Messages in this thread | | | Date | Tue, 30 Sep 2008 21:12:24 +0100 | From | Russell King - ARM Linux <> | Subject | Re: wrong usage of MAX_DMA_ADDRESS in bootmem.h |
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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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