linux-kernel - Re: [Q] Why does dma_alloc_coherent() of ia64 GFP

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Message-Id: <20081112104048C.fujita.tomonori@lab.ntt.co.jp>
Date:	Wed, 12 Nov 2008 10:40:18 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	cl@...ux-foundation.org
Cc:	fujita.tomonori@....ntt.co.jp, y-goto@...fujitsu.com,
	tony.luck@...el.com, linux-ia64@...r.kernel.org,
	linux-kernel@...r.kernel.org, joerg.roedel@....com
Subject: Re: [Q] Why does dma_alloc_coherent() of ia64 GFP_DMA?

On Tue, 11 Nov 2008 14:32:46 -0600 (CST)
Christoph Lameter <cl@...ux-foundation.org> wrote:

> On Tue, 11 Nov 2008, FUJITA Tomonori wrote:
> 
> > Is it because it does some kinda address translation
> > (provider->dma_map_consistent) later? The zone flag is meaningless if
> > you do sorta address translation (e.g. hardware IOMMU like VT-d).
> 
> Yes it can do address translation. Therefore a < 4G address can show up at
> any 64 bit address. So no need for a special DMA zone. The same is true
> for more x86_64 platforms that have an IOMMU.

Yes, with address translation hardware such as IOMMU, the zone is
meaningless. The IOMMU drivers ignore the zone flag
(e.g. intel_alloc_coherent).

But the GFP_DMA in IA64's platform_dma_alloc_coherent() is still
necessary for swiotlb with devices that don't have DMA_64BIT_MASK
coherent_dma_mask. They need a < 4G address.

This is exactly what x86 and x86_64 do, dma_alloc_coherent in
arch/x86/include/asm/dma-mapping.h. It sets GFP_DMA and GFP_DMA32 for
swiotlb and pci-nommu.c when necessary.
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