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    FromKeir Fraser <>
    SubjectRe: [PATCH] Xen/i386 cleanups - AGP bus/phys cleanups
    DateThu, 17 Mar 2005 09:16:55 +0000
    On 16 Mar 2005, at 22:06, Jesse Barnes wrote:
    
    >> The distinction is that physical addresses are what are used to access
    >> physical memory, whereas bus addresses are what appears on some
    >> external bus (usually PCI).  The GATT sits between an external (AGP)
    >> bus and memory, so while the GATT is indexed using bus addresses, its
    >> entries contain physical addresses.  So in fact virt_to_phys is the
    >> correct thing to use to calculate values to put in GATT entries.
    >
    > Thanks for the explanation Paul, now the code actually makes sense.
    > Converting to the DMA mapping API doesn't make sense at all in this 
    > context
    > then, since we're basically programming the GATT (an IOMMU type table) 
    > with
    > physical addresses.  Ken, are you sure you need to make these changes 
    > at all?
    > Does Xen break w/o them?
    
    Yes, Xen will break w/o them, because physical addresses are an 
    illusory trick that the guest OS plays on itself to give itself the 
    impression of a contiguous memory map. We use _to_machine and _to_bus 
    macros to get 'real' physical addresses.
    
    For allocating the GATT itself, using dma_alloc_coherent() as done in 
    my patch certainly seems sane -- the bus base address of that table is 
    poked into a chipset register, right?
    
    As for poking entries into the GATT, I guess I'm not sure what ought to 
    be used. virt_to_phys() doesn't sound right to me: the GART is a bridge 
    between two buses, so some sort of bus mapping would still be in order 
    imo. Perhaps Linux should allow mapping requests to be tagged with a 
    bridge id, like in *BSD? :-)
    
    So: I would very much like you to take the patches I made to generic.c 
    that replace __get_free_pages() calls with dma_alloc_coherent(). For 
    now, the patch lines that poke into the GATT I guess stay as they are. 
    We can maintain an out-of-tree patch for Xen, or perhaps if 
    virt_to_phys() is not used very much we can override its definition.
    
      -- Keir
    
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