Re: [PATCH for-4.22 1/5] x86/time: use RTC century byte when available

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 5/12/26 4:58 PM, Jan Beulich wrote:
> Without this the present logic will misbehave from 2070 onwards.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Generally it looks like 4.22 won't be used in 2070 or higher (I am 
curious do we have similar use cases now that very old Xen version is 
used nowadays?) but the patch looks pretty straightforward:
 Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

Thanks.

~ Oleksii

> --- a/xen/arch/x86/time.c
> +++ b/xen/arch/x86/time.c
> @@ -1257,6 +1257,7 @@ struct rtc_time {
>   static bool __get_cmos_time(struct rtc_time *rtc)
>   {
>       s_time_t start, t1, t2;
> +    unsigned int century = 0;
>       unsigned long flags;
>   
>       spin_lock_irqsave(&rtc_lock, flags);
> @@ -1280,6 +1281,8 @@ static bool __get_cmos_time(struct rtc_t
>       rtc->day  = CMOS_READ(RTC_DAY_OF_MONTH);
>       rtc->mon  = CMOS_READ(RTC_MONTH);
>       rtc->year = CMOS_READ(RTC_YEAR);
> +    if ( acpi_gbl_FADT.century && acpi_gbl_FADT.century < 0x80 )
> +        century = CMOS_READ(acpi_gbl_FADT.century);
>       
>       if ( RTC_ALWAYS_BCD || !(CMOS_READ(RTC_CONTROL) & RTC_DM_BINARY) )
>       {
> @@ -1293,7 +1296,12 @@ static bool __get_cmos_time(struct rtc_t
>   
>       spin_unlock_irqrestore(&rtc_lock, flags);
>   
> -    if ( (rtc->year += 1900) < 1970 )
> +    if ( century )
> +    {
> +        BCD_TO_BIN(century);
> +        rtc->year += century * 100;
> +    }
> +    else if ( (rtc->year += 1900) < 1970 )
>           rtc->year += 100;
>   
>       return t1 <= SECONDS(1) && t2 < MILLISECS(3);