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

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Message-ID: <87ziemm0fb.fsf@eliezer.anholt.net>
Date:   Tue, 09 May 2017 09:59:04 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Phil Elwell <phil@...pberrypi.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <marc.zyngier@....com>,
        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

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.

Acked-by: Eric Anholt <eric@...olt.net>

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