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From:   Victor Ding <victording@google.com>
Date:   Fri, 14 Aug 2020 16:10:13 +1000
Message-ID: <CANqTbdZhZL--JebFhZPkf2+VuCUs2b=Me-BoBHgAtt_MvQBX3A@mail.gmail.com>
Subject: Re: [PATCH] rtc: cmos: initialize rtc time when reading alarm
To:     Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Alessandro Zummo <a.zummo@towertech.it>,
        linux-rtc@vger.kernel.org
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Hi Alexandre,

On Thu, Aug 13, 2020 at 5:33 PM Alexandre Belloni
<alexandre.belloni@bootlin.com> wrote:
>
> Hi,
>
> On 13/08/2020 15:41:34+1000, Victor Ding wrote:
> > cmos_read_alarm() may leave certain fields of a struct rtc_time
> > untouched; therefore, these fields contain garbage if not properly
> > initialized, leading to inconsistent values when converting into
> > time64_t.
> > This patch to set all fields of a struct rtc_time to -1 before calling
> > cmos_read_alarm().
> >
>
> I don't think this actually helps with the conversion as mktime64
> is taking unsigned int so I would think you need the whole logic that is
> in __rtc_read_alarm

It's true that this change does not produce a correct time64_t; however,
it isn't the intention either. The proposed change only produces a
consistent value: calling obtaining identical struct rtc_time if the CMOS
wakealarm is unchanged. In the case of suspend/resume, a correct value
time64_t is not necessary; a consistent value is sufficient to correctly
perform an equality test for `t_current_expires` and `t_saved_expires`.
Logic to deduce a correct time64_t is expensive and hence I would like to
avoid __rtc_read_alarm's logic here.

Prior to this patch, the struct rtc_time is uninitialized. After calling
cmos_read_alarm(), the tm_year field is always left untouched and hence
contains only garbage. On platforms without enhanced RTC timers, the
tm_mon and tm_mday fields are left with garbage as well. Therefore,
`t_current_expires` and `t_saved_expires` from garbage data, which leads
to incorrect equality test results.

>
> > Signed-off-by: Victor Ding <victording@google.com>
> > ---
> >
> >  drivers/rtc/rtc-cmos.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/drivers/rtc/rtc-cmos.c b/drivers/rtc/rtc-cmos.c
> > index bcc96ab7793f..c99af567780d 100644
> > --- a/drivers/rtc/rtc-cmos.c
> > +++ b/drivers/rtc/rtc-cmos.c
> > @@ -1006,6 +1006,7 @@ static int cmos_suspend(struct device *dev)
> >                       enable_irq_wake(cmos->irq);
> >       }
> >
> > +     memset(&cmos->saved_wkalrm.time, -1, sizeof(struct rtc_time));
> >       cmos_read_alarm(dev, &cmos->saved_wkalrm);
> >
> >       dev_dbg(dev, "suspend%s, ctrl %02x\n",
> > @@ -1054,6 +1055,7 @@ static void cmos_check_wkalrm(struct device *dev)
> >               return;
> >       }
> >
> > +     memset(&current_alarm.time, -1, sizeof(struct rtc_time));
> >       cmos_read_alarm(dev, &current_alarm);
> >       t_current_expires = rtc_tm_to_time64(&current_alarm.time);
> >       t_saved_expires = rtc_tm_to_time64(&cmos->saved_wkalrm.time);
> > --
> > 2.28.0.236.gb10cc79966-goog
> >
>
> --
> Alexandre Belloni, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com

Best regards,
Victor Ding