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

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Date:	Mon, 29 Feb 2016 10:37:40 +0000
From:	Robin Murphy <robin.murphy@....com>
To:	Arnd Bergmann <arnd@...db.de>, Joerg Roedel <jroedel@...e.de>,
	Joerg Roedel <joro@...tes.org>
Cc:	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

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?

> 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.

> 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.

>   	select IOMMU_API
>   	select IOMMU_DMA

As above, this is already selected on arm64, and isn't used on 32-bit*, 
so could probably just be removed, especially if it leads to build issues.

Robin.

*yet, of course. I need to have a proper look over Marek's RFC ;)

>   	select IOMMU_IO_PGTABLE_ARMV7S
>