linux-kernel - Re: [PATCH] Clarify pci_iomap() usage for MMIO-only devices

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Date:	Tue, 18 Sep 2007 13:30:25 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Luis R. Rodriguez" <mcgrof@...il.com>
cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jeff Garzik <jeff@...zik.org>,
	"John W. Linville" <linville@...driver.com>,
	linux-wireless <linux-wireless@...r.kernel.org>
Subject: Re: [PATCH] Clarify pci_iomap() usage for MMIO-only devices

On Tue, 18 Sep 2007, Luis R. Rodriguez wrote:
> 
> An extra branch is created on MMIO-only devices on read/writes on the
> IO_COND macro using this interface -- or is this optimized out?

Umm. Does anybody actually have any performance numbers? 

The thing is, those things are *cheap* compared to the IO. And any 
high-performance device will be using DMA for the real IO, so we're not 
generally even talking about any performance-critical stuff. 

Quite frankly, if performance is a _real_ reason to avoid 
ioread*/iowrite*, I'll happily accept read*/write*, but it would be needed 
to be backed up by real numbers. Can you even measure it?

I would definitely *not* encourage the notion that people should use 
readl/writel because of "performance reasons". That may be valid for some 
fbcon driver, but those drivers go to other extremes (ie they use 
"__raw_writel()" etc, and MTRR's etc).

If you don't use write-combining memory regions, the performance argument 
is not really valid.

			Linus
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