Hi,

Le mar. 28 sept. 2021 à 15:31, Daniel Palmer [off-list ref] a écrit :

Hi Colin,

On Tue, 28 Sept 2021 at 21:39, Colin King [off-list ref] wrote:

quoted

Shifting the u16 value returned by readw by 16 bits to the left
will be promoted to a 32 bit signed int and then sign-extended
to an unsigned long. If the top bit of the readw is set then
the shifted value will be sign extended and the top 32 bits of
the result will be set.

Good catch !

Ah,.. C is fun in all the wrong places. :)
These chips are full of 32bit registers that are split into two 16
registers 4 bytes apart when seen from the ARM CPU so we probably have
this same mistake in a few other places.

A similar pattern is used a bit later on in the same file to read the counter:

seconds = readw(priv->rtc_base + REG_RTC_CNT_VAL_L)
| (readw(priv->rtc_base + REG_RTC_CNT_VAL_H) << 16);

I guess it works at the moment because the top bit won't be set until 2038.

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 :) (probably
because *this case* specifically did not happen while running the test)

1. https://git.kernel.org/pub/scm/linux/kernel/git/abelloni/rtc-tools.git/tree/rtc-range.c

Thanks,
Regards,
Romain

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