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

From: Arnd Bergmann
Date: Wed Oct 29 2014 - 17:41:28 EST


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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/