Re: [PATCH] Xen/i386 cleanups - AGP bus/phys cleanups

[Keir Fraser]

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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