[PATCH v4] clocksource: arch_timer: Allow the device tree to specify uninitialized timer registers

[Doug Anderson]

Daniel,

On Wed, Oct 8, 2014 at 12:33 AM, Sonny Rao <sonnyrao at chromium.org> wrote:
> From: Doug Anderson <dianders at chromium.org>
>
> 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 (CNTVOFF)
>   between the virtual and physical counters.  Each core gets a
>   different random offset.
>
> * The device boots in "Secure SVC" mode.
>
> * Nothing has touched the reset value of CNTHCTL.PL1PCEN or
>   CNTHCTL.PL1PCTEN (both default to 1 at reset)
>
> 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.
>
> This adds an optional property which can inform the kernel of this
> situation, and firmware is free to remove the property if it is going
> to initialize the CNTVOFF registers when each CPU comes out of reset.
>
> Currently, the best course of action in this case is to use the
> physical timer, which is why it is important that CNTHCTL hasn't been
> changed from its reset value and it's a reasonable assumption given
> that the firmware has never entered HYP mode.
>
> Note that it's been said that on 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.
>
> Signed-off-by: Doug Anderson <dianders at chromium.org>
> Signed-off-by: Sonny Rao <sonnyrao at chromium.org>
> Reviewed-by: Mark Rutland <mark.rutland at arm.com>
> ---
> Changes in v2:
> - Add "#ifdef CONFIG_ARM" as per Will Deacon
>
> Changes in v3:
> - change property name to arm,cntvoff-not-fw-configured and specify
>   that the value of CNTHCTL.PL1PC(T)EN must still be the reset value
>   of 1 as per Mark Rutland
>
> Changes in v4:
> - change property name to arm,cpu-registers-not-fw-configured and
>   specify that all cpu registers must have architected reset values
>   per Mark Rutland
> - change from "#ifdef CONFIG_ARM" to "if (IS_ENABLED(CONFIG_ARM))" per
>   Arnd Bergmann
> ---
>  Documentation/devicetree/bindings/arm/arch_timer.txt | 8 ++++++++
>  drivers/clocksource/arm_arch_timer.c                 | 8 ++++++++
>  2 files changed, 16 insertions(+)

Do you know what the status of this patch is?  This patch and Sonny's
patch at <https://patchwork.kernel.org/patch/5051901/> are needed on
Rockchip rk3288 for some specific things:

1. To make SMP happy with coreboot.

2. To (I assume) make SMP happy after S2R (no matter which firmware is
used since I don't think anyone has PSCI for rk3288).

...we still need a DTS entry atop these patches, but that's trivial to
add once these land.

Thanks!