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

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Date:	Tue, 11 Nov 2008 14:39:18 +0900
From:	Yasunori Goto <y-goto@...fujitsu.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	"Luck, Tony" <tony.luck@...el.com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	linux-ia64@...r.kernel.org,
	Linux Kernel ML <linux-kernel@...r.kernel.org>,
	Joerg Roedel <joerg.roedel@....com>
Subject: Re: [Q] Why does dma_alloc_coherent() of ia64 GFP_DMA?

> On Mon, 10 Nov 2008, Yasunori Goto wrote:
> 
> > Even if a device allows over 4G access and the driver doesn't specify
> > GFP_DMA, dma_alloc_coherent() returns under 4G area.
> 
> GFP_DMA can become 0 for configurations that have
> !CONFIG_ZONE_DMA. Then all of memory is available.
>
> The call is subarch specific. So f.e. Altix sn_dma_alloc_coherent does
> not set __GFP_DMA.

I heard that Altix has IOMMU and __GFP_DMA is not necessary.

> If you have an IA64 arch that only support 32bit I/O then __GFP_DMA in
> dma_alloc_coherent makes sense.

Agree. 
But our box supports both of 32bit I/O and 64bit I/O without IOMMU.
Is it abnormal platform? New interface is necessary for our box like Altix?


Bye.

-- 
Yasunori Goto 


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