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

[Christopher Covington <cov@codeaurora.org>]

On 09/11/2014 12:04 PM, Doug Anderson wrote:
> Christopher,
> 
> On Thu, Sep 11, 2014 at 8:58 AM, Christopher Covington
> <cov@codeaurora.org> wrote:
>> Hi Doug,
>>
>> On 09/11/2014 11:52 AM, 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.
>>>
>>> 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.
>>>
>>> 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@chromium.org>
>>> Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
>>> ---
>>>  Documentation/devicetree/bindings/arm/arch_timer.txt | 6 ++++++
>>>  drivers/clocksource/arm_arch_timer.c                 | 3 +++
>>>  2 files changed, 9 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.
>>> +
>>> +
>>>  Example:
>>>
>>>       timer {
>>> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
>>> index 5163ec1..8ca07a9 100644
>>> --- a/drivers/clocksource/arm_arch_timer.c
>>> +++ b/drivers/clocksource/arm_arch_timer.c
>>> @@ -649,6 +649,9 @@ static void __init arch_timer_init(struct device_node *np)
>>>               arch_timer_ppi[i] = irq_of_parse_and_map(np, i);
>>>       arch_timer_detect_rate(NULL, np);
>>>
>>> +     if (of_property_read_bool(np, "arm,use-physical-timer"))
>>> +             arch_timer_use_virtual = false;
>>> +
>>>       /*
>>>        * If HYP mode is available, we know that the physical timer
>>>        * has been configured to be accessible from PL1. Use it, so
>>>
>>
>> How's the VDSO supposed to deal with this? It currently does:
>>
>> cycle_now = arch_counter_get_cntvct()
> 
> I don't see that line of code anywhere when I do a "git grep" on linux
> or linuxnext.  Can you give me a clearer pointer?

As Nathan mentioned, the code on the 32 bit side isn't merged yet.

http://www.spinics.net/lists/arm-kernel/msg356336.html

Christopher

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