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

William Jhun
2002-06-10T18:10:26Z

On Sun, Jun 09, 2002 at 09:27:05PM -0700, David S. Miller wrote:
> From: Paul Mackerras <paulus@samba.org>
> Date: Sun, 9 Jun 2002 19:51:58 +1000 (EST)
>
> This is the problem scenario. Suppose we are doing DMA to a buffer B
> and also independently accessing a variable X which is not part of B.
> Suppose that X and the beginning of B are both in cache line C.
>
> I see what the problem is. Hmmm...
>
> I'm trying to specify this such that knowledge of cachelines and
> whatnot don't escape the arch specific code, ho hum... Looks like
> that isn't possible.

Perhaps provide macros in asm/pci.h that will:

- Take a buffer size and add an appropriate amount (one cache line for
alignment and the remainder to fill out the last cache line) to be
used for kmalloc(), etc, eg:

#define DMA_SIZE_ROUNDUP(size) \
((size + 2*SMP_CACHE_BYTES - 1) & ~(SMP_CACHE_BYTES - 1))

- Take a buffer address (as returned from kmalloc() with the modified
size from above) and round it up to a cacheline boundary, eg:

#define DMA_BUFFER_ALIGN(ptr) \
(((unsigned long)ptr + SMP_CACHE_BYTES - 1) & ~(SMP_CACHE_BYTES - 1))

These two, in conjunction, would provide a buffer that's aligned on a
cacheline boundary and ends on a cacheline boundary. Kind of ugly, but
would be sufficient and would hide the cacheline size specifics.
Cache-coherent platforms would just returned the original argument.

Thanks,
Will