linux-kernel - DMA descriptor alignment

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Message-ID: <4737EF9C.4010207@lwfinger.net>
Date:	Mon, 12 Nov 2007 00:15:56 -0600
From:	Larry Finger <Larry.Finger@...inger.net>
To:	LKML <linux-kernel@...r.kernel.org>
Subject: DMA descriptor alignment

For those variants of BCM43xx cards that use 64-bit DMA, there is a requirement that all descriptor
rings must be aligned on an 8K boundary and must fit within an 8K page. On the x86_64 architecture
where the page size is 4K, I was getting addresses like 0x67AF000 when using dma_alloc_coherent
calls. From the description of the dma_pool_create and dma_pool_allocate routines, I thought they
would fix my problems; however, even with a dma_pool_create(name, dev, 8192, 8192, 8192) call, I'm
still getting 4K rather than 8K alignment, which results in DMA errors.

Is there a bug in these routines, am I using them incorrectly, or do I have a misunderstanding of
what it takes to get this kind of alignment?

Thanks,

Larry
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