linux-kernel - Re: [PATCH 01/11] irqchip: Allow irq_reg_{readl,writel} to use __raw_{readl

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Message-ID: <alpine.DEB.2.11.1410292226050.5308@nanos>
Date:	Wed, 29 Oct 2014 22:31:06 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Arnd Bergmann <arnd@...db.de>
cc:	Kevin Cernekee <cernekee@...il.com>,
	Florian Fainelli <f.fainelli@...il.com>,
	Jason Cooper <jason@...edaemon.net>,
	Ralf Baechle <ralf@...ux-mips.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Maxime Bizon <mbizon@...ebox.fr>,
	Jonas Gorski <jogo@...nwrt.org>,
	Linux MIPS Mailing List <linux-mips@...ux-mips.org>
Subject: Re: [PATCH 01/11] irqchip: Allow irq_reg_{readl,writel} to use
 __raw_{readl_writel}

On Wed, 29 Oct 2014, Arnd Bergmann wrote:
> On Wednesday 29 October 2014 13:09:47 Kevin Cernekee wrote:
> > generic-chip.c already has a fair amount of indirection, with pointers
> > to saved masks, user-specified register offsets, and such.  Is there a
> > concern that introducing, say, a pair of readl/writel function
> > pointers, would cause an unacceptable performance drop?
> 
> I don't know. Thomas' reply suggests that it isn't. Doing byteswap
> in software at a register access is usually free in terms of CPU
> cycles, but an indirect function call can be noticeable if we do
> that a lot.

I did not say that it is free. I merily said that I prefer to have
this solved at the core level rather than at the driver level. So you
have several options to do so:

1) Indirections

2) Different functions for the different access modes

3) Alternatives

#1 Is the simplest solution, but imposes the overhead of an indirect
   function call for something trivial

#2 The most efficient and flexible way if you have to provide
   different access modes for different drivers. But it comes with the
   price of increasing the text foot print.

#3 Smart and efficient, but requires that on a particular system all
   drivers use the same access mode.

Thanks,

	tglx



  
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