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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-06-25 at 16:46 +0100, Julien Grall wrote:
> On 06/25/2013 04:31 PM, Ian Campbell wrote:
> 
> > 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?
> 
> 
> Timers on ARM seems a bit complex. The current issue is with arch timer
> (which is also used by dom0).

Oh, sorry, I somehow thought we were talking about the platform timers,
never mind me!

> 
> As I understand, there are also a bunch of clocks for different device
> (UARTs, I2C,...). If theses clocks is not "pass-through" to dom0 nothing
> will work.
> 



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