linux-kernel - Re: [PATCH V3 00/14] genirq endian fixes; bcm7120/brcmstb IRQ updates

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Date:	Mon, 03 Nov 2014 12:56:52 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Kevin Cernekee <cernekee@...il.com>
Cc:	f.fainelli@...il.com, tglx@...utronix.de, jason@...edaemon.net,
	ralf@...ux-mips.org, linux-sh@...r.kernel.org,
	sergei.shtylyov@...entembedded.com, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, mbizon@...ebox.fr, jogo@...nwrt.org,
	linux-mips@...ux-mips.org
Subject: Re: [PATCH V3 00/14] genirq endian fixes; bcm7120/brcmstb IRQ updates

On Saturday 01 November 2014 18:03:47 Kevin Cernekee wrote:
> V2->V3:
> 
>  - Move updated irq_reg_{readl,writel} functions back into <linux/irq.h>
>    so they can be called by irqchip drivers
> 
>  - Add gc->reg_{readl,writel} function pointers so that irqchip
>    drivers like arch/sh/boards/mach-se/{7343,7722}/irq.c can override them
> 
>  - CC: linux-sh list in lieu of Paul's defunct linux-sh.org email address
> 
>  - Fix handling of zero L2 status in bcm7120-l2.c
> 
>  - Rebase on Linus' head of tree

Looks all great. I also looked at the series now and am very happy
about how it turned out.

>  - Drop GENERIC_CHIP / GENERIC_CHIP_BE compile-time optimizations
> 
> For the latter item, I ran a quick benchmark to see if the extra
> indirection in irq_reg_{readl,write} had any perceptible effect on
> register access times.  The MIPS BE case did show a small performance
> hit from using the read wrapper, but on ARM LE the only differences
> were attributed to the presence/absence of a barrier:
>
>
> BCM3384 (UBUS architecture, MIPS BE, IRQ_GC_BE_IO):
> 
> irq_reg_readl       : 207 ns
> readl               : 186 ns
> __raw_readl         : 186 ns
> ioread32be          : 195 ns
> 
> irq_reg_writel      : 177 ns
> writel              : 177 ns
> __raw_writel        : 177 ns
> iowrite32be         : 177 ns
> 
> 
> BCM7445 (GISB architecture, ARM LE, standard LE readl):
> 
> irq_reg_readl       : 519 ns
> readl               : 519 ns
> __raw_readl         : 482 ns
> ioread32be          : 519 ns
> 
> irq_reg_writel      : 500 ns
> writel              : 500 ns
> __raw_writel        : 482 ns
> iowrite32be         : 500 ns
> 

Yes, good idea to check this. 43ns is probably not significant to
warrant optimizing this, but if we wanted to, a driver could now
override the accessors using readl_relaxed()/writel_relaxed().

Note that the cost of the barriers can depend a lot on the hardware
setup and on the state of the system. I believe synchronizing the
L2 cache on some Cortex-A9 machines can be particularly expensive.

Anyway, the existing code doesn't do it, so we can leave that as
a possible optimization.

	Arnd
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