linux-kernel - RE: [PATCH 1/1] irq: add IRQF_TRIGGER

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Date:   Fri, 31 Mar 2017 12:07:06 +0000
From:   Aniruddha Banerjee <aniruddhab@...dia.com>
To:     Marc Zyngier <marc.zyngier@....com>,
        Thomas Gleixner <tglx@...utronix.de>
CC:     Jonathan Hunter <jonathanh@...dia.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thierry Reding <treding@...dia.com>,
        "Stephen Warren" <swarren@...dia.com>
Subject: RE: [PATCH 1/1] irq: add IRQF_TRIGGER_MASK on PPI by default

> On 31/03/17 , Marc Zyngier wrote:
> On 31/03/17 09:01, Thomas Gleixner wrote:
> > On Thu, 30 Mar 2017, Aniruddha Banerjee wrote:
> >
> >> add IRQF_TRIGGER_MASK on PPI by default so that the PPIs are not
> >> configured as edge-triggered, which may be wrong for certain GIC
> >> implementations such as the GIC-400
> >
> > The above is just useless blurb.
> >
> > I can't figure out at all WHY a generic interface has anything to do
> > with edge trigger configuration.
> >
> > I assume this is (Nvidia) GIC specific nonsense, so why are you
> > inflicting this on every caller of this interface unconditionally w/o
> > explaining what the impact of this change might be and why it does not
> > cause havoc for any existing caller?
> >
> > This is function is implemented in kernel/irq/ not in foo/gic/ so you
> > better come up with some coherent explanation.
> 
> Indeed. I'm not aware of anything wrong so far with GIC400, so this is most likely
> referring to an integration issue.
> 
> Furthermore, PPI triggers are usually not configurable on GIC400. My bet is that this is
> only a DT issue, but in the absence of any coherent justification, it is hard to make an
> educated guess...

That was an awesome guess and we were in fact doing something very wrong in the DT. 
In the GIC-400 implementation, the PPI triggers are read-only. I was trying to configure
the PPI as edge-triggered, and the writes were dropped in the process.
A big thank you to Jon Hunter and Marc for pointing this out.

Regards,
Aniruddha.