Re: how does one disable processor cache on memory allocated with get_free_pages?

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Fri, 2004-02-27 at 03:25, Ross Tyler wrote:
> Andrew,
> 
> Thank you for taking the time to reply. I really appreciate your help.
> 
> My understanding of ioremap_nocache is that it falls short of what I 
> need to do.
> It is appropriate for, say, mapping physical memory on a PCI device that 
> is marked prefetchable (and otherwise subject to caching when mapped 
> with ioremap) as non-caching.
> Can you confirm my understanding?

No, ioremap always maps non-cacheable, this has nothing to do with
the prefetching attribute

> If so, it will not work for me as I am not mapping physical memory but 
> memory allocated by get_free_pages.
> Do you concur?

get_free_pages() returns you physical memory...

> AFAIK, the only way to access this memory without using processor cache 
> is to have a device driver memory map it for a process like 
> drivers/char/mem.c does.
> When the memory is accessed through these memory mapped pages, the 
> access will not be cached.
> When the memory is accessed through the get_free_pages pages, the access 
> will be cached.
> Concur?

get_free_page allocates cacheable physical memory, nothing to do
with your PCI device... ioremap maps your device non-cacheable
into the kernel address space.
 
> In order to access this process mapped memory from outside the context 
> of the process it was mapped for, one either needs to independently 
> remap it for the current process (what to do in interrupt code?) or set 
> up a kiobuf to the memory.
> It has been my experience, however, that the pages referenced by the 
> kiobuf are the same pages returned by get_free_pages.
> I expect these same pages have the same (caching) attributes associated 
> with them which would not work.

What are you trying to do exactly ?

Ben.