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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 28/07/14 14:32, Jan Beulich wrote:
>>>> 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

Silly me. I had mentally included the break in the if ( rtc.sec !=
seconds ) scope.

In which case, Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Unfortunately we don't have affected hardware to test with.

~Andrew

>
>>>>> +        {
>>>>> +            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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