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

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Message-ID: <ce824cd4-4ae4-d080-ed99-b8cef13f5482@raspberrypi.org>
Date:   Tue, 9 May 2017 09:30:29 +0100
From:   Phil Elwell <phil@...pberrypi.org>
To:     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: [PATCH] irq_bcm2836: Send event when onlining sleeping cores

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;
 }

-- 
1.9.1