linux-kernel - Re: [PATCH] m68k: Fix off-by-one calendar month

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Message-ID: <20180424103737.GA19011@piout.net>
Date:   Tue, 24 Apr 2018 12:37:37 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Finn Thain <fthain@...egraphics.com.au>,
        Sam Creasey <sammy@...my.net>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] m68k: Fix off-by-one calendar month

On 24/04/2018 12:06:30+0200, Geert Uytterhoeven wrote:
> Hi Finn,
> 
> On Mon, Apr 23, 2018 at 3:02 AM, Finn Thain <fthain@...egraphics.com.au> wrote:
> > This fixes a bug in read_persistent_clock() which causes the system
> > clock to lag the Real Time Clock by one month. The problem was noticed
> > on a Mac, but theoretically it must also affect Atari, BVME6000 and Q40.
> >
> > The tm_mon value in the struct rtc_time passed to mach_hwclk() is
> > zero-based, and atari_mste_hwclk(), atari_tt_hwclk(), bvme6000_hwclk(),
> > mac_hwclk() and q40_hwclk() all make this adjustment. Unfortunately,
> > dn_dummy_hwclk(), mvme147_hwclk(), mvme16x_hwclk(), sun3_hwclk() and
> > sun3x_hwclk() fail to decrement tm_mon.
> >
> > Bring these platforms into line and fix read_persistent_clock() so it
> > works correctly on all m68k platforms.
> >
> > The datasheets for the RTC devices found on the affected platforms
> > all confirm that the year is stored as a value in the range 0-99 and
> > the month is stored as a value in the range 1-12. Please refer to the
> > datasheets for MC146818 (Apollo), DS1643 (MVME), ICM7170 (Sun 3)
> > and M48T02 (Sun 3x).
> >
> > Reported-by: Stan Johnson <userm57@...oo.com>
> > Signed-off-by: Finn Thain <fthain@...egraphics.com.au>
> 
> Thanks, applied and queued for v4.18.
> 
> <snip>
> 
> > --- a/arch/m68k/kernel/time.c
> > +++ b/arch/m68k/kernel/time.c
> > @@ -74,17 +74,17 @@ static irqreturn_t timer_interrupt(int irq, void *dummy)
> >  void read_persistent_clock(struct timespec *ts)
> >  {
> >         struct rtc_time time;
> > +
> >         ts->tv_sec = 0;
> >         ts->tv_nsec = 0;
> >
> > -       if (mach_hwclk) {
> > -               mach_hwclk(0, &time);
> > +       if (!mach_hwclk)
> > +               return;
> > +
> > +       mach_hwclk(0, &time);
> >
> > -               if ((time.tm_year += 1900) < 1970)
> > -                       time.tm_year += 100;

Note that this change may break existing users. I'm perfectly fine with
it as doing this is generally wrong anyway and this is something I'd
like to see eliminated.

> > -               ts->tv_sec = mktime(time.tm_year, time.tm_mon, time.tm_mday,
> > -                                     time.tm_hour, time.tm_min, time.tm_sec);
> > -       }
> > +       ts->tv_sec = mktime(time.tm_year + 1900, time.tm_mon + 1, time.tm_mday,
> > +                           time.tm_hour, time.tm_min, time.tm_sec);
> 
> That might explain why my Amiga spends so much time on file system checks
> since I wrote the rp5c01 RTC driver...
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> -- 
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
> 
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds

-- 
Alexandre Belloni, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com