Messages in this thread | | | From | "Adam J. Richter" <> | Date | Tue, 5 Mar 2002 06:43:35 -0800 | Subject | Re: Does kmalloc always return address below 4GB? |
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> | If you acquired your memory via the page allocator > | (i.e. __get_free_page*()) or the generic memory allocators > | (i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from > | that memory using the addresses returned from those routines. [...] >Probably it should qualify what it means with "and you used >GFP_KERNEL". Because that was the intention.
That change alone would result in a setence that I still would not understand. Does GFP_KERNEL guarantee address below 4GB on all architectures? The GFP_KERNEL mask that currently includes __GFP_IO, but I'm not sure that that even means "below 4GB" on all architectures. If __GFP_IO really does guarantee "below 4GB and physical = virtual" on all architectures, then it sounds like I am back to not having to use pci_{,un}map_single in a number of places in advansys.c, because the Scsi_Request structurs are allocated with GFP_ATOMIC, which also includes __GFP_IO.
>I'll fix that.
>However, you can use GFP_HIGH memory with pci_map_page _iff_ >you set your DMA mask to allow 64-bits.
>The original impetus for that quoted bit of DMA-mapping.txt >was to make sure nobody used vmalloc() or kmap() pointers.
Just to be clear, I assume that you mean that you cannot simply cast these virtual addresses to dma_addr_t and that the underlying physical memory is not guaranteed to be below 4GB, but that you can use that memory with pci_map_single if your PCI device can handle 64 bit addresses.
If I got it right, then here is some proposed replacement text, to possibly save you a little effort:
| You cannot directly use an address in an area returned by | vmalloc() or kmap(), because these routines are *not* guaranteed to | return addresses where virtual == cpu memory bus == pci. You | cannot even use those addresses with PCI device that only have | 32 bit addressing, because the underlying physical memory may be | above 4GB. | | However, a PCI card that does 64-bit addressing can | access memory returned by vmalloc or kmap via the dma_addr_t | address returned by pci_map_single or pci_map_page.
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