linux-kernel - Re: [PATCH -mm 1/2] scsi: remove dma_is

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Message-Id: <20100630113548K.fujita.tomonori@lab.ntt.co.jp>
Date:	Wed, 30 Jun 2010 11:39:12 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	James.Bottomley@...senPartnership.com
Cc:	fujita.tomonori@....ntt.co.jp, akpm@...ux-foundation.org,
	grundler@...isc-linux.org, lethal@...ux-sh.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH -mm 1/2] scsi: remove dma_is_consistent usage in 53c700

On Tue, 29 Jun 2010 08:37:35 -0500
James Bottomley <James.Bottomley@...senPartnership.com> 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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