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

["Amit D. Chaudhary" <amit_c@comcast.net>]

Deepak,

Thanks a lot for the inputs. kmalloc with GFP_DMA is not to be used for 
pci DMA.
I continued trying it and it does work, there was some alignment for the 
header which was incorrect and that I limited the size to 128k.

Amit

Amit D. Chaudhary wrote:
> 
> 
> 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
>