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From:   Dmitry Torokhov <dtor@google.com>
Date:   Wed, 2 Oct 2019 08:20:34 -0700
Message-ID: <CAE_wzQ9AodXUEANpDEQM+VYMVuxWmLoF0_1k-m5HdAfx+=01-A@mail.gmail.com>
Subject: Re: [PATCH v3] rtc: wilco-ec: Handle reading invalid times
To:     Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc:     Nick Crews <ncrews@chromium.org>,
        Alessandro Zummo <a.zummo@towertech.it>,
        linux-rtc@vger.kernel.org, lkml <linux-kernel@vger.kernel.org>,
        Pavel Machek <pavel@ucw.cz>, enric.balletbo@collabora.com,
        Benson Leung <bleung@chromium.org>, dlaurie@chromium.org,
        Daniel Kurtz <djkurtz@chromium.org>
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On Wed, Oct 2, 2019 at 3:32 AM Alexandre Belloni
<alexandre.belloni@bootlin.com> wrote:
>
> On 01/10/2019 13:42:24-0700, Dmitry Torokhov wrote:
> > On Tue, Oct 1, 2019 at 12:53 PM Alexandre Belloni
> > <alexandre.belloni@bootlin.com> wrote:
> > >
> > > Hi Nick,
> > >
> > > On 25/09/2019 14:32:09-0600, Nick Crews wrote:
> > > > If the RTC HW returns an invalid time, the rtc_year_days()
> > > > call would crash. This patch adds error logging in this
> > > > situation, and removes the tm_yday and tm_wday calculations.
> > > > These fields should not be relied upon by userspace
> > > > according to man rtc, and thus we don't need to calculate
> > > > them.
> > > >
> > > > Signed-off-by: Nick Crews <ncrews@chromium.org>
> > > > ---
> > > >  drivers/rtc/rtc-wilco-ec.c | 13 +++++++++----
> > > >  1 file changed, 9 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/drivers/rtc/rtc-wilco-ec.c b/drivers/rtc/rtc-wilco-ec.c
> > > > index 8ad4c4e6d557..53da355d996a 100644
> > > > --- a/drivers/rtc/rtc-wilco-ec.c
> > > > +++ b/drivers/rtc/rtc-wilco-ec.c
> > > > @@ -110,10 +110,15 @@ static int wilco_ec_rtc_read(struct device *dev, struct rtc_time *tm)
> > > >       tm->tm_mday     = rtc.day;
> > > >       tm->tm_mon      = rtc.month - 1;
> > > >       tm->tm_year     = rtc.year + (rtc.century * 100) - 1900;
> > > > -     tm->tm_yday     = rtc_year_days(tm->tm_mday, tm->tm_mon, tm->tm_year);
> > > > -
> > > > -     /* Don't compute day of week, we don't need it. */
> > > > -     tm->tm_wday = -1;
> > > > +     /* Ignore other tm fields, man rtc says userspace shouldn't use them. */
> > > > +
> > > > +     if (rtc_valid_tm(tm)) {
> > > > +             dev_err(dev,
> > > > +                      "Time from RTC is invalid: second=%u, minute=%u, hour=%u, day=%u, month=%u, year=%u, century=%u",
> > > > +                      rtc.second, rtc.minute, rtc.hour, rtc.day, rtc.month,
> > > > +                      rtc.year, rtc.century);
> > >
> > > Do you mind using %ptR? At this point you already filled the tm struct
> > > anyway and if you print century separately, you can infer tm_year.
> >
> > I do not think this is a good idea: we have just established that tm
> > does not contain valid data. Does %ptR guarantee that it handles junk
> > better than, let's say, rtc_year_days(), and does not crash when
> > presented with garbage?
> >
>
> It is safe to use. You can also use %ptRr if you want to ensure no
> extra operations are done on the value before printing them out.

OK, I'll keeo this in mind then.

>
> I'm still not convinced it is useful to have an error in dmesg when the
> time is invalid, as long as userspace knows it is invalid. What is the
> course of action for the end user when that happens?

Report it, or, in our case, we will see it in the feedback logs.
However I do agree that it is not the best option, even if we report
error to userspace I am not sure if it will handle it properly. What
userspace is supposed to do when presented with -EIO or similar?

Nick, do we know the root cause of the EC/RTC reporting invalid time?

Thanks,
Dmitry