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Date: Wed, 11 Sep 2024 13:54:11 +0100
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Subject: Re: [PATCH] dma-mapping: reliably inform about DMA support for IOMMU
To: Leon Romanovsky <leon@kernel.org>, Joerg Roedel <joro@8bytes.org>,
 Will Deacon <will@kernel.org>, Christoph Hellwig <hch@lst.de>,
 Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Leon Romanovsky <leonro@nvidia.com>, iommu@lists.linux.dev,
 linux-kernel@vger.kernel.org,
 =?UTF-8?B?TsOtY29sYXMgRi4gUi4gQS4gUHJhZG8=?= <nfraprado@collabora.com>
References: <7bbedc085ce87b121b9d0cb33eca8fba2fbdddbc.1726049194.git.leonro@nvidia.com>
From: Robin Murphy <robin.murphy@arm.com>
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In-Reply-To: <7bbedc085ce87b121b9d0cb33eca8fba2fbdddbc.1726049194.git.leonro@nvidia.com>
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On 2024-09-11 11:15 am, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@nvidia.com>
> 
> If the DMA IOMMU path is going to be used, the appropriate check should
> return that DMA is supported.

Oof, indeed the significance of what iommu_dma_ops *didn't* implement is 
a subtle one...

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

> Fixes: b5c58b2fdc42 ("dma-mapping: direct calls for dma-iommu")
> Closes: https://lore.kernel.org/all/181e06ff-35a3-434f-b505-672f430bd1cb@notapiano
> Reported-by: Nícolas F. R. A. Prado <nfraprado@collabora.com> #KernelCI
> Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> ---
>   kernel/dma/mapping.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index 1a7de37bd643..38d7b3239dbb 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -824,6 +824,9 @@ static int dma_supported(struct device *dev, u64 mask)
>   
>   	if (WARN_ON(ops && use_dma_iommu(dev)))
>   		return false;
> +
> +	if (use_dma_iommu(dev))
> +		return true;
>   	/*
>   	 * ->dma_supported sets the bypass flag, so we must always call
>   	 * into the method here unless the device is truly direct mapped.