linux-kernel - Re: [PATCH 1/2] iommu/mediatek: select ARM_DMA_USE

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Date:	Mon, 29 Feb 2016 11:53:46 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Robin Murphy <robin.murphy@....com>
Cc:	Joerg Roedel <jroedel@...e.de>, Joerg Roedel <joro@...tes.org>,
	linux-arm-kernel@...ts.infradead.org,
	Yong Wu <yong.wu@...iatek.com>,
	iommu@...ts.linux-foundation.org,
	linux-mediatek@...ts.infradead.org,
	Matthias Brugger <matthias.bgg@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] iommu/mediatek: select ARM_DMA_USE_IOMMU

On Monday 29 February 2016 10:37:40 Robin Murphy wrote:
> Hi Arnd,
> 
> On 29/02/16 09:19, Arnd Bergmann wrote:
> > The newly added Mediatek IOMMU driver uses the IOMMU_DMA infrastructure,
> > but unlike other such drivers, it does not select 'ARM_DMA_USE_IOMMU',
> > which is a prerequisite, leading to a link error:
> >
> > warning: (MTK_IOMMU) selects IOMMU_DMA which has unmet direct dependencies (IOMMU_SUPPORT && NEED_SG_DMA_LENGTH)
> 
> Going off on a tangent, is it actually right for that to depend on a 
> NEED_* symbol, or should that really be a select instead?

ARM_DMA_USE_IOMMU gets this right, it selects NEED_SG_DMA_LENGTH as it
actually needs it.

The IOMMU_DMA symbol is a bit strange, and the dependency can probably
get dropped altogether, but at least here it told us what went wrong.

> > drivers/iommu/built-in.o: In function `iommu_put_dma_cookie':
> > mtk_iommu.c:(.text+0x11fe): undefined reference to `put_iova_domain'
> > drivers/iommu/built-in.o: In function `iommu_dma_init_domain':
> > mtk_iommu.c:(.text+0x1316): undefined reference to `init_iova_domain'
> > drivers/iommu/built-in.o: In function `__iommu_dma_unmap':
> > mtk_iommu.c:(.text+0x1380): undefined reference to `find_iova'
> >
> > This adds the same select that the other drivers have. On a related
> > note, I wonder if we should just always select ARM_DMA_USE_IOMMU
> > whenever any IOMMU driver is enabled. Are there any cases where
> > we would enable an IOMMU but not use it?
> 
> You could use one solely for VFIO without caring about DMA ops - I think 
> that's mostly how ARM SMMUs are being used in practice at the moment - 
> but DMA-focused 'media' IOMMUs vastly outnumber 'virtualisation' IOMMUs 
> on ARM, so it would probably make sense. We already have the equivalent 
> "select IOMMU_DMA if IOMMU_SUPPORT" on arm64.

Ok.

> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > Fixes: 0df4fabe208d ("iommu/mediatek: Add mt8173 IOMMU driver")
> > ---
> >   drivers/iommu/Kconfig | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> > index b325954cf8f8..ea0998921702 100644
> > --- a/drivers/iommu/Kconfig
> > +++ b/drivers/iommu/Kconfig
> > @@ -341,6 +341,7 @@ config MTK_IOMMU
> >   	bool "MTK IOMMU Support"
> >   	depends on ARM || ARM64
> >   	depends on ARCH_MEDIATEK || COMPILE_TEST
> > +	select ARM_DMA_USE_IOMMU
> 
> If going down this route, I'd be inclined to add an "if ARM" there, just 
> for clarity.

That would run into the NEED_SG_DMA_LENGTH problem on other architectures
that don't already set it, right?

	Arnd