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Date:   Fri, 4 Dec 2020 10:51:38 +0100
From:   Alexandre Belloni <alexandre.belloni@bootlin.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>,
        Miroslav Lichvar <mlichvar@redhat.com>,
        linux-kernel@vger.kernel.org, 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] rtc: adapt allowed RTC update error
Message-ID: <20201204095138.GG74177@piout.net>
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On 04/12/2020 10:34:13+0100, Thomas Gleixner wrote:
> On Thu, Dec 03 2020 at 23:00, Alexandre Belloni wrote:
> > On 03/12/2020 22:05:09+0100, Thomas Gleixner wrote:
> >> 2) I2C/SPI ...
> >> 
> >>    tsched t0                 t1                     t2
> >>           transfer(newsec)   RTC update (newsec)    RTC increments seconds
> >> 
> >>    Lets assume that ttransfer = t1 - t0 is known.
> >
> > Note that ttransfer is one of the reason why setting set_offset_nsec
> > from the RTC driver is not a good idea. The same RTC may be on busses
> > with different rates and there is no way to know that. I think that was
> > one of my objections at the time.
> >
> > ttransfer is not a function of the RTC model but rather of how it is
> > integrated in the system.
> 
> Yes, but it's the right place to store that information.
> 
> It's a fundamental problem of the RTC driver because that's the one
> which has to be able to tell the caller about it. The caller has
> absolutely no way to figure it out because it does not even know what
> type of RTC is there.
> 
> So either the RTC knows the requirements for tsched, e.g. the MC14xxx
> datasheet, or it can retrieve that information from DT or by querying
> the underlying bus mechanics for the xfer time estimate or just by
> timing an xfer for reference.
> 

What I do from userspace is that the caller is the one estimating the
transfer time and this works very well. I really think that the ntp code
could do just the same.


-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com