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

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Date:   Tue, 09 May 2017 11:08:56 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Marc Zyngier <marc.zyngier@....com>,
        Phil Elwell <phil@...pberrypi.org>,
        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

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.

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