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

[Doug Anderson]

Will,

On Thu, Sep 11, 2014 at 9:47 AM, Will Deacon <will.deacon@xxxxxxx> 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.

Sure.  I added two more bullet points.


>> 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.

OK.


>> In order for this patch to do anything useful, we also need Sonny's
>> patch at <https://patchwork.kernel.org/patch/4790921/>
>>
>> Signed-off-by: Doug Anderson <dianders@xxxxxxxxxxxx>
>> Signed-off-by: Sonny Rao <sonnyrao@xxxxxxxxxxxx>
>> ---
>> Changes in v2:
>> - Add "#ifdef CONFIG_ARM" as per Will Deacon
>>
>>  Documentation/devicetree/bindings/arm/arch_timer.txt | 6 ++++++
>>  drivers/clocksource/arm_arch_timer.c                 | 5 +++++
>>  2 files changed, 11 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/arm/arch_timer.txt b/Documentation/devicetree/bindings/arm/arch_timer.txt
>> index 37b2caf..876d32b 100644
>> --- a/Documentation/devicetree/bindings/arm/arch_timer.txt
>> +++ b/Documentation/devicetree/bindings/arm/arch_timer.txt
>> @@ -22,6 +22,12 @@ to deliver its interrupts via SPIs.
>>  - always-on : a boolean property. If present, the timer is powered through an
>>    always-on power domain, therefore it never loses context.
>>
>> +** Optional properties:
>> +
>> +- arm,use-physical-timer : Don't ever use the virtual timer, just use the
>> +  physical one.  Not supported for ARM64.
>
> I'd say `Only supported for ARM' to better match what we've done. Probably
> also worth mentioning that this relies on the hypervisor/firmware having set
> CNTHCTL.PL1PCEN and CNTHCTL.EL1PCTEN (but assumedly made a mess of CNTVOFF
> ;) if you want to boot on the non-secure side (e.g. as a guest).

Note that the reset value of CNTHCTL.PL1PCEN and CNTHCTL.PL1PCTEN are
both 1 in my version of the ARM ARM.  On the other hand CNTVOFF is
documented to have an UNKNOWN reset value.  If only ARM had guaranteed
that CNTVOFF started out as 0 (which seems like it would have been
sensible) we wouldn't be in this mess.  :-/

I've adjusted the wording.  Hopefully it looks good to you.

-Doug
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