linux-kernel - Re: [RFC PATCH 1/3] genirq: add support for per-cpu dev

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Date:	Sun, 25 Sep 2011 18:58:10 -0700
From:	Abhijeet Dharmapurikar <adharmap@...eaurora.org>
To:	Marc Zyngier <marc.zyngier@....com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 1/3] genirq: add support for per-cpu dev_id interrupts

On 09/25/2011 06:31 PM, Abhijeet Dharmapurikar wrote:
> On 09/19/2011 02:28 AM, Marc Zyngier wrote:
>> On 19/09/11 00:20, Abhijeet Dharmapurikar wrote:
>>> On 09/15/2011 09:52 AM, Marc Zyngier wrote:
> ...
>>> > + * @devname: An ascii name for the claiming device
>>> > + * @dev_id: A percpu cookie passed back to the handler function
>>> > + *
>>> > + * This call allocates interrupt resources, but doesn't
>>> > + * automatically enable the interrupt. It has to be done on each
>>> > + * CPU using enable_percpu_irq().
>>> > + *
>>> > + * Dev_id must be globally unique. It is a per-cpu variable, and
>>> > + * the handler gets called with the interrupted CPU's instance of
>>> > + * that variable.
>>> > + */
>>> > +int request_percpu_irq(unsigned int irq, irq_handler_t handler,
>>> > + const char *devname, void __percpu *dev_id)
>>>
>>> Can we add irqflags argument. I think it will be useful to pass flags,
>>> at least the IRQF_TRIGGER_MASK since it ends up calling __setup_irq().
>>> The chip could use a set_type callback for ppi's too.
>>
>> We're entering dangerous territory here. While this would work with the
>> GIC (the interrupt type is at the distributor level), you could easily
>> imagine an interrupt controller with the PPI configuration at the CPU
>> interface level... In that case, calling set_type from __setup_irq()
>> would end up doing the wrong thing, and I'd hate the API to give the
>> idea it can do things it may not do in the end...
>>
>> Furthermore, do we actually have a GIC implementation where PPI
>> configuration isn't read-only? I only know about the ARM implementation,
>> and the Qualcomm may well be different (the spec says it's
>> implementation defined).
>
> Yes, you are exactly right, Qualcomm's GIC has configurable PPIs. The
> default configuration for PPI's is level triggered, but we change the
> timer PPI to edge trigger. Without this we wont even boot (no timer
> interrupts). We do this trigger type setting in board specific code.
>
> Although I agree with your concern, I would still request to provide a
> facility to set the trigger flags. All the PPI's request will have that
> argument set to zero, except for msm timer (and few other msm
> interrupts). Additionally we can add that concern as a comment in
> request_percpu_irq so the user of request_percpu_irq is aware of it.
>

I need to correct myself a tad bit. As Russell King pointed in the other 
email, the trigger type register in the GIC is banked per cpu for PPI 
interrupts. So, on those lines, enable_percpu_irq should take this 
irqflags parameter (and call set_type on the chip) instead of 
request_percpu_irq.

Thanks,
Abhijeet


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