[PATCH v2] arm: zynq: Fix system clock with multi_v7_defconfig

[Arnd Bergmann]

On Friday 10 April 2015 17:09:01 Ola Jeppsson wrote:
> On 2015-04-10 15:44, Arnd Bergmann wrote:
> > On Monday 23 March 2015 08:05:45 Michal Simek wrote:
> >> Hi,
> >>
> >> On 03/23/2015 02:39 AM, Ola Jeppsson wrote:
> >>> As mentioned in this commit:
> >>> arm: zynq: Don't use arm_global_timer with cpufreq
> >>> 61f1fc7e9258a169ac8afb5ddf657a181e60d052
> >>>
> >>> arm_global_timer depends on the CPU frequency. With cpufreq altering the
> >>> CPU frequency arm_global_timer will not maintain a stable time base. So
> >>> arm_global_timer must not be the clocksource when cpufreq is enabled.
> >>>
> >>> The above commit tries to solve this at build time by only selecting
> >>> CONFIG_ARM_GLOBAL_TIMER if CONFIG_CPU_FREQ is disabled. This is not
> >>> always sufficient because other machs can also enable
> >>> CONFIG_ARM_GLOBAL_TIMER.
> >>>
> >>> Therefore: If built with CONFIG_CPU_FREQ and CONFIG_ARM_GLOBAL_TIMER,
> >>> disable (on Zynq) the arm_global_timer devicetree node at boot before
> >>> clock sources are initialized. This ensures that arm_global_timer will
> >>> not be selected clocksource.
> >>>
> >>> Signed-off-by: Ola Jeppsson <ola at adapteva.com>
> >
> >> Arnd: Waiting for your thoughts on this one?
> >> It is some sort of arch/arm/mach-mvebu/board-v7.c quirk code.
> >>
> >> I don't think it can be done via Kconfig because it will affect others.
> > (catching up with old email)
> >
> > I think it's ok to work around the problem like this in principle.
> >
> > However, I have one concern about the way this condition is detected.
> > At the moment, you assume that all zynq variants suffer from this
> > problem and no other chip has it.
> >
> > How would you handle the situation if a future zynq variant fixes
> > the problem?
> The check is only applicable on Cortex A9 zynqs.
> Assume for now that it is broken on all those variants. If it gets fixed on some variant add a
> of_machine_is_compatible() check?

I think it would be better to avoid those if we can.

> > What if it turns out to be a more common problem and we actually
> > want to work around it in the arm global timer implementation
> > by dynamically adapting to CPU frequency changes? I believe some
> > other drivers (x86 tsc?) do this. Would we be able to detect this case
> > on zynq?
> >
> If  the arm global timer implementation gets cpufreq scaling support,
> this check isn't needed and should be removed?

The timer code however would still need to find out whether to scale the
frequence along with the CPU or not.

> The only thing that happens if it is left in is that you end up with
> an inferior, but correct, clock source (which is what you already had
> before cpufreq support in arm-global-timer).

I've just had another idea: how about introducing a new compatible string
for the global timer that gets used for timers that have their frequency
tied to the CPU (alternatively a bool property in that node). This can
be checked by the clocksource driver, which will then be able to either
skip the device if cpufreq is in use, or implement a more sophisticated
workaround.

This would keep the hack outside of platform code and make it immediately
available to any platform with the same problem. The only zynq specific
hack you need is to modify that property on old dtb files that you want
to keep supporting with new kernels (if any).

	Arnd