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

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 06/24/2013 03:52 PM, Stefano Stabellini wrote:

> On Mon, 24 Jun 2013, 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.
> 
> Linux gets the frequency from:
> 
> 1) device tree
> 2) the arch timer register
> 
> in this order.
> Either way it should work.
> 
> Could it be that Xen is failing to set the frequency on the versatile
> express? Therefore the hardware is running the timer at a different
> frequency than Xen is expecting it to?



Lots of place in the DTS and the board configuration use a frequency of
24Mhz.
On the arndale, the timer frequency is 24Mhz, why do we need 100Mhz on
the versatile express?

It seems U-boot uses SP810 to modify the clock frequency on the
versatile express but I can't find documentation about the different
registers.

-- 
Julien

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