Re: PCI DMA to small buffers on cache-incoherent arch

[Roland Dreier <roland@topspin.com>]

>>>>> "David" == David S Miller <davem@redhat.com> writes:

    David> Wait a second, forget all of this cache alignment crap.  If
    David> we can avoid drivers seeing it, we should by all means
    David> necessary.

    David> We should just tell people to use PCI pools and be done
    David> with it.  That way all the complexity about buffer
    David> alignment and all this other crapola lives strictly inside
    David> of the PCI pool code.

That's fine but there are drivers (USB, etc) doing

	struct something {
	        int field1;
	        char dma_buffer[SMALLER_THAN_CACHE_LINE];
	        int field2;
	};

	struct something *dev = kmalloc(sizeof *dev, GFP_KERNEL);

Do they have to change to

	struct something {
	        int field1;
	        char *dma_buffer;
	        int field2;
	};

	struct something *dev = kmalloc(sizeof *dev, GFP_KERNEL);
        dev->dma_buffer = kmalloc(SMALLER_THAN_CACHE_LINE, GFP_KERNEL);

(This is always safe because as you said kmalloc can never return a
slab that's not safe for DMA)  I don't see how PCI pools help here.

Best,
  Roland