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

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Message-ID: <875zqvu1l3.fsf@tarshish>
Date:   Tue, 30 Apr 2019 15:20:08 +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 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?

baruch

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