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

[Amit D. Chaudhary]

Deepak Saxena wrote:
> 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.
Thanks, noted and verified.

This chip cannot DMA with a memory buffer returned by kmalloc without a 
GFP_DMA flag, that is memory addresses like 0xf67e0000, it works with 
0xcxxx xxxx.

I verified it by modifying the code and trying it out.

> > 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.
This was a backup approach, I mentioned to provide details about the 
memory being allocated. I would like to avoid this approach. See reasons 
above.

Amit
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