Re: MAX_DMA_ADDRESS in include/asm/asm-i386/dma.h (2.6.x and 2.4.x)

[Deepak Saxena]

On Jul 16 2004, at 15:11, Amit D. Chaudhary was caught saying:
> Deepak,
> 
> I am missing what you are directing me to.
> 
> If it is,
> pci_alloc_consistent(), linux-2.4.25/arch/i386/kernel/pci-dma.c
> dma_alloc_coherent(), linux-2.6.8-rc1/arch/i386/kernel/pci-dma.c
> 
> They internally seem to __get_free_pages()

Correct, but take a second look at the code (2.6):

        void *ret;
        /* ignore region specifiers */
        gfp &= ~(__GFP_DMA | __GFP_HIGHMEM);

        if (dev == NULL || (dev->coherent_dma_mask < 0xffffffff))
                gfp |= GFP_DMA;

        ret = (void *)__get_free_pages(gfp, get_order(size));

It uses GFP_DMA iff your coherent_dma_mask is != 0xffffffff.  Assuming
your device can address a the full 32-bit PCI address space, you
need to set the coherent_dma_mask appropriately and you will get
buffers from all addressable lowmem. I don't do much x86, so not
sure how you go about allocating highmem DMA buffers.

> The memory need not be page size, as a matter of fact, using a large 
> consecutive block, for example using alloc_bootmem_low() during kernel 
> bootup, will simplify the data transfer and result in no internal 
> fragmentation, it does introduce inflexibility in changing the size and 
> other issues.

If you are using alloc_bootmem_low(), all you should have to do after
allocating the memory is call pci_dma_map_single()/map_sg() to get PCI-DMA 
addresses. You still should have no reason to touch MAX_DMA_ADDRESS.

~Deepak

-- 
Deepak Saxena - dsaxena at plexity dot net - http://www.plexity.net/

"Unlike me, many of you have accepted the situation of your imprisonment and
 will die here like rotten cabbages." - Number 6
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