linux-kernel - Re: [PATCH 2/4] rtc: digicolor: set range

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Message-ID: <87y33rsef3.fsf@tarshish>
Date:   Tue, 30 Apr 2019 18:25:52 +0300
From:   Baruch Siach <baruch@...s.co.il>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     linux-rtc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/4] rtc: digicolor: set range

Hi Alexandre,

On Tue, Apr 30 2019, Alexandre Belloni wrote:
> On 30/04/2019 15:20:08+0300, Baruch Siach wrote:
>> On Tue, Apr 30 2019, Alexandre Belloni wrote:
>> > On 30/04/2019 14:36:24+0300, Baruch Siach wrote:
>> >> Hi Alexandre,
>> >>
>> >> On Tue, Apr 30 2019, Alexandre Belloni wrote:
>> >>
>> >> > While the range of REFERENCE + TIME is actually 33 bits, the counter
>> >> > itself (TIME) is a 32-bits seconds counter.
>> >> >
>> >> > Signed-off-by: Alexandre Belloni <alexandre.belloni@...tlin.com>
>> >> > ---
>> >> >  drivers/rtc/rtc-digicolor.c | 1 +
>> >> >  1 file changed, 1 insertion(+)
>> >> >
>> >> > diff --git a/drivers/rtc/rtc-digicolor.c b/drivers/rtc/rtc-digicolor.c
>> >> > index 5bb14c56bc9a..e6e16aaac254 100644
>> >> > --- a/drivers/rtc/rtc-digicolor.c
>> >> > +++ b/drivers/rtc/rtc-digicolor.c
>> >> > @@ -206,6 +206,7 @@ static int __init dc_rtc_probe(struct platform_device *pdev)
>> >> >  	platform_set_drvdata(pdev, rtc);
>> >> >
>> >> >  	rtc->rtc_dev->ops = &dc_rtc_ops;
>> >> > +	rtc->rtc_dev->range_max = U32_MAX;
>> >>
>> >> Where can I find documentation on the meaning and usage of the range_max
>> >> value? I could not find anything in the kernel source.
>> >
>> > It should be set to the maximum UNIX timestamp the RTC can be set to
>> > while keeping range_min to range_max contiguous.
>> >
>> > In the digicolor case, you could go up to 8589934590 (Wed Mar 16
>> > 12:56:30 UTC 2242) but the driver only writes DC_RTC_REFERENCE and I'm
>> > not sure it can also update DC_RTC_TIME safely.
>> 
>> DC_RTC_TIME resets to zero whenever dc_rtc_write writes CMD_RESET to the
>> DC_RTC_CONTROL register. DC_RTC_REFERENCE keeps the value that
>> dc_rtc_write stores there. So the driver will return values larger than
>> U32_MAX if you happen to cross this point in time between dc_rtc_write
>> and dc_rtc_read. But you can't store a value larger than U32_MAX in
>> DC_RTC_REFERENCE.
>> 
>> Will the core RTC code handle the U32_MAX cross correctly?
>
> Yes, this is ok to return a valid value that is higher than range_max.
> However, at that time, you will not be able to set any alarms anymore as
> the core doesn't allow to set alarms after range_max.
>
> I would think that this is fine because this will happen in 2106 and we
> have a way to offset the time (the whole goal of setting the range)
> using device tree.

That's the sort of documentation that I'm missing. The 'start-year'
property is mentioned in the DT binding documentation. But I don't see
where range_max is documented as a facility for the time offset feature.

Anyway,

Acked-by: Baruch Siach <baruch@...s.co.il>

Thanks,
baruch

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