On Sat, Jun 15, 2002 at 02:27:53AM -0400, Dan Malek wrote:

Paul Mackerras wrote:

quoted

"Consistent" means "the cpu and the device see the same stuff in
memory".  On a cache-coherent system that is true of all normal
memory.  Surely that is obvious?

Not really.  You have seen enough messages about people wanting to
get "uncached" memory, even though this isn't necessary.

We also don't want to be remapping 'consistent' space on those
processors that don't require it, one of the primary functions
of consistent_alloc() so it works properly on noncoherent processors.

We don't.

Basically, David Gibson moved an 'ifdef' from one place to another,
and then had to write a consistent_alloc function for processors that
didn't need it before, placing more code in an include file no less.
How does that add value to the kernel?  It just complicated the
maintenance of a function that is similar across multiple architectures.

Oh fer cryin' out loud.  Before any non-PCI driver that wanted DMA
consistent memory and wanted to work on both cache-coherent and
non-cache coherent would have to have an ifdef on
CONFIG_NOT_COHERENT_CACHE.  If it was on, it would have to use
consistent_alloc(), other wise it could just allocate the memory
normally (with kmalloc() or get_free_pages()) since consistency comes
for free on cache coherent processors.

I just made consistent_alloc() do for cache coherent processors
exactly what the driver would have to do anyway if
CONFIG_NOT_COHERENT_CACHE were not set.  The cache coherent
implementation of consistent_alloc() is utterly trivial, and does not
remapping.  If we really don't want to be calling consistent_alloc()
on cache coherent processors it should BUG() or cause a link error,
rather than just failing.

Admittedly pci_alloc_consistent() was (so far) the only place this
happened, because every other user of consistent_alloc() was basically
in a driver which would only ever be used on non cache coherent
processors.

--
David Gibson			| For every complex problem there is a
david@gibson.dropbear.id.au	| solution which is simple, neat and
				| wrong.  -- H.L. Mencken
http://www.ozlabs.org/people/dgibson

** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/