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Date: Tue, 10 Jul 2018 11:18:34 +1000 (AEST)
From: Finn Thain <fthain@telegraphics.com.au>
To: Arnd Bergmann <arnd@arndb.de>
cc: Meelis Roos <mroos@linux.ee>, Mathieu Malaterre <malat@debian.org>,
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Subject: Re: [PATCH 1/3] [v2] powerpc: mac: fix rtc read/write functions
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On Mon, 9 Jul 2018, Arnd Bergmann wrote:

> 
> The most likely explanation I have here is that the RTC was indeed set 
> to an incorrect date, either because of a depleted battery (not unlikely 
> for a ~15 year old box) or because it was previously stored incorrectly.

The PowerMac stores the GMT offset in NVRAM, and this gets used to 
initialize timezone_offset.

If timezone_offset was negative and now.tv_sec was zero, I think this 
could store a 1969 date in the RTC:

int update_persistent_clock64(struct timespec64 now)
{
        struct rtc_time tm;

        if (!ppc_md.set_rtc_time)
                return -ENODEV;

        rtc_time64_to_tm(now.tv_sec + 1 + timezone_offset, &tm);

        return ppc_md.set_rtc_time(&tm);
}

But maybe now.tv_sec can be shown to be greater than timezone_offset.

Then, what would happen when the timezone in /etc/localtime disagrees with 
the timezone_offset stored in NVRAM (PRAM)?

Besides that, if the battery went flat, what use is a backtrace? Why not 
scrap the WARN_ON()?

--