8] ntp/rtc: Fixes and cleanups Thomas Gleixner
2020-12-06 21:46 ` [patch 1/8] rtc: mc146818: Prevent reading garbage Thomas Gleixner
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2021-01-25 18:40   ` [patch 1/8] rtc: mc146818: Prevent reading garbage - bug Mickaël Salaün
2021-01-26 13:26     ` Thomas Gleixner
2021-01-26 14:17       ` Mickaël Salaün
2021-01-26 15:35         ` Thomas Gleixner
2021-01-26 17:02           ` [PATCH V2] rtc: mc146818: Detect and handle broken RTCs Thomas Gleixner
2021-01-26 18:00             ` Alexandre Belloni
2021-01-27  8:41             ` [tip: timers/urgent] " tip-bot2 for Thomas Gleixner
2021-01-31 10:59             ` [PATCH V2] " Dirk Gouders
2021-02-01 13:53               ` Serge Belyshev
2021-02-01 19:09                 ` [PATCH] rtc: mc146818: Dont test for bit 0-5 in Register D Thomas Gleixner
2021-02-01 19:24                   ` [PATCH V2] " Thomas Gleixner
2021-02-01 19:32                     ` Linus Torvalds
2021-02-01 19:40                       ` Thomas Gleixner
2021-02-11 23:09                         ` Maciej W. Rozycki
2021-02-01 19:38                     ` Alexandre Belloni
2021-02-01 20:09                     ` Borislav Petkov
2021-02-01 20:15                     ` Dirk Gouders
2021-02-02  4:22                     ` Len Brown
2021-02-02 19:40                     ` [tip: timers/urgent] " tip-bot2 for Thomas Gleixner
2021-02-03 13:00                     ` [PATCH V2] " Mickaël Salaün
2020-12-06 21:46 ` [patch 2/8] rtc: mc146818: Reduce spinlock section in mc146818_set_time() Thomas Gleixner
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2020-12-06 21:46 ` [patch 3/8] rtc: cmos: Make rtc_cmos sync offset correct Thomas Gleixner
2020-12-07 20:50   ` Jason Gunthorpe [this message]
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2020-12-06 21:46 ` [patch 4/8] rtc: core: Make the sync offset default more realistic Thomas Gleixner
2020-12-07 20:55   ` Jason Gunthorpe
2020-12-10 23:59   ` Alexandre Belloni
2020-12-11  0:23     ` Thomas Gleixner
2020-12-11  0:28       ` Alexandre Belloni
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2020-12-06 21:46 ` [patch 5/8] ntp: Make the RTC synchronization more reliable Thomas Gleixner
2020-12-07 12:47   ` Miroslav Lichvar
2020-12-07 20:59   ` Jason Gunthorpe
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2020-12-29 19:41   ` [patch 5/8] " Geert Uytterhoeven
2021-01-11 10:12     ` Thomas Gleixner
2021-01-11 10:40       ` Geert Uytterhoeven
2020-12-06 21:46 ` [patch 6/8] ntp, rtc: Move rtc_set_ntp_time() to ntp code Thomas Gleixner
2020-12-07 20:59   ` Jason Gunthorpe
2020-12-09  3:51     ` Alexandre Belloni
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2020-12-06 21:46 ` [patch 7/8] ntp: Make the RTC sync offset less obscure Thomas Gleixner
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2020-12-06 21:46 ` [patch 8/8] ntp: Consolidate the RTC update implementation Thomas Gleixner
2020-12-07 21:05   ` Jason Gunthorpe
2020-12-11  9:23     ` Thomas Gleixner
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2020-12-09  0:33 ` [patch 0/8] ntp/rtc: Fixes and cleanups Thomas Gleixner
2020-12-09  4:01   ` Alexandre Belloni
2020-12-09 12:39     ` Thomas Gleixner

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