[microblaze-uclinux] RE: [PATCH] Microblaze: implement dma-coherent API andrefactorcache flush code.

["Stephen Neuendorffer" <stephen.neuendorffer@xxxxxxxxxx>]

> -----Original Message-----
> From: John Williams [mailto:john.williams@xxxxxxxxxxxxx]
> Sent: Monday, May 05, 2008 5:15 PM
> To: Stephen Neuendorffer
> Cc: arnd@xxxxxxxx; linux-arch@xxxxxxxxxxxxxxx; John Linn;
matthew@xxxxxx; will.newton@xxxxxxxxx;
> drepper@xxxxxxxxxx; microblaze-uclinux@xxxxxxxxxxxxxx;
grant.likely@xxxxxxxxxxxx; Michal Simek;
> linux-kernel@xxxxxxxxxxxxxxx
> Subject: RE: [PATCH] Microblaze: implement dma-coherent API
andrefactorcache flush code.
> 
> 
> > > Does the DMA API insist upon the dma_cache_sync call to guarantee
> > > sensible results?  If so, your implementation looks fine.  If not,
> > then
> > > the results will clearly be bogus as there's nothing magical about
the
> > > memory being allocated in dma_alloc.
> >
> > Yes, in fact this is one of the keys to getting the lltemac driver
to
> > work right.  see Documentation/DMA-API.txt
> >
> > > To that end, can it just call kmalloc(), and similarly kfree() for
> > > dma_free?
> >
> > My understanding is that on other architectures (x86, for instance)
> > 'dma' memory ensures other things, like it's accessible in PCI
memory
> > space.  On microblaze, there's nothing really special about dma
memory,
> > but you get the API as a chunk.
> 
> Sure - what I meant is can dma_alloc just call kmalloc to do it's
work?

I scanned through Linux Device Drivers, and it appears that calling
get_free_pages is:
1) more efficient than kmalloc for large allocations
2) allocates physically contiguous memory, which kmalloc doesn't
necessarily do if there's an mmu.

Steve


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