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

From: Amit D. Chaudhary
Date: Fri Jul 16 2004 - 18:40:58 EST

Deepak Saxena wrote:
On Jul 16 2004, at 15:11, Amit D. Chaudhary was caught saying:


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.

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