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

[Scott Wood]

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, 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