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

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Message-ID: <93a69cf4-cbf6-e0a7-b7e3-d0e53040efe4@arm.com>
Date:   Tue, 9 May 2017 19:53:09 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Phil Elwell <phil@...pberrypi.org>, 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 09/05/17 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.

	M.
-- 
Jazz is not dead. It just smells funny...