Re: [Qemu-devel] [PATCH v2]booke: timer: Deactivate timer for target_bit above 61

[Alexander Graf]

On 10.06.2013, at 19:20, Scott Wood wrote:

> On 06/10/2013 09:26:18 AM, Alexander Graf wrote:
>> On 06/10/2013 02:47 PM, Bhushan Bharat-R65777 wrote:
>>>> -----Original Message-----
>>>> From: Andreas Färber [mailto:address@hidden
>>>> Sent: Monday, June 10, 2013 5:43 PM
>>>> To: Bhushan Bharat-R65777
>>>> Cc: address@hidden; address@hidden; address@hidden; Wood Scott-
>>>> B07421; Bhushan Bharat-R65777
>>>> Subject: Re: [Qemu-devel] [PATCH v2]booke: timer: Deactivate timer for
>>>> target_bit above 61
>>>> So IIUC we can only allow 63 bits due to signedness, thus a maximum of
>>>> (1<<  62), thus target_bit<= 61.
>>>> Any chance at least the comment can be worded to explain that any better? 
>>>> Maybe
>>>> also use (target-bit + 1>= 63) or period>  INT64_MAX as condition?
>>> How about this:
>>>     /* QEMU timer supports a maximum timer of INT64_MAX 
>>> (0x7fffffff_ffffffff).
>>>      * Run booke fit/wdog timer when
>>>      * ((1ULL<<  target_bit + 1)<  0x40000000_00000000), i.e target_bit = 
>>> 61.
>>>      * Also the time with this maximum target_bit (with current range of
>>>      * CPU frequency PowerPC supports) will be many many years. So it is
>>>      * pretty safe to stop the timer above this threshold. */
>> How about
>>  /* This timeout will take years to trigger. Treat the timer as disabled. */
> 
> There should be at least a brief mention that it's because the QEMU timer 
> can't handle larger values,

If it can't handle higher values, maybe it's better to just set the timer value 
to INT64_MAX when we detect an overflow? That would make the code plainly 
obvious.


Alex

> with the detailed explanation in the changelog.  A better lower bound on the 
> number of years would be nice as well (e.g. "hundreds of years").
> 
> -Scott