linux-kernel - Re: [PATCH] rtc: adapt allowed RTC update error

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Message-ID: <87im9hc3u2.fsf@nanos.tec.linutronix.de>
Date:   Fri, 04 Dec 2020 10:34:13 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        Miroslav Lichvar <mlichvar@...hat.com>,
        linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org>,
        Prarit Bhargava <prarit@...hat.com>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        linux-rtc@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] rtc: adapt allowed RTC update error

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.

Thanks,

        tglx