linux-kernel - Re: [PATCH v2 1/6] ARM/dma-mapping: Remove iommu_detach

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Date:   Mon, 20 Feb 2023 14:21:00 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>, iommu@...ts.linux.dev
Cc:     Joerg Roedel <joro@...tes.org>, Jason Gunthorpe <jgg@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/6] ARM/dma-mapping: Remove iommu_detach_device()

On 2023-02-17 09:47, Lu Baolu wrote:
> iommu_detach_device() attaches the default domain to the device, or if
> default domain is not supported by the IOMMU driver, it calls its
> set_platform_dma_ops callback. If the default domain is supported or
> the IOMMU driver is not iommu-dma aware, iommu_detach_device() is
> actually a noop.
> 
> The 64-bit ARM drivers always support default domain and iommu-dma is
> even not enabled for 32-bit ARM. This turns out that iommu_detach_device()
> is always a noop in arm_iommu_detach_device(). Remove it to avoid dead
> code.

Huh? This call clearly balances the iommu_attach_device() call in 
arm_iommu_attach_device() - it has nothing to do with default domains.

> The bonus is that it also removes a obstacle of arm_iommu_detach_device()
> re-entering the iommu core during release_device. With this removed, the
> iommu core code could be simplified a lot.

That needs to be worked around in those release paths, not by breaking 
the public API. Should probably just be a case of doing as much "detach" 
as necessary directly, then calling arm_iommu_release_mapping(). Just 
beware that arm_teardown_iommu_dma_ops() may or may not have done some 
of it already, depending on whether a driver ever bound to the device.

Thanks,
Robin.

> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---
>   arch/arm/mm/dma-mapping.c | 1 -
>   1 file changed, 1 deletion(-)
> 
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index 8bc01071474a..dcbc2f4586d4 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -1699,7 +1699,6 @@ void arm_iommu_detach_device(struct device *dev)
>   		return;
>   	}
>   
> -	iommu_detach_device(mapping->domain, dev);
>   	kref_put(&mapping->kref, release_iommu_mapping);
>   	to_dma_iommu_mapping(dev) = NULL;
>   	set_dma_ops(dev, NULL);