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SubjectRe: [PATCH v2 1/6] ARM/dma-mapping: Remove iommu_detach_device()
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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@linux.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);

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