linux-kernel - Re: [RFC PATCH v1] irqchip: add support for SMP irq router

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Date:	Wed, 6 Jul 2016 12:49:24 +0200
From:	Sebastian Frias <sf84@...oste.net>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Marc Zyngier <marc.zyngier@....com>
Cc:	Jason Cooper <jason@...edaemon.net>, Mason <slash.tmp@...e.fr>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v1] irqchip: add support for SMP irq router

Hi,

On 07/06/2016 11:30 AM, Thomas Gleixner wrote:
> On Wed, 6 Jul 2016, Marc Zyngier wrote:
>> On 05/07/16 20:24, Thomas Gleixner wrote:
>>> On Tue, 5 Jul 2016, Marc Zyngier wrote:
>>>> Hardcoded? No way. You simply implement a route allocator in your
>>>> driver, assigning them as needed. And yes, if you have more than 24
>>>> interrupts, they get muxed.
>>>
>>> There is one caveat though. Under some circumstances (think RT) you want to
>>> configure which interrupts get muxed and which not. We really should have that
>>> option, but yes for anything which has less than 24 autorouting is the way to
>>> go.
>>
>> Good point. I can see two possibilities for that:
>>
>> - either we describe this DT with some form of hint, indicating what are
>> the inputs that can be muxed to a single output. Easy, but the DT guys
>> are going to throw rocks at me for being Linux-specific.
> 
> That's not necessarily Linux specific. The problem arises with any other OS as
> well.
> 
>> - or we have a way to express QoS in the irq subsystem, and a driver can
>> request an interrupt with a "make it fast" flag. Of course, everybody
>> and his dog are going to ask for it, and we're back to square one.
> 
> That and the driver does not know about the particular application
> scenario/system configuration.
>  
>> Do we have a way to detect which interrupt is more likely to be
>> sensitive to muxing? My hunch is that if it is requested with
>> IRQF_SHARED, then it is effectively muxable. Thoughts?
> 
> That's too late. request_irq happens _after_ the interrupt is set up and the
> routing established.
> 

What about using 3 values for the interrupt description like the GIC does?
When connecting to the GIC we say "interrupts = <GIC_SPI 2 IRQ_TYPE_LEVEL_HIGH>;"
If devices using this driver (the one from the RFC) requested the interrupt like:
"interrupts = <0 38 IRQ_TYPE_LEVEL_HIGH>;"
"interrupts = <2 27 IRQ_TYPE_LEVEL_HIGH>;"
etc.
with the first field being the "group", then the driver could create a domain
for the device's IRQ (or associate it to an existing one if it has already been
created). It would thus serve as a hint on how to create domains and how to
share IRQs into the same line (domain).

I guess I can get such information from the .translate and .alloc callbacks
from a newly created domain hierarchy attached to the GIC, right?

What do you think?

Best regards,

Sebastian