linux-kernel - Re: [PATCH] rtc: ds1307: fix century bit support

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Date:	Thu, 14 Jul 2016 21:54:45 +0200
From:	arno@...isbad.org (Arnaud Ebalard)
To:	Alexandre Belloni <alexandre.belloni@...e-electrons.com>
Cc:	Alessandro Zummo <a.zummo@...ertech.it>,
	rtc-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rtc: ds1307: fix century bit support

Hi Alexandre,

Alexandre Belloni <alexandre.belloni@...e-electrons.com> writes:

> Add an option to properly support the century bit of ds1337 and compatibles
> and ds1340.
> Because the driver had a bug until now, it is not possible to switch users
> to the fixed code directly as RTCs in the field will wrongly have the
> century bit set.
>
> Signed-off-by: Alexandre Belloni <alexandre.belloni@...e-electrons.com>
> ---
>
> Arnaud, do you mind testing that patch, I still don't have the necessary
> hardware but I think this is the proper course of action.

I did some tests w/ all patches applied (the one below and the ones in
the other series). I had to fix two errors: one pointed in the patch
below and the other one in a patch in the other series (I'll reply to
the patch).

With both RTC_DRV_DS1307 and RTC_DRV_DS1307_CENTURY enabled, this works
as expected on my side: booting on a system which had only seen an
ISL12057 driver, the time on the system at boot (or via hwclock -r) is
correct. Playing w/ -s and -w options works as expected. Booting back on
a kernel w/ ISL12057 driver, things are also ok. So, AFAICT, your
changes do the work.

One small thing though: for user which have always used RTC_DRV_ISL12057
config knob, the change will not be transparent. Would it be possible to
somehow make that old kernel config knob some kind of alias for
RTC_DRV_DS1307 and RTC_DRV_DS1307_CENTURY in Kconfig?

Cheers,

a+

> +#ifdef CONFIG_RTC_DRV_DS1307_CENTURY
> +	switch (ds1307->type) {
> +	case ds_1337:
> +	case ds_1339:
> +	case ds_3231:
> +		if (ds1307->regs[DS1307_REG_MONTH] && DS1337_BIT_CENTURY)

                                                   ^^

> +			t->tm_year += 100;
> +		break;
> +	case ds_1340:
> +		if (ds1307->regs[DS1307_REG_HOUR] && DS1340_BIT_CENTURY)

                                                  ^^