linux-kernel - Re: [PATCH] irq_bcm2836: Send event when onlining sleeping cores

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Message-ID: <06fad93c-a850-a99a-ae60-7306f9179032@arm.com>
Date:   Wed, 10 May 2017 08:42:50 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Phil Elwell <phil@...pberrypi.org>, Eric Anholt <eric@...olt.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        bcm-kernel-feedback-list@...adcom.com,
        linux-kernel@...r.kernel.org, linux-rpi-kernel@...ts.infradead.org
Subject: Re: [PATCH] irq_bcm2836: Send event when onlining sleeping cores

On 09/05/17 20:02, Phil Elwell wrote:
> On 09/05/2017 19:53, Marc Zyngier wrote:
>> On 09/05/17 19:52, Phil Elwell wrote:
>>> On 09/05/2017 19:14, Marc Zyngier wrote:
>>>> On 09/05/17 19:08, Eric Anholt wrote:
>>>>> Marc Zyngier <marc.zyngier@....com> writes:
>>>>>
>>>>>> On 09/05/17 17:59, Eric Anholt wrote:
>>>>>>> Phil Elwell <phil@...pberrypi.org> writes:
>>>>>>>
>>>>>>>> In order to reduce power consumption and bus traffic, it is sensible
>>>>>>>> for secondary cores to enter a low-power idle state when waiting to
>>>>>>>> be started. The wfe instruction causes a core to wait until an event
>>>>>>>> or interrupt arrives before continuing to the next instruction.
>>>>>>>> The sev instruction sends a wakeup event to the other cores, so call
>>>>>>>> it from bcm2836_smp_boot_secondary, the function that wakes up the
>>>>>>>> waiting cores during booting.
>>>>>>>>
>>>>>>>> It is harmless to use this patch without the corresponding change
>>>>>>>> adding wfe to the ARMv7/ARMv8-32 stubs, but if the stubs are updated
>>>>>>>> and this patch is not applied then the other cores will sleep forever.
>>>>>>>>
>>>>>>>> See: https://github.com/raspberrypi/linux/issues/1989
>>>>>>>>
>>>>>>>> Signed-off-by: Phil Elwell <phil@...pberrypi.org>
>>>>>>>> ---
>>>>>>>>  drivers/irqchip/irq-bcm2836.c | 3 +++
>>>>>>>>  1 file changed, 3 insertions(+)
>>>>>>>>
>>>>>>>> diff --git a/drivers/irqchip/irq-bcm2836.c b/drivers/irqchip/irq-bcm2836.c
>>>>>>>> index e10597c..6dccdf9 100644
>>>>>>>> --- a/drivers/irqchip/irq-bcm2836.c
>>>>>>>> +++ b/drivers/irqchip/irq-bcm2836.c
>>>>>>>> @@ -248,6 +248,9 @@ static int __init bcm2836_smp_boot_secondary(unsigned int cpu,
>>>>>>>>  	writel(secondary_startup_phys,
>>>>>>>>  	       intc.base + LOCAL_MAILBOX3_SET0 + 16 * cpu);
>>>>>>>>
>>>>>>>> +	dsb(sy); /* Ensure write has completed before waking the other CPUs */
>>>>>>>> +	sev();
>>>>>>>> +
>>>>>>>>  	return 0;
>>>>>>>>  }
>>>>>>>
>>>>>>> This is also the behavior that the standard arm64 spin-table method has,
>>>>>>> which we unfortunately can't quite use.
>>>>>>
>>>>>> And why is that so? Why do you have to reinvent the wheel (and hide the
>>>>>> cloned wheel in an interrupt controller driver)?
>>>>>>
>>>>>> That doesn't seem right to me.
>>>>>
>>>>> The armv8 stubs (firmware-supplied code in the low page that do the
>>>>> spinning) do actually implement arm64's spin-table method.  It's the
>>>>> armv7 stubs that use these registers in the irqchip instead of plain
>>>>> addresses in system memory.
>>>>
>>>> Let's put ARMv7 aside for the time being. If your firmware already
>>>> implements spin-tables, why don't you simply use that at least on arm64?
>>>
>>> We do.
>>
>> Obviously not the way it is intended if you have to duplicate the core
>> architectural code in the interrupt controller driver, which couldn't
>> care less.
> 
> If we were using this method on arm64 then the other cores would not start up
> because armstub8.S has always included a wfe. Nothing in the commit mentions
> arm64 - this is an ARCH=arm fix.

Thanks for the clarification, which you could have added to the commit
message.

The question still remains: why do we have CPU bring-up code in an
interrupt controller, instead of having it in the architecture code?

The RPi-2 is the *only* platform to have its SMP bringup code outside of
arch/arm, so the first course of action would be to move that code where
it belongs.

Thanks,

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