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

[Doug Anderson]

Christopher,

On Thu, Sep 11, 2014 at 8:58 AM, Christopher Covington
<cov@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxx>
>> Signed-off-by: Sonny Rao <sonnyrao@xxxxxxxxxxxx>
>> ---
>>  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?

Also, I just want to make sure you saw the part of the patch
description that says:
  In order for this patch to do anything useful, we also need Sonny's
  patch at <https://patchwork.kernel.org/patch/4790921/>

-Doug
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