Re: SCSI breakage on non-cache coherent architectures

Roland Dreier
2007-11-20T02:10:22Z

> 2) Add the __dma_cacheline_aligned tag.
>
> But note that with #2 it could get quite ugly because the
> alignment and size both have a minimum that needs to be
> enforced, not just the alignment alone. So either:
>
> struct foo {
> unsigned int other_unrelated_stuff;
>
> struct object dma_thing __dma_cacheline_aligned;
>
> unsigned int more_nondma_stuff __dma_cacheline_aligned;
> };
>
> or:
>
> struct foo {
> unsigned int other_unrelated_stuff;
>
> union {
> struct object dma_thing __dma_cacheline_aligned;
> char __pad[(sizeof(object) + DMA_CACHELINE_SIZE &
> ~DMA_CACHELINE_SIZE)];
> } u;
>
> unsigned int more_nondma_stuff __dma_cacheline_aligned;
> };

I wrapped this ugliness up inside the macro back in what I posted in
2002 (http://lkml.org/lkml/2002/6/12/234):

#define __dma_buffer __dma_buffer_line(__LINE__)
#define __dma_buffer_line(line) __dma_buffer_expand_line(line)
#define __dma_buffer_expand_line(line) \
__attribute__ ((aligned(L1_CACHE_BYTES))); \
char __dma_pad_ ## line [0] __attribute__ ((aligned(L1_CACHE_BYTES)))

then you just need to tag the actual member like:

char foo[3] __dma_buffer;

- R.