Messages in this thread | | | Date | Fri, 6 Dec 2002 10:54:41 +1100 | From | David Gibson <> | Subject | Re: [RFC] generic device DMA implementation |
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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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