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

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Message-ID: <f1394925-0651-0390-b646-1c489bfe6b4d@raspberrypi.org>
Date:   Wed, 10 May 2017 09:27:10 +0100
From:   Phil Elwell <phil@...pberrypi.org>
To:     Marc Zyngier <marc.zyngier@....com>, 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 10/05/2017 08:42, Marc Zyngier wrote:
> 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.

You were CC'd on the commit (41f4988cc287e5f836d3f6620c9f900bc9b560e9) that
introduced bcm2836_smp_boot_secondary - it seems strange to start objecting
now. Yes, I think it is odd that it didn't go into arch/arm/mach-bcm, but in
the interests of making changes in small, independent steps, do you have a
problem with this commit?

Phil