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From:   Daniel Palmer <daniel@0x0f.com>
Date:   Wed, 29 Sep 2021 21:56:36 +0900
Message-ID: <CAFr9PXna7YsYnfdtu1jvJhkVSX0SiyixYr_bTsx3tDepeHqcMg@mail.gmail.com>
Subject: Re: [PATCH][next] rtc: msc313: Fix unintentional sign extension issue
 on left shift of a u16
To:     Romain Perier <romain.perier@gmail.com>
Cc:     Colin King <colin.king@canonical.com>,
        Daniel Palmer <daniel@thingy.jp>,
        Alessandro Zummo <a.zummo@towertech.it>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        Nobuhiro Iwamatsu <iwamatsu@nigauri.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-rtc@vger.kernel.org, kernel-janitors@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Hi Romain,

On Tue, 28 Sept 2021 at 22:55, Romain Perier <romain.perier@gmail.com> wrot=
e:
>
> Hi,
>
> Le mar. 28 sept. 2021 =C3=A0 15:31, Daniel Palmer <daniel@0x0f.com> a =C3=
=A9crit :
> The crazy stuff being, I ran rtctest from selftests and rtc-range (1)
> that tests a variety
> of dates including 2038 and 2106 for example. Both tests passed :) (proba=
bly
> because *this case* specifically did not happen while running the test)

I suspect it works because for reading the time because seconds is a
u32 not unsigned long like the other functions.
So if the high word of the register is read, is promoted to a wider
type and sign extended it doesn't actually matter because it gets
truncated to 32 bits so the sign extended part is gone.

Cheers,

Daniel