From: Arnd Bergmann <arnd@arndb.de>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Kevin Cernekee <cernekee@gmail.com>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Jason Cooper <jason@lakedaemon.net>,
        Ralf Baechle <ralf@linux-mips.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        Maxime Bizon <mbizon@freebox.fr>, Jonas Gorski <jogo@openwrt.org>,
        Linux MIPS Mailing List <linux-mips@linux-mips.org>
Subject: Re: [PATCH 01/11] irqchip: Allow irq_reg_{readl,writel} to use __raw_{readl_writel}
Date: Wed, 29 Oct 2014 22:41:03 +0100
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On Wednesday 29 October 2014 22:31:06 Thomas Gleixner wrote:
> 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.

Yes, I understood that.

> 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.

Right. The option that I was explaining earlier basically combines #1 and
#3: For all kernels on which we know the endianess of all generic-irqchip
users at compile time, we hardcode that, and we use indirections of
some sort for the cases where we build a kernel that needs both.

	Arnd
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