linux-kernel - Re: [PATCH] libata-sff: Don't call bmdma

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Message-ID: <45B95D3A.10108@pobox.com>
Date:	Thu, 25 Jan 2007 20:45:30 -0500
From:	Jeff Garzik <jgarzik@...ox.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	David Woodhouse <dwmw2@...radead.org>,
	Alan <alan@...rguk.ukuu.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] libata-sff: Don't call bmdma_stop on non DMA capable
 controllers

Linus Torvalds wrote:
> 
> On Fri, 26 Jan 2007, David Woodhouse wrote:
>> You're thinking of MMIO, while the case we were discussing was PIO. My
>> laptop is perfectly happy to assign PIO resources from zero.
> 
> I was indeed thinking MMIO, but I really think it should extend to PIO 
> also. It certainly is (again) true on PC's, where the low IO space is 
> special and reserved for motherboard/system devices.

Via the even-less-of-an-excuse-than-you-thought department:

Many (most?) non-x86 handle PIO via special mappings and additional 
serialization instructions, but otherwise treat PIO register space in a 
very similar manner to MMIO.

Thus, it's /easier/ on non-x86 to ensure that PIO addresses never land 
at zero, because you must remap /anyway/.  It's only on x86 that PIO 
register spaces are accessed by vastly different CPU instructions.  Most 
other arches convert PIO accesses into massage+mmio R/W+massage.

On sparc64, for example, after I pointed this out to DaveM, he was able 
to implement the new iomap interface without the 'if (pio-mem-area)' 
branch present on x86.

	Jeff

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