[PATCH 03/17] ARM: gic: Use cpu pm notifiers to save gic state

[Colin Cross]

On Thu, Jul 21, 2011 at 3:46 AM, Santosh Shilimkar
<santosh.shilimkar at ti.com> wrote:
> On 7/21/2011 3:57 PM, Lorenzo Pieralisi wrote:
>>
>> On Thu, Jul 21, 2011 at 09:32:12AM +0100, Santosh Shilimkar wrote:
>>>
>>> Lorenzo, Colin,
>>>
>>> On 7/7/2011 9:20 PM, Lorenzo Pieralisi wrote:
>>>>
>>>> From: Colin Cross<ccross at android.com>
>>>>
>>>> When the cpu is powered down in a low power mode, the gic cpu
>>>> interface may be reset, and when the cpu complex is powered
>>>> down, the gic distributor may also be reset.
>>>>
>>>> This patch uses CPU_PM_ENTER and CPU_PM_EXIT notifiers to save
>>>> and restore the gic cpu interface registers, and the
>>>> CPU_COMPLEX_PM_ENTER and CPU_COMPLEX_PM_EXIT notifiers to save
>>>> and restore the gic distributor registers.
>>>>
>>>> Signed-off-by: Colin Cross<ccross at android.com>
>>>> ---
>>>>   arch/arm/common/gic.c |  212
>>>> +++++++++++++++++++++++++++++++++++++++++++++++++
>>>>   1 files changed, 212 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/arch/arm/common/gic.c b/arch/arm/common/gic.c
>>>> index 4ddd0a6..8d62e07 100644
>>>> --- a/arch/arm/common/gic.c
>>>> +++ b/arch/arm/common/gic.c
>>>
>>> [...]
>>>
>>> I missed one more comment in the last review.
>>>
>>>> +static int gic_notifier(struct notifier_block *self, unsigned long cmd,
>>>>        void *v)
>>>> +{
>>>> +       int i;
>>>> +
>>>> +       for (i = 0; i<   MAX_GIC_NR; i++) {
>>>> +               switch (cmd) {
>>>> +               case CPU_PM_ENTER:
>>>> +                       gic_cpu_save(i);
>>>
>>> On OMAP, GIC cpu interface context is lost only when CPU cluster
>>> is powered down.
>>
>> Yes, it's true, but that's the only chance we have to save the GIC CPU IF
>> state if the GIC context is lost, right ?
>> It is a private memory map per processor; I agree, it might be useless
>> if just one CPU is shutdown, but at that point in time you do not know
>> the state of other CPUs. If the cluster moves to a state where GIC context
>> is lost at least you had the GIC CPU IF state saved. If we do not
>> save it, well, there is no way to do that anymore since the last CPU
>> cannot
>> access other CPUs GIC CPU IF registers (or better, banked GIC distributor
>> registers).
>> If you force hotplug on CPUs other than 0 (that's the way it is done on
>> OMAP4
>> in cpuidle, right ?) to hit deep low-power states you reinit the GIC CPU
>> IF
>> state as per cold boot, so yes, it is useless there.
>>
> Actually, on OMAP there is no need to save any CPU interface registers.
>
> For my OMAP4 PM rebasing, for time-being I will go with exported
> GIC functions so that I don't have too many redundancies with GIC
> save/restore code.

I think you should try to balance cpu idle latency with reuse of
common code.  In this case, you are avoiding restoring 7 registers by
reimplementing the bare minimum that is necessary for OMAP4, which is
unlikely to make a measurable impact on wakeup latency.  Can you try
starting with reusing all the common code, and add some timestamps
during wakeup to measure where the longest delays are, to determine
where you should diverge from the common code and use omap-optimized
code?