linux-kernel - Re: [PATCH v2 02/10] irqchip: add irqchip driver for nuc900

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Date:	Thu, 21 Jul 2016 18:45:34 +0000
From:	Jason Cooper <jason@...edaemon.net>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Wan ZongShun <mcuos.com@...il.com>, Wan Zongshun <vw@...mu.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	Russell King <linux@...linux.org.uk>,
	devicetree@...r.kernel.org, linux-clk@...r.kernel.org,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Rob Herring <robh@...nel.org>, p.zabel@...gutronix.de
Subject: Re: [PATCH v2 02/10] irqchip: add irqchip driver for nuc900

Wan ZongShun,

On Fri, Jul 15, 2016 at 12:02:55PM +0200, Arnd Bergmann wrote:
> On Friday, July 15, 2016 5:44:50 PM CEST Wan ZongShun wrote:
> > 2016-07-15 15:00 GMT+08:00 Arnd Bergmann <arnd@...db.de>:
> > > On Friday, July 15, 2016 1:15:58 PM CEST Wan Zongshun wrote:
...
> > > That assumes that REG_AIC_IPER contains a 32-bit value with one single
> > > bit set to indicate which IRQ was triggered.
> > >
> > > If the difference is only in performance, you could try measuring which
> > > of the two ends up being faster.
> > 
> > It seems hard to measure. I think Do IO operation should be slower
> > than shift 2. 
> 
> It depends on how fast that particular I/O path is. A lot of readl()
> operations are awfully slow, but the hardware design for the interrupt
> controller may in fact have optimized this to be reasonably fast.
> 
> Another option would be to avoid the shift and just use the raw value
> of the REG_AIC_IPER register as the hwirq, with a custom map()
> callback that turns shifts the number read from the DT two bits
> so it matches the register value.

Good idea.  Are the two lsb bits constant or do they need to be masked?

thx,

Jason.