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

[Tim Deegan <tim@xxxxxxx>]

At 11:24 +0100 on 26 Jun (1372245863), Stefano Stabellini wrote:
> 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?

Yes -- in the case where the firmware has done it, we should be reading
this register rather than writing it. 

On the software VE model the code was developed on, there is no firmware
and so Xen has to set this up along with the other things that we'd
expect firmware to do.  So we can move that write to the start-fo-day
board-specific asm hacks, and the actual driver code can just read it.

Cheers,

Tim.

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