On Thu, Dec 05, 2002 at 09:03:24AM -0600, James Bottomley wrote:
> david@gibson.dropbear.id.au said:
> > But if you have the sync points, you don't need a special allocater
> > for the memory at all - any old RAM will do. So why not just use
> > kmalloc() to get it.
>
> Because with kmalloc, you have to be aware of platform
> implementations. Most notably that cache flush/invalidate
> instructions only operate at the level of certain block of memory
> (called the cache line width). If kmalloc returns less than a cache
> line width you have the potential for severe cockups because of the
> possibility of interfering cache operations on adjacent kmalloc
> regions that share the same cache line.
Having debugged a stack corruption problem when attempting to use USB
on a PPC 4xx machine, which was due to improperly aligned DMA buffers,
I am well aware of this issue.
> the dma_alloc... function guarantees to avoid this for you by passing the
> allocation to the platform layer which knows the cache characteristics and
> requirements for the machine (and dma controller) you're using.
Ok - now I begin to see the point of this: I was being misled by the
emphasis on a preference for consistent allocation and the original
"alloc_consistent" name you suggested. When consistent memory isn't
strictly required it's as likely as not that it won't be preferred
either.
Given this, and Miles example, I can see the point of a DMA mallocater
that applies DMA constraints that are not to do with consistency.
Then consistency could also be specified, but that's a separate issue.
So, to remove the misleading emphasis on the point of the allocated
being consistent memory (your name change was a start, this goes
further), I'd prefer to see something like:
void *dma_malloc(struct device *bus, unsigned long size, int flags,
dma_addr_t *dma_addr);
Which returns virtual and DMA pointers for a chunk of memory
satisfying any DMA conditions for the specified bus. Then if flags
includes DMA_CONSISTENT (or some such) the memory will be allocated
consistent in addition to those constraints.
If DMA_CONSISTENT is not specified, the memory might be consistent,
and there would be a preference for consistent only on platforms where
consistent memory is actually preferable (I haven't yet heard of one).
-- David Gibson | For every complex problem there is a david@gibson.dropbear.id.au | solution which is simple, neat and | wrong. http://www.ozlabs.org/people/dgibson - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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