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SubjectRe: [BK PATCH] PCI reorg fix
From said:
> If your are at it: I was always itching my had what
> pci_alloc_inconsistent and pci_free_inconsistent is supposed to be?
> Negating semantically the consistent attribute shows nicely that the
> _consistent is a bad nomenclatures. Perhaps something more related to
> the purpose of it would help. Like

> ioalloc() and iofree()

> Could be even abstracted from the bus implementation.

> And of course much less typing...

I'm all for less typing, but "consistent" memory has a definite meaning to
some non-x86 architectures (and inconsistent also has a definite and
semantically opposite meaning).

The x86 is nice because it is fully coherent architecture: the CPUs snoop the
I/O busses and do CPU cache invalidations for data transferred from an I/O
device directly to memory and CPU cache flushes when a device reads directly
from memory.

If the CPU doesn't snoop I/O transfers, you have to manually invalidate or
flush the necessary CPU cache lines. The pci_sync_... functions are for doing
the cache invalidations and flushes correctly. Sometimes you can obtain a
region of memory which is "consistent" meaning that you no longer need to do
the invalidations/flushes manually because the CPU will operate coherently on
this region. "inconsistent" means that you must control the CPU cache
yourself. In an incoherent memory architecture, the normal memory allocations
give you "inconsistent" regions.

The royal pain on some CPUs is that they only have small consistent regions,
so pci_alloc_consistent can fail and you have to know this and fall back to
doing all the manual cache operations. For this reason, some cross platform
drivers simply choose not to bother with consistent memory at all because the
cache manipulation operations are optimised away on fully consistent platforms.

I'd like to say that this is totally unrelated to the bus type, but some
architectures place MMUs on their busses which means that memory consistency
(and even memory addressability) can indeed be bus specific depending on what
the MMU actually does.


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