Re: [PATCH v5 4/4] irqchip/gic-v3-its: add ability to resend MAPC on resume

[Marc Zyngier]

On 08/02/18 00:00, dbasehore . wrote:
> On Wed, Feb 7, 2018 at 3:22 PM, Brian Norris <briannorris@xxxxxxxxxxxx> wrote:
>> Hi Marc,
>>
>> I'm really not an expert on this, so take my observations with a large
>> grain of salt:
>>
>> On Wed, Feb 07, 2018 at 08:46:42AM +0000, Marc Zyngier wrote:
>>> On 07/02/18 01:41, Derek Basehore wrote:
>>>> This adds functionality to resend the MAPC command to an ITS node on
>>>> resume. If the ITS is powered down during suspend and the collections
>>>> are not backed by memory, the ITS will lose that state. This just sets
>>>> up the known state for the collections after the ITS is restored.
>>>>
>>>> This is enabled via the reset-on-suspend flag in the DTS for an ITS
>>>> that has a non-zero number of collections stored in it.
>>>>
>>>> Signed-off-by: Derek Basehore <dbasehore@xxxxxxxxxxxx>
>>>> ---
>>>>  drivers/irqchip/irq-gic-v3-its.c   | 80 ++++++++++++++++++++------------------
>>>>  include/linux/irqchip/arm-gic-v3.h |  1 +
>>>>  2 files changed, 43 insertions(+), 38 deletions(-)
>>>>
>>>> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
>>>> index 5e63635e2a7b..dd6cd6e68ed0 100644
>>>> --- a/drivers/irqchip/irq-gic-v3-its.c
>>>> +++ b/drivers/irqchip/irq-gic-v3-its.c
>>>> @@ -1942,52 +1942,53 @@ static void its_cpu_init_lpis(void)
>>>>     dsb(sy);
>>>>  }
>>>>
>>>> -static void its_cpu_init_collection(void)
>>>> +static void its_cpu_init_collection(struct its_node *its)
>>
>> ...
>>
>>>> @@ -3127,6 +3128,9 @@ static void its_restore_enable(void)
>>>>                     its_write_baser(its, baser, baser->val);
>>>>             }
>>>>             writel_relaxed(its->ctlr_save, base + GITS_CTLR);
>>>> +
>>>> +           if (GITS_TYPER_HWCOLLCNT(gic_read_typer(base + GITS_TYPER)) > 0)
>>>> +                   its_cpu_init_collection(its);
>>>
>>> This isn't correct. Think of a system where half the collections are in
>>> HW, and the other half memory based (nothing in the spec forbids this).
>>> You must evaluate the CID of each collection and replay the MAPC *only*
>>> if it falls into the range [0..HCC-1]. The memory-based collections are
>>> already mapped, and remapping an already mapped collection requires
>>> extra care (see MAPC and the UNPREDICTABLE behaviour when V=1), so don't
>>> go there.
>>
>> IIUC, this is only run on CPU0 (it's in syscore resume), so implicitly,
>> CID is 0. Thus, the current condition is already doing what you ask:
>>
>>         HCC > 0 == CID
>>
>> which is equivalent to:
>>
>>         HCC - 1 >= CID
>>
>> Or should we really double check what CPU we're running on?
> 
> There seems to be the edge case where you hotplug CPU 0 before
> suspending. In that case, I believe you're on the lowest number CPU
> left?

I don't think the core code makes any guarantee in that respect. This is
probably what happens in practice, but I wouldn't bet anything on this
being set in stone.

> It seems that all of the CPUs that are disabled have the ITS
> reinitialized from scratch via enable_nonboot_cpus(). This code runs
> on only the CPU that firmware resumes with. If that CPU isn't CPU 0
> for whatever reason, we need to make sure that it's processor ID is
> less than HCC.

Exactly, thanks for putting it better than I initially did.

	M.
-- 
Jazz is not dead. It just smells funny...