linux-kernel - Re: [PATCH v2 1/5] irqchip: add dumb demultiplexer implementation

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Date:	Wed, 14 Jan 2015 09:22:35 +0100
From:	Boris Brezillon <boris.brezillon@...e-electrons.com>
To:	Rob Herring <robherring2@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 1/5] irqchip: add dumb demultiplexer implementation

Hi Rob,

On Tue, 13 Jan 2015 21:26:42 -0600
Rob Herring <robherring2@...il.com> wrote:

> On Tue, Jan 13, 2015 at 12:46 PM, Boris Brezillon
> <boris.brezillon@...e-electrons.com> wrote:
> > Some interrupt controllers are multiplexing several peripheral IRQs on
> > a single interrupt line.
> > While this is not a problem for most IRQs (as long as all peripherals
> > request the interrupt with IRQF_SHARED flag set), multiplexing timers and
> > other type of peripherals will generate a WARNING (mixing IRQF_NO_SUSPEND
> > and !IRQF_NO_SUSPEND is prohibited).
> >
> > Create a dumb irq demultiplexer which simply forwards interrupts to all
> > peripherals (exactly what's happening with IRQ_SHARED) but keep a unique
> > irq number for each peripheral, thus preventing the IRQF_NO_SUSPEND
> > and !IRQF_NO_SUSPEND mix on a given interrupt.
> 
> This really seems like a work-around for how IRQF_SHARED works. It
> seems like what is really desired is just per handler disabling.

Like what I proposed here [1] ?

> It is
> fragile in that devices can deadlock the system if the drivers don't
> disable the interrupt source before calling disable_irq.

Not exactly deadlock since spurious interrupt detection is implemented,
but yes, things won't work as expected.

> But unlike
> IRQF_SHARED, there is nothing explicit in the driver indicating it is
> designed to work properly with a shared interrupt line.
> 
> I see no reason to accept this into DT either. We already can support
> shared lines and modeling an OR gate as an interrupt controller is
> pointless.

Okay, I guess I'll let DT and irq maintainers decide what is preferable
here (I already spent much time than I first expected to remove this
warning in a proper way).

Best Regards,

Boris

[1]https://lkml.org/lkml/2014/12/15/552

-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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