linux-kernel - Re: [PATCH 3/3] x86/iommu: use __GFP

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Date:	Tue, 30 Sep 2008 13:03:19 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	joerg.roedel@....com, mingo@...hat.com, tglx@...utronix.de,
	hpa@...or.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] x86/iommu: use __GFP_ZERO instead of memset for
	GART


* FUJITA Tomonori <fujita.tomonori@....ntt.co.jp> wrote:

> > yes - could you please make a new option for it, 
> > CONFIG_IOMMU_DEBUG_FORCE=y or so - and cover all iommus that support 
> > it?
> 
> I'm not sure we are talking about the same thing. Surely, I and Joerg 
> are talking different things.
> 
> GART driver doesn't need to use the IOMMU hardware at all times. GART 
> does a virtual mapping only when necessary (a device needs to handle 
> an address that it can't access to). But as I wrote, if you use 
> iommu=force, GART driver always use the IOMMU hardware.

yes - but iommu=force is not randconfig covered, hence it never gets 
tested by -tip testing. So my suggestion was a really simple patch: a 
new Kconfig entry that makes iommu=force default. 

CONFIG_BOOTPARAM_IOMMU_FORCE=y would be the right name for it. (disabled 
by default, of course)

> Other x86 IOMMU drivers always use the IOMMU hardware. Except for 
> Intel VT-D, they manage free virtual I/O space in the round-robin 
> manner with the bitmap algorithm to avoid frequent IOTLB flush. Joerg 
> said he tested AMD IOMMU driver with the round-robin manner disabled 
> so AMD IOMMU driver uses the same virtual I/O space with lots of IOTLB 
> flush.

all i'm suggesting is to please expose existing debug capabilities in 
the .config space, so that it can be tested in automated setups.

	Ingo
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