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[156.34.48.30]) by smtp.gmail.com with ESMTPSA id j21sm1524129qtp.10.2020.12.03.08.16.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 03 Dec 2020 08:16:23 -0800 (PST) Received: from jgg by mlx with local (Exim 4.94) (envelope-from ) id 1kkrHK-005XLH-RZ; Thu, 03 Dec 2020 12:16:22 -0400 Date: Thu, 3 Dec 2020 12:16:22 -0400 From: Jason Gunthorpe To: Thomas Gleixner Cc: Alexandre Belloni , Miroslav Lichvar , linux-kernel@vger.kernel.org, John Stultz , Prarit Bhargava , Alessandro Zummo , linux-rtc@vger.kernel.org, Peter Zijlstra Subject: Re: [PATCH] rtc: adapt allowed RTC update error Message-ID: <20201203161622.GA1317829@ziepe.ca> References: <20201201171420.GN1900232@localhost> <20201201173540.GH5487@ziepe.ca> <87mtywe2zu.fsf@nanos.tec.linutronix.de> <20201202162723.GJ5487@ziepe.ca> <87a6uwdnfn.fsf@nanos.tec.linutronix.de> <20201202205418.GN5487@ziepe.ca> <874kl3eu8p.fsf@nanos.tec.linutronix.de> <87zh2vd72z.fsf@nanos.tec.linutronix.de> <20201203021047.GG3544@piout.net> <87pn3qdhli.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87pn3qdhli.fsf@nanos.tec.linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 03, 2020 at 04:39:21PM +0100, Thomas Gleixner wrote: > The logic in sync_cmos_clock() and rtc_set_ntp_time() is different as I > pointed out: sync_cmos_clock() hands -500ms to rtc_tv_nsec_ok() and > rtc_set_ntp_time() uses +500ms, IOW exactly ONE second difference in > behaviour. I understood this is because the two APIs work differently, rmk explained this as: > 1. kernel/time/ntp.c assumes that all RTCs want to be told to set the > time at around 500ms into the second. > > 2. drivers/rtc/systohc.c assumes that if the time being set is >= 500ms, > then we want to set the _next_ second. ie one path is supposed to round down and one path is supposed to round up, so you get to that 1s difference.. IIRC this is also connected to why the offset is signed.. Jason