linux-kernel - Re: [PATCH] iommu: provide of_xlate pointer unconditionally

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Message-ID: <56E939A5.6040905@arm.com>
Date:	Wed, 16 Mar 2016 10:47:01 +0000
From:	Robin Murphy <robin.murphy@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Yong Wu <yong.wu@...iatek.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Joerg Roedel <jroedel@...e.de>,
	Alex Williamson <alex.williamson@...hat.com>,
	Will Deacon <will.deacon@....com>,
	Magnus Damm <damm+renesas@...nsource.se>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH] iommu: provide of_xlate pointer unconditionally

On 15/03/16 21:37, Arnd Bergmann wrote:
> iommu drivers that support the standard DT bindings use a of_xlate
> callback pointer, but that is only part of struct iommu_ops when
> CONFIG_OF_IOMMU is enabled, leading to build errors in randconfig
> builds when that is not provided:
>
> drivers/iommu/mtk_iommu.c:497:2: error: unknown field 'of_xlate' specified in initializer
>    .of_xlate = mtk_iommu_of_xlate,
>    ^
> drivers/iommu/mtk_iommu.c:497:14: error: initialization from incompatible pointer type [-Werror=incompatible-pointer-types]
>    .of_xlate = mtk_iommu_of_xlate,
>                ^~~~~~~~~~~~~~~~~~
> drivers/iommu/mtk_iommu.c:497:14: note: (near initialization for 'mtk_iommu_ops.domain_get_attr')
>
> We can work around it by adding more #ifdefs in each driver, but
> it seems nicer to just allow setting the pointer even if it is
> unused. This makes the driver code look nicer, and it gives better
> compile-time coverage when test building on other architectures.

Makes sense. I don't know what the exact plan is for ACPI IORT, but I 
would imagine that that could also tie in to the same mechanism.

> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> Fixes: 0df4fabe208d ("iommu/mediatek: Add mt8173 IOMMU driver")

Reviewed-by: Robin Murphy <robin.murphy@....com>

FWIW the Exynos IOMMU driver has also been setting .of_xlate 
unconditionally for a while, but that isn't covered by COMPILE_TEST.

> ---
>   include/linux/iommu.h | 2 --
>   1 file changed, 2 deletions(-)
>
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index a5c539fa5d2b..ef7a6ecd8584 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -195,9 +195,7 @@ struct iommu_ops {
>   	/* Get the number of windows per domain */
>   	u32 (*domain_get_windows)(struct iommu_domain *domain);
>
> -#ifdef CONFIG_OF_IOMMU
>   	int (*of_xlate)(struct device *dev, struct of_phandle_args *args);
> -#endif
>
>   	unsigned long pgsize_bitmap;
>   	void *priv;
>