Messages in this thread | | | Subject | Re: _PAGE_PCD bit in DMAable memory | From | Roland Dreier <> | Date | 16 May 2004 19:20:14 -0700 |
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fc> Hi, I am trying to get DMAable memory on i386 fc> (kmalloc(GFP_DMA,)) but seems _PAGE_PCD is not set on the fc> pages that i get back the memory from. Am I getting the fc> correct results? If yes, then is it not that GFP_DMA should fc> give me non-cacheable memory (equivalent to setting _PAGE_PCD fc> along with _PAGE_PWT on i386). Can anyone confirm for me that fc> GFP_DMA will *always* give me non-cacheable and contiguous fc> memory.
On i386, the GFP_DMA flag controls _where_ the memory will be allocated, namely in the low 16 MB. This is really a relic of the days of 24-bit ISA DMA.
On i386, the memory does not have to be non-cacheable, since in the PC architecture the bus controller will maintain consistency by snooping the CPU. However, to be portable, your code should use pci_alloc_consistent() to get memory for DMAing. This will do the right thing on all platforms, including making sure that the memory is non-cacheable on architectures where the bus controller does not snoop.
(By the way, kmalloc() will always return contiguous memory, but you should still use pci_alloc_consistent for DMA memory)
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