Re: [Xen-devel] [PATCH] xen/arm: Set up Versatile Express timer frequency to 24 Mhz

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 26 Jun 2013, Tim Deegan wrote:
> At 09:05 +0900 on 26 Jun (1372237555), Sengul Thomas wrote:
> > >
> > > Timers on ARM seems a bit complex. The current issue is with arch timer
> > > (which is also used by dom0).
> > 
> > Just out of curiosity, what is the difference between arch timer and
> > platform timer?
> 
> The arch timer has its interface specified in the CPU manual and
> required (IIRC) for any processor that supports the virt extensions.
> The platform timers are whatever other time sources come on the SoC
> you're running on.
> 
> Having thought about this a bit, ISTR that the register in Julien's
> patch doesn't control anything -- it's there to tell the arch timer's
> users how fast that timer is going, so it ought to be set up by whatever
> sets up the timer itself (turning on oscillators &c).
> 
> So the right thing to do is to move that write into more board-specific
> start-of-day code (and eventually into the boot-wrapper), and rely on
> the bootloader/firmware to have done it in the general case.

I think that you are right


> On a real
> TC2 is that register already correct at (xen's) boot time?
> 
> But presumably just changing it from 100KHz to 24KHz for all vexpresses
> is wrong because that will break things on the software models in the
> opposite way.

I think we should read CNTFRQ at boot time rather than trying to write
it. We should be able to cope with whatever frequency has been setup by
the firmware, right?

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