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

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Message-ID: <43e72e890709181402x7335d0acp306cf576fac6c070@mail.gmail.com>
Date:	Tue, 18 Sep 2007 17:02:06 -0400
From:	"Luis R. Rodriguez" <mcgrof@...il.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
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 9/18/07, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>
>
> 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.

ACK but do we really need to benchmark this if we *know* we are
creating unnecessary branches? How about Jeff's suggestion of
introducing pci_mmio_map() ? This way we leave the current
documenation as it is and just introduce this for MMIO-only devices.
No benchmarks are necessary then, and no need to confuse people with
the old API.

  Luis
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