On Tue, 29 Jun 2010 08:37:35 -0500 James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: > > How about using ARCH_KMALLOC_MINALIGN instead of L1_CACHE_BYTES? (snip) > Actually, I'd rather not do this. The reason is that L1_CACHE_ALIGN is > quite a big performance optimisation on x86 for the driver. Without it, > it's functionally correct, but the DMA use of the mailboxes really > thrashes the cache which damages performance (x86 has > ARCH_KMALLOC_MINALIGN set to 8 ... the default) Ah, I see. If slab.h doesn't define ARCH_KMALLOC_MINALIGN for architectures that don't define it, the driver could do something like: #ifdef ARCH_KMALLOC_MINALIGN #define DMA_ALIGN(x) ALIGN(x, ARCH_KMALLOC_MINALIGN) #else #define DMA_ALIGN(x) ALIGN(x, L1_CACHE_BYTES) #endif Seems that it's better to rename ARCH_KMALLOC_MINALIGN to something like ARCH_DMA_MINALIGN and make ARCH_KMALLOC_MINALIGN the slab internal thing. > The only correctness problem, which the BUG is checking for is mismatch > in dma alignment ... as I said, I'm happy just to rely on that being > correct on every incoherent platform the driver operates on. Ok, it's fine by me too. let's simply remove the BUG_ON. I think that you want to document that dma_get_cache_alignment() cannot be greater than the L1 cache stride. However, seems that dma_get_cache_alignment() is greater than L1_CACHE_BYTES on some architectures (they have some reasons, I assume). So I'll just remove the BUG_ON. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
