linux-kernel - Re: [patch] x86, voyager: fix ioremap

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Date:	Sun, 27 Apr 2008 15:52:02 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	David Miller <davem@...emloft.net>
CC:	jeff@...zik.org, James.Bottomley@...senPartnership.com,
	mingo@...e.hu, tglx@...utronix.de, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org
Subject: Re: [patch] x86, voyager: fix ioremap_nocache()

David Miller wrote:
> From: Jeff Garzik <jeff@...zik.org>
> Date: Sun, 27 Apr 2008 18:39:24 -0400
> 
>> I disagree with this semantics change.  A number of code places _and 
>> drivers_ GET IT RIGHT, and these are all broken now?
> 
> [ Note, James's patch that you quoted is about mapping DMA
>   memory, in dma_declare_coherent_memory(), rather than devices.
>   But I know what you are trying to talk about Jeff. :-) ]
> 
> Wrt. ioremap() semanics, it is important to realize that if
> the implementation of this on x86 has been giving non-cached
> I/O mappings out up until recently, you can expect that there
> are hundreds of drivers that might now be broken.
> 
> That's the sad fact of the ubiquity of x86, and it doesn't matter how
> we defined the API is some document.
> 
> Anyways, my point is that this angle should be strongly considered in
> any discussion about ioremap() behavior.
> 

There are pretty much three reasons to default to uncached, as far as I 
understand:

1. De facto historical practice (follow the MTRRs);
2. Conservatism (failure mode less drastic);
3. Compatibility with other architectures.

Arguably, the right thing is to not even have ioremap() anymore, just 
ioremap_{cache,nocache,wc} and consider any unconverted ioremap() as a 
flag to audit that particular piece of code.

I don't think that can be done "instantly", though, so defaulting 
ioremap() to uncached (as it apparently has been on other arches 
already?) is the conservative option in the meantime.

	-hpa
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