linux-kernel - Re: wrong usage of MAX_DMA

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Message-ID: <20080930201224.GL15911@flint.arm.linux.org.uk>
Date:	Tue, 30 Sep 2008 21:12:24 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Nicolas Pitre <nico@....org>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: wrong usage of MAX_DMA_ADDRESS in bootmem.h

On Tue, Sep 30, 2008 at 02:56:26PM -0500, Christoph Lameter wrote:
> Nicolas Pitre wrote:
> > I have implemented highmem for ARM.  To catch wrong usage of __pa() and 
> >
> > One issue is that bootmem.h uses __pa(MAX_DMA_ADDRESS). However 
> > MAX_DMA_ADDRESS on ARM is defined as 0xffffffff because there is usually 
> > no restriction on the maximum DMA-able address.
> > 
> > RMK suggested that those places should be using ISA_DMA_THRESHOLD 
> > instead -- here's an excert of our conversation on this topic:
> >
> 
> ok so do
> 
> #define MAX_DMA_ADDRESS ISA_DMA_THRESHOLD

Not correct.  MAX_DMA_ADDRESS is a virtual address.  ISA_DMA_THRESHOLD
is the last byte of _physical_ memory which ISA DMA can transfer:

include/asm-x86/scatterlist.h:#define ISA_DMA_THRESHOLD (0x00ffffff)

So what you've just suggested is completely insane.

>  >> I suspect all those places which are doing __pa(MAX_DMA_ADDRESS) really
> >> want to be using ISA_DMA_THRESHOLD - that's something to raise on LKML
> >> if it's causing problems.
> 
> MAX_DMA_ADDRESS is the highest address used for ZONE_DMA / GFP_DMA

Incorrect.  MAX_DMA_ADDRESS is the highest possible virtual DMA address:

include/asm-x86/dma.h:#define MAX_DMA_ADDRESS      (PAGE_OFFSET + 0x1000000)

Think about what that means on an x86 machine with less than 16MB of RAM.

> Does ISA_DMA_THRESHOLD have any meaning on ARM? If you use old ISA stuff then
> you need CONFIG_ZONE_DMA and therefore also MAX_DMA_ADDRESS.

As we have already covered in the past, CONFIG_ZONE_DMA has to always
be enabled on ARM because ARM always puts all memory in the first zone.
To do otherwise introduces lots of special cases, and I steadfastly
refuse to make the memory initialisation any more complicated than it
already is.

And besides, this has nothing to do with that issue.
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