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

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Date:   Tue, 9 May 2017 20:02:29 +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 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.

Phil