Re: [Qemu-ppc] [PATCH 1.7 v2 2/2] PPC: BookE: Make FIT/WDT timers at best millisecond grained

[Scott Wood]

On Mon, 2013-12-02 at 10:50 -0600, Tom Musta wrote:
> On 11/25/2013 3:46 PM, Alexander Graf wrote:
> > The default granularity for the FIT timer on 440 is on every 0x1000th
> > transition of TB from 0 to 1. Translated that means 48828 times a second.
> > 
> > Since interrupts are quite expensive for 440 and we don't really care
> > about the accuracy of the FIT to that significance, let's force FIT and
> > WDT to at best millisecond granularity.
> > 
> > This basically restores behavior as it was in QEMU 1.6, where timers
> > could only deal with millisecond granularities at all.
> > 
> > This patch greatly improves performance with the 440 target and restores
> > roughly the same performance level that QEMU 1.6 had for me.
> > 
> 
> @Reviewed-by: Tom Musta <address@hidden>
> 
> Alexander:
> 
> Is the problem the default?  If so, would it be better to change the default?
> (for example, set the default TCR_FP for the 440 to 0b10).  Of course, if your
> software is setting this back to 0b00, you will still have a problem.
> 
> The downside to your approach is that it eliminates the possibility for fine
> grained timer interrupts for all Book E implementations and all kernels and/or
> hypervisors.  I am not aware of any such system so my comment is purely
> theoretical.

FWIW, the decrementer is much more likely to be used than FIT/WDT for
fine grained timers.

-Scott