Re: Inefficient PCI DMA usage (was: [experimental patch] UHCI updates)

[Russell King <rmk@arm.linux.org.uk>]

Manfred Spraul writes:
> Not yet, but that would be a 2 line patch (currently it's hardcoded to
> BYTES_PER_WORD align or L1_CACHE_BYTES, depending on the HWCACHE_ALIGN
> flag).

I don't think there's a problem then.  However, if slab can be told "I want
1024 bytes aligned to 1024 bytes" then I can get rid of
arch/arm/mm/small_page.c (separate problem to the one we're discussing
though) ;)

> But there are 2 other problems:
> * kmem_cache_alloc returns one pointer, pci_alloc_consistent 2 pointers:
> one dma address, one virtual address.
> * The code relies on the virt_to_page() macro.

What I'm wondering is what about a wrapper around the slab allocator, in
a similar way to pci_alloc_consistent() is a wrapper around gfp.  Since
the slab allocator returns "pointers" in the same space as gfp returns
page references, there shouldn't be a problem (Linus may complain here).

ie, we could make pci_alloc_consistent() a little more intelligent and
allocate from the slab for small sizes, but use gfp for larger sizes?

Comments, anyone (DaveM, Linus, et al) ?
   _____
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  |   |         Russell King        rmk@arm.linux.org.uk      --- ---
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