linux-kernel - Re: [PATCH v2 1/2] rtc: wilco-ec: Remove yday and wday calculations

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Date:   Mon, 23 Sep 2019 11:20:42 -0600
From:   Nick Crews <ncrews@...omium.org>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Pavel Machek <pavel@....cz>, Benson Leung <bleung@...omium.org>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Duncan Laurie <dlaurie@...omium.org>
Subject: Re: [PATCH v2 1/2] rtc: wilco-ec: Remove yday and wday calculations

On Sun, Sep 22, 2019 at 1:05 PM Alexandre Belloni
<alexandre.belloni@...tlin.com> wrote:
>
> On 22/09/2019 18:13:06+0200, Pavel Machek wrote:
> > On Mon 2019-09-16 12:12:15, Nick Crews wrote:
> > > The tm_yday and tm_wday fields are not used by userspace,
> > > so since they aren't needed within the driver, don't
> > > bother calculating them. This is especially needed since
> > > the rtc_year_days() call was crashing if the HW returned
> > > an invalid time.
> > >
> > > Signed-off-by: Nick Crews <ncrews@...omium.org>
> > > ---
> > >  drivers/rtc/rtc-wilco-ec.c | 4 ----
> > >  1 file changed, 4 deletions(-)
> > >
> > > diff --git a/drivers/rtc/rtc-wilco-ec.c b/drivers/rtc/rtc-wilco-ec.c
> > > index 8ad4c4e6d557..e84faa268caf 100644
> > > --- a/drivers/rtc/rtc-wilco-ec.c
> > > +++ b/drivers/rtc/rtc-wilco-ec.c
> > > @@ -110,10 +110,6 @@ 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;
> > >
> > >     return 0;
> >
> > Are you sure? It would be bad to pass unititialized memory to userspace...
> >
>
> This problem doesn't exist because userspace is passing the memory, not
> the other way around.
>
> > If userspace does not need those fields, why are they there?
> >
>
> This is coming from struct tm, it is part of C89 but I think I was not
> born when this decision was made. man rtc properly reports that those
> fields are unused and no userspace tools are actually making use of
> them. Nobody cares about the broken down representation of the time.
> What is done is use the ioctl then mktime to have a UNIX timestamp.
>
> "The mktime function ignores the specified contents of the tm_wday,
> tm_yday, tm_gmtoff, and tm_zone members of the broken-down time
> structure. It uses the values of the other components to determine the
> calendar time; it’s permissible for these components to have
> unnormalized values outside their normal ranges. The last thing that
> mktime does is adjust the components of the brokentime structure,
> including the members that were initially ignored."

This is very non-obvious and I only knew this from talking to you,
Alexandre. Perhaps we should add this note to the RTC core,
such as in the description for rtc_class_ops?

For this patch, do you want me to make any further changes?

Thanks,
Nick

>
>
> --
> Alexandre Belloni, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com