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

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Date:   Tue, 9 May 2017 19:52:08 +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 19:14, 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.