linux-kernel - Re: [PATCH 5/5] iommu/sun50i: Ensure the IOMMU can be used for DMA

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Date:   Thu, 28 Apr 2022 12:12:39 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Samuel Holland <samuel@...lland.org>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        iommu@...ts.linux-foundation.org
Cc:     devicetree@...r.kernel.org, Heiko Stuebner <heiko@...ech.de>,
        linux-kernel@...r.kernel.org,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Chen-Yu Tsai <wens@...e.org>, Rob Herring <robh+dt@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Maxime Ripard <mripard@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        linux-riscv@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 5/5] iommu/sun50i: Ensure the IOMMU can be used for DMA

On 2022-04-28 02:04, Samuel Holland wrote:
> So far, the driver has relied on arch/arm64/Kconfig to select IOMMU_DMA.
> Unsurprisingly, this does not work on RISC-V, so the driver must select
> IOMMU_DMA itself.

No, IOMMU_DMA should only be selected by the architecture code that's 
also responsible for calling iommu_setup_dma_ops(). Without that, this 
select will do nothing other than add some unused code to the kernel image.

I appreciate that the current state of the x86 IOMMU drivers being 
tightly-coupled to the x86 arch code might be confusing (which reminds 
me I'd totally forgotten about [1]). I'm about to start reworking the 
whole area anyway, but for now please just follow the existing intent.

Thanks,
Robin.

[1] 
https://lore.kernel.org/linux-iommu/9ba6f2e8568a3ff6a94fade66668d99705433c44.1631536879.git.robin.murphy@arm.com/

> Signed-off-by: Samuel Holland <samuel@...lland.org>
> ---
> 
>   drivers/iommu/Kconfig | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> index c79a0df090c0..70a0bfa6d907 100644
> --- a/drivers/iommu/Kconfig
> +++ b/drivers/iommu/Kconfig
> @@ -223,6 +223,7 @@ config SUN50I_IOMMU
>   	depends on ARCH_SUNXI || COMPILE_TEST
>   	select ARM_DMA_USE_IOMMU
>   	select IOMMU_API
> +	select IOMMU_DMA
>   	help
>   	  Support for the IOMMU introduced in the Allwinner H6 SoCs.
>