Re: [PATCH -mm 1/2] scsi: remove dma_is_consistent usage in 53c700

[FUJITA Tomonori]

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.
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