Re: Cache coherency and snooping

[Ralf Baechle]

On Tue, Aug 13, 2002 at 06:00:43PM -0700, Imran Badr wrote:

> Please advise if following sequence of operations are going to help:
> 
> alloc memory
> reserve the page
> flush every cache

That's an extremly expensive operation on some platforms and there's no
portable kernel API to do it.

Note that the flush_cache_*() functions are not suitable for I/O as you
want to do it.  Many platforms such as i386 implement these functions
as empty functions.

> call ioremap_nocache

You said you intend to remap memory, that is RAM.  The ioremap*() functions
will refuse to remap RAM; they're only suitable for other types of memory
such as PCI boards.  If the latter was your intension then ioremap_nocache()
will work as intended.

Again, unless your hardware is fucked beyond recognition it'll do a better
job at keeping cache coherence than software.

  Ralf