Subject: [PATCH v2] clocksource: arch_timer: Allow the device tree to specify the physical timer

From: [email-masked] (Marc Zyngier)
Date: Thu, 11 Sep 2014 18:00:24 +0100

On 11/09/14 17:47, Will Deacon wrote:
> On Thu, Sep 11, 2014 at 05:16:44PM +0100, Doug Anderson wrote:
>> Some 32-bit (ARMv7) systems are architected like this:
>>
>> * The firmware doesn't know and doesn't care about hypervisor mode and
>>   we don't want to add the complexity of hypervisor there.
>>
>> * The firmware isn't involved in SMP bringup or resume.
>>
>> * The ARCH timer come up with an uninitialized offset between the
>>   virtual and physical counters.  Each core gets a different random
>>   offset.
>>
>> On systems like the above, it doesn't make sense to use the virtual
>> counter.  There's nobody managing the offset and each time a core goes
>> down and comes back up it will get reinitialized to some other random
>> value.
> 
> You probably need to rephrase this slightly, as there *is* still a
> requirement on the hypervisor/firmware (actually, two!). See below.
> 
>> Let's add a property to the device tree to say that we shouldn't use
>> the virtual timer.  Firmware could potentially remove this property
>> before passing the device tree to the kernel if it really wants the
>> kernel to use a virtual timer.
>>
>> Note that it's been said that ARM64 (ARMv8) systems the firmware and
>> kernel really can't be architected as described above.  That means
>> using the physical timer like this really only makes sense for ARMv7
>> systems.
> 
> I'd go further: this only makes sense if you're booting in secure SVC
> mode.

If that's the case, what's the problem? Enter monitor mode, set SCR.NS
to one, nuke CNTVOFF, revert, job done.

What am I missing?

	M.
-- 
Jazz is not dead. It just smells funny...