linux-kernel - Re: [patch] x86: optimize __pa() to be linear again on 64-bit x86

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Message-ID: <20090227065717.GA5207@elte.hu>
Date:	Fri, 27 Feb 2009 07:57:17 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Tejun Heo <tj@...nel.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	rusty@...tcorp.com.au, tglx@...utronix.de, x86@...nel.org,
	linux-kernel@...r.kernel.org, hpa@...or.com, jeremy@...p.org,
	cpw@....com
Subject: Re: [patch] x86: optimize __pa() to be linear again on 64-bit x86


* Tejun Heo <tj@...nel.org> wrote:

> Hello,
> 
> Ingo Molnar wrote:
> > Yeah, we can do this complete conversion.
> >
> > I'll clean it up some more. I think the best representation of 
> > this will be via a virt_to_sym() and sym_to_virt() space. That 
> > makes it really clear when we are moving from the symbol space 
> > to the linear space and back.
> 
> For arch code, maybe it's maintainable but with my driver developer
> hat on I gotta say virt_to_page() not working on .data/.bss is quite
> scary. [...]

Well, we have a debug mechanism in place.

As i suggested it in my first mail we can run with debug enabled 
for a cycle and then turn on the optimization by default (with 
the debug option still available too).

Drivers doing DMA on .data/.bss items is rather questionable 
anyway (and dangerous as well, on any platform where there's 
coherency problems if DMA is misaligned, etc.), and a quick look 
shows there's at most 2-3 dozen examples of that in all of 
drivers/*.

	Ingo
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