[PATCH v2 00/15] Make SMP timers standalone

[Shilimkar, Santosh]

On Thu, Jan 5, 2012 at 12:19 PM, Marc Zyngier <marc.zyngier at arm.com> wrote:
> On 05/01/12 11:13, Shilimkar, Santosh wrote:
>> On Thu, Jan 5, 2012 at 12:08 PM, Marc Zyngier <marc.zyngier at arm.com> wrote:
>>> On 05/01/12 00:00, Linus Walleij wrote:
>>>> However IIRC this situation does not occur in the ARM reference
>>>> designs, notably Versatile Express since it simply isn't designed to
>>>> be power-agressive in this way and does not gate the clock or
>>>> power down the CPU power domain, it's always on, always
>>>> clocked (albeit with shifting frequency).
>>>>
>>>> The Vexpress seem to register a clockevent for its SP804
>>>> timer, and even though I've never used this machine I guess
>>>> it would tick a few ticks during boot and then as TWD is
>>>> registered it switches to that (due to higher .rate) and
>>>> no IRQ is ever fired on the SP804 again.
>>>>
>>>> Does this correspond to what is seen in /proc/interrupts
>>>> on the Vexpress?
>>>
>>> Yes. You get about 8 ticks worth of SP804, and then switch to TWD for good.
>>>
>>>> And does the system really work if you simply delete the
>>>> code registering the SP804 clockevent from
>>>> mach-vexpress/ct-ca9x4.c?
>>>
>>> No, because you need the global timer to calibrate the TWD on this
>>> platform. But on the VE with a Cortex-A15 tile, the system works
>>> perfectly with the architected timers being the only one in the system
>>> (the SP804 is left unconfigured).
>>>
>>> But now that your cpufreq-aware patches are in -next, I can boot a Panda
>>> without a single tick of the global timer (OMAP4 provides the "smp-twd"
>>> clock).
>>>
>> That's only because CPUIDLE low power states are not enabled. You
>> do that and without global timer, system will just die
>
> Global timer is still present, configured and enabled. The kernel just
> happens to select TWD as a better timer. Should TWD be turned off,
> gp_timer will be selected again.
>
> This is also what happens with the current implementation, and my
> patches don't change that.
>
Great. Thanks for clarifying Marc.

Regards
Santosh