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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2013-06-24 at 14:51 +0100, Tim Deegan wrote:
> At 14:10 +0100 on 24 Jun (1372083058), Julien Grall wrote:
> > On TC2, the timer frequency was set to 100 Mhz which slows down the whole
> > platform.
> > When Linux is running on bare metal, the frequency is 24 Mhz.
> > 
> > "sleep 60" in dom0 takes:
> >   - 4 mins with a frequency equals to 100 Mhz
> >   - 1 min with a frequency equals to 24 Mhz
> > 
> > Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> 
> Is this a problem with the vexpress (e.g. the timer runs at 24MHz
> regardless of this setting, which would explain the otherwise odd
> numbers you give above), or just that linux code assumes the timer is
> always 24MHz?
> 
> If it's the latter, I think it needs to be fixed on the linux side.  Xen
> can't let the guest OS dictate things like this, since we might want to
> run two guests with different OSes.  Also, if linux changes its choice,
> we'd have trouble with old and new linux running together.

Isn't this timer only available to dom0, as the owner of most
peripherals?

Guests use the arch timers, because that is all they see.

Xen doesn't use this timer at all for any purpose AFAIK.

That said, it is a bit of an odd quirk that dom0 can see platform timers
too, especially if it can cause breakage, but maybe not as critical as
all that?

Ian.


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