linux-kernel - Re: [PATCH v4 2/2] irqchip: add J-Core AIC driver

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Date:	Wed, 27 Jul 2016 13:08:27 -0400
From:	Rich Felker <dalias@...c.org>
To:	Mark Rutland <mark.rutland@....com>
Cc:	linux-kernel@...r.kernel.org, linux-sh@...r.kernel.org,
	devicetree@...r.kernel.org, Jason Cooper <jason@...edaemon.net>,
	Marc Zyngier <marc.zyngier@....com>,
	Rob Herring <robh+dt@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4 2/2] irqchip: add J-Core AIC driver

On Wed, Jul 27, 2016 at 02:27:54PM +0100, Mark Rutland wrote:
> On Wed, Jul 27, 2016 at 09:08:21AM -0400, Rich Felker wrote:
> > On Wed, Jul 27, 2016 at 11:15:38AM +0100, Mark Rutland wrote:
> > > On Wed, Jul 27, 2016 at 05:35:09AM +0000, Rich Felker wrote:
> > > > For simplicity, there is no aic1-specific logic in the driver beyond
> > > > setting the priority register, which is necessary for interrupts to
> > > > work at all. Eventually aic1 will likely be phased out, but it's
> > > > currently in use in deployments and all released bitstream binaries.
> > > 
> > > [...]
> > > 
> > > > +	if (!of_device_is_compatible(node, "jcore,aic2")) {
> > > 
> > > If this is only meant to run for AIC1, it would be better to check for
> > > the "jcore,aic1" compatible string explicitly.
> > > 
> > > While that shouldn't matter much currently, it better matches the intent
> > > described in the commit message, and avoids surprises and/or churn in
> > > future if you have AIC3+.
> > 
> > My intent in doing this was to support a DT that might claim an aic2
> > is aic1-compatible as a fallback "compatible" property. The hardware
> > is designed such that this works (ignoring the spurious writes to
> > unused prio registers) as long as the DT still has the right irq
> > numbers for attached devices.
> 
> Ok.
> 
> If the HW ignores it, what's the cost of those one-off spurious writes?
> If it's not noticeable, you could allow the kernel to perform them
> regardless.

Indeed, it essentially costs nothing. My motivation was more just to
document that it's not needed/used for aic2.

> Otherwise, please add a comment above the check, explaining why we do
> the check this way around.

Since the intent is documenting this might be the best approach.

Rich