Re: [PATCH: x86] Add dma_mmap_coherent()

[William Lee Irwin III <wli@holomorphy.com>]

On Sun, Mar 28, 2004 at 03:40:47AM -0800, William Lee Irwin III wrote:
>> I think we're just missing an API for it e.g. break_compound_page();
>> I don't see why we shouldn't arrange for it. Alternatively, clearing
>> PG_compound by hand is possible. Though I guess it raises the question
>> of what we're trying to do; e.g. why not allocate pieces separately?

On Sun, Mar 28, 2004 at 01:36:33PM +0100, Russell King wrote:
> dma_alloc_coherent() is supposed to allocate a contiguous memory area.
> However, x86 can get around this problem if it uses the ->nopage
> method of mapping the pages into memory.
> This all brings up one fundamental question: what really are the
> semantics of dma_alloc_coherent() - if it returns a kernel direct
> mapped address, are the pages supposed to be marked reserved or not.
> And this leads on to this question: if the pages are not marked
> reserved, how do we mmap() the pages given that remap_page_range()
> won't touch them?

There isn't really an API for mapping discontiguous physical memory any
higher-level than pagetable etc. manipulations. Maybe it's supposed to
do that internally when remap_page_range() doesn't suffice.


-- wli