linux-kernel - Re: [PATCH 1/3] [v2] powerpc: mac: fix rtc read/write functions

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Date:   Wed, 27 Jun 2018 14:32:17 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Arnd Bergmann <arnd@...db.de>, Paul Mackerras <paulus@...ba.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Joshua Thompson <funaho@...ai.org>
Cc:     Mathieu Malaterre <malat@...ian.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        linux-m68k@...ts.linux-m68k.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, y2038@...ts.linaro.org,
        Meelis Roos <mroos@...ux.ee>,
        Andreas Schwab <schwab@...ux-m68k.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 1/3] [v2] powerpc: mac: fix rtc read/write functions

Arnd Bergmann <arnd@...db.de> writes:

> As Mathieu pointed out, my conversion to time64_t was incorrect and resulted
> in negative times to be read from the RTC. The problem is that during the
> conversion from a byte array to a time64_t, the 'unsigned char' variable
> holding the top byte gets turned into a negative signed 32-bit integer
> before being assigned to the 64-bit variable for any times after 1972.
>
> This changes the logic to cast to an unsigned 32-bit number first for
> the Macintosh time and then convert that to the Unix time, which then gives
> us a time in the documented 1904..2040 year range. I decided not to use
> the longer 1970..2106 range that other drivers use, for consistency with
> the literal interpretation of the register, but that could be easily
> changed if we decide we want to support any Mac after 2040.
>
> Just to be on the safe side, I'm also adding a WARN_ON that will trigger
> if either the year 2040 has come and is observed by this driver, or we
> run into an RTC that got set back to a pre-1970 date for some reason
> (the two are indistinguishable).
>
> For the RTC write functions, Andreas found another problem: both
> pmu_request() and cuda_request() are varargs functions, so changing
> the type of the arguments passed into them from 32 bit to 64 bit
> breaks the API for the set_rtc_time functions. This changes it
> back to 32 bits.
>
> The same code exists in arch/m68k/ and is patched in an identical way now
> in a separate patch.
>
> Fixes: 5bfd643583b2 ("powerpc: use time64_t in read_persistent_clock")
> Reported-by: Mathieu Malaterre <malat@...ian.org>
> Reported-by: Andreas Schwab <schwab@...ux-m68k.org>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  arch/powerpc/platforms/powermac/time.c | 29 ++++++++++++++++++++---------
>  1 file changed, 20 insertions(+), 9 deletions(-)

So I think I can take this patch in isolation via the powerpc tree as a
fix for 4.18.

I'll leave the other two alone.

cheers