[PATCH] ARM: EXYNOS: Fix coupled CPU idle freeze on Exynos4210

[Marc Zyngier]

On 21/03/18 10:18, Marek Szyprowski wrote:
> Hi Marc,
> 
> On 2018-03-21 11:05, Marc Zyngier wrote:
>> On 21/03/18 09:45, Marek Szyprowski wrote:
>>> Since commit 04c8b0f82c7d ("irqchip/gic: Make locking a BL_SWITCHER only
>>> feature") coupled CPU idle freezes from time to time on Exynos4210. Later
>>> commit 313c8c16ee62 ("PM / CPU: replace raw_notifier with atomic_notifier")
>>> changed the context in which the CPU idle code is executed, what results
>>> in fully reproducible freeze all the time. However, almost the same coupled
>>> CPU idle code works fine on Exynos3250 regarless of the changes made in
>>> the mentioned commits.
>>>
>>> It turned out that the IPI call used on Exynos4210 is conflicting with the
>>> change done in the first mentioned commit in GIC. Fix this by using the
>>> same code path as for Exynos3250, instead of the IPI call for
>>> synchronization with second CPU core, call dsb_sev() directly.
>>>
>>> Tested on Exynos4210-based Trats and Origen boards.
>>>
>>> Signed-off-by: Marek Szyprowski <m.szyprowski at samsung.com>
>>> CC: stable at vger.kernel.org # v4.13+
>>> ---
>>>   arch/arm/mach-exynos/pm.c | 6 +-----
>>>   1 file changed, 1 insertion(+), 5 deletions(-)
>>>
>>> diff --git a/arch/arm/mach-exynos/pm.c b/arch/arm/mach-exynos/pm.c
>>> index dc4346ecf16d..a1055a2b8d54 100644
>>> --- a/arch/arm/mach-exynos/pm.c
>>> +++ b/arch/arm/mach-exynos/pm.c
>>> @@ -271,11 +271,7 @@ static int exynos_cpu0_enter_aftr(void)
>>>   				goto fail;
>>>   
>>>   			call_firmware_op(cpu_boot, 1);
>>> -
>>> -			if (soc_is_exynos3250())
>>> -				dsb_sev();
>>> -			else
>>> -				arch_send_wakeup_ipi_mask(cpumask_of(1));
>>> +			dsb_sev();
>>>   		}
>>>   	}
>>>   fail:
>>>
>> I'm a bit puzzled here.
>>
>> If the GIC change broke something on your platform, it probably also
>> broke something for all other users of arch_send_wakeup_ipi_mask. But
>> nobody reported anything until now. Also, it doesn't look like 4210 is a
>> big-little platform, so it is unlikely to use the big-little switcher.
>>
>> What is specific to this system that makes misbehave? Were you
>> implicitly relying on the BL lock to perform some serialization?
> 
> My hypothesis, after some internal discussion, is that dsb_sev() should be
> there from the beginning, but instead there was an 
> arch_send_wakeup_ipi_mask()
> call, which indirectly called dsb_sev() as a part of GIC spinlock internals.
> When spinlock has been removed from GIC, there is no dsb_sev() call anymore,
> what causes synchronization issue.

That's pretty scary. Glad you pinpointed it in the end.

FWIW:

Acked-by: Marc Zyngier <marc.zyngier at arm.com>

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