Re: [PATCH][next] rtc: msc313: Fix unintentional sign extension issue on left shift of a u16

From: Daniel Palmer
Date: Tue Sep 28 2021 - 09:31:22 EST

Hi Colin,

On Tue, 28 Sept 2021 at 21:39, Colin King <colin.king@xxxxxxxxxxxxx> wrote:
>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.

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.