Re: [Xen-devel] [PATCH] x86/ACPI: allow CMOS RTC use even when ACPI says there is none

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.07.14 at 15:04, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 28/07/14 13:47, Jan Beulich wrote:
>>
>>>> +
>>>> +        start = NOW();
>>>> +        do { /* must try at least 2.228 ms */
>>>> +            t2 = NOW() - start;
>>>> +        } while ( (CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP) &&
>>>> +                  t2 < MILLISECS(3) );
>>>> +
>>>> +        __get_cmos_time(&rtc);
>>>> +
>>>> +        spin_unlock_irqrestore(&rtc_lock, flags);
>>>> +
>>>> +        if ( likely(!cmos_rtc_probe) ||
>>>> +             t1 > SECONDS(1) || t2 >= MILLISECS(3) ||
>>>> +             rtc.sec >= 60 || rtc.min >= 60 || rtc.hour >= 24 ||
>>>> +             !rtc.day || rtc.day > 31 ||
>>>> +             !rtc.mon || rtc.mon > 12 )
>>>>              break;
>>>> -    for ( i = 0 ; i < 1000000 ; i++ ) /* must try at least 2.228 ms */
>>>> -        if ( !(CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP) )
>>>> +
>>>> +        if ( seconds < 60 )
>>> Seconds doesn't appear to be updated before this point, meaning that we
>>> will reprobe even if we find a plausible RTC.
>> But that's exactly the point: We want to go through the loop twice.
>> Only if the second round results in updated seconds do we consider
>> the RTC okay for use.
> 
> Right, but in the case that the RTC is handing back static values (which
> is slightly more likely if we are probing something which might not be a
> CMOS RTC), we will sit in the loop forever.

We won't: If seconds is an invalid value, we bail in the first iteration.
If seconds is a valid value but unchanged on the second iteration,
we bail there (see the still available context below.

Jan

>>>> +        {
>>>> +            if ( rtc.sec != seconds )
>>>> +                cmos_rtc_probe = 0;
>>>>              break;
>>>> +        }
>>>> +
>>>> +        process_pending_softirqs();
>>>> +
>>>> +        seconds = rtc.sec;
>>>> +    }
>>>>  
>>>> -    res = __get_cmos_time();
>>>> +    if ( unlikely(cmos_rtc_probe) )
>>>> +        panic("No CMOS RTC found - system must be booted from EFI");



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