linux-kernel - Re: [PATCH] iommu/amd: Fix amd_iommu_detect() (does not fix any issues).

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Message-ID: <20151027005329.GB3569@gmail.com>
Date:	Tue, 27 Oct 2015 09:53:30 +0900
From:	Jerome Glisse <j.glisse@...il.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	linux-kernel@...r.kernel.org,
	Jérôme Glisse <jglisse@...hat.com>,
	Joerg Roedel <jroedel@...e.de>,
	iommu@...ts.linux-foundation.org
Subject: Re: [PATCH] iommu/amd: Fix amd_iommu_detect() (does not fix any
 issues).

On Tue, Oct 27, 2015 at 09:47:48AM +0900, Jerome Glisse wrote:
> On Mon, Oct 26, 2015 at 12:07:17PM -0400, Konrad Rzeszutek Wilk wrote:
> > On Mon, Aug 31, 2015 at 06:13:03PM -0400, j.glisse@...il.com wrote:
> > > From: Jérôme Glisse <jglisse@...hat.com>
> > > 
> > > Fix amd_iommu_detect() to return positive value on success, like
> > > intended, and not zero. This will not change anything in the end
> > > as AMD IOMMU disable swiotlb and properly associate itself with
> > 
> > Not sure how it disables SWIOTLB? The AMD Vi does not seem to
> > change 'swiotlb'. While 'gart_iommu_init' does. Did you mean
> > the AMD GART code?
> 
> So this is convoluted and painfull, each i look back at that it takes
> me time to figure out of thing happen. Basicly amd_iommu_init_dma_ops()
> will replace dma_ops to no_mmu unless passthrough, and when the AMD
> iommu associate itself with each device it will set the archdata.dma_ops
> again this unbind the default of swiotlb that is initialize before
> hw IOMMU.
> 
> > 
> > > devices even if detect() doesn't return a positive value.
> > 
> > Returning positive will mean that the pci_iommu_alloc will stop
> > processing _all_ other IOMMUs.
> >
> > While returning 0 will let it detect the other IOMMUs.
> 
> No see the IOMMU_FINISH_IF_DETECTED flags in pci_iommu_alloc().
> Which is not set for AMD hence my patch should not change anything
> it (AFAICT and from testing but i do not have all AMD hw the ever
> existed).
> 
> So i am just making the detect function do what the API doc says it
> should do. See line 72 to 80 of : arch/x86/include/asm/iommu_table.h
> 
> > 
> > Granted on an AMD machine there can be two 'IOMMU's - the GART
> > and the AMD Vi. The detection is always to call gart_iommu_hole_init
> > first, then amd_iommu_detect.
> > 
> > I presume if there was one more type on AMD we would run into trouble.
> 
> No because of IOMMU_FINISH_IF_DETECTED flag.
> 
> Hope this clarify thing this spagethi mix :)

Ok my bad amd actualy is using IOMMU_INIT_FINISH() so it finish before
trying other. Which make sense for AMD as AMD driver will call the gart
init gart_iommu_init() if it fails to initialize. If we ever end up with
a platform with multiple IOMMU beside AMD then we need to switch to the
IOMMU_INIT() instead of the finish one.

Cheers,
Jérôme
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