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

[Julien Grall <julien.grall@linaro.org>]

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@linaro.org>
>>
>> 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).

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.

-- 
Julien