Re: [PATCH 2/3] ARM: dma-mapping: reset DMA ops before detaching from the IOMMU

On 30/07/2026 2:29 pm, Dmitry Baryshkov wrote:
> arm_iommu_attach_device() installs the IOMMU-aware dma_map_ops only
> after iommu_attach_device() has run, so a driver that builds its page
> tables during attach (msm_iommu via io-pgtable) maps them while the
> device still has the direct DMA ops: the resulting dma_handle equals the
> physical address.
> 
> arm_iommu_detach_device() does the reverse in the wrong order. It calls
> iommu_detach_device() first and clears the DMA ops afterwards. When
> detaching frees those page tables (msm_iommu_identity_attach() ->
> free_io_pgtable_ops()), the freeing dma_unmap now goes through the still
> installed IOMMU ops,

Hang on, that sounds majorly wrong...

> which treat the stored physical address as an IOVA.
> iommu_unmap() warns "region not mapped" and __free_iova() indexes an
> unallocated bitmap extension, dereferencing NULL:
> 
>    __bitmap_clear from arm_iommu_unmap_phys
>    arm_iommu_unmap_phys from dma_unmap_phys

*this* dma_unmap is for the IOMMU device itself - the client device's 
ops should be irrelevant, and the IOMMU device should never be trying to 
translate itself through its own ops either.

 From a quick look, the real issue appears to be that the assignment of 
priv->dev in msm_iommu_attach_dev() and subsequent use in 
msm_iommu_domain_config() is completely bogus.

Thanks,
Robin.

>    dma_unmap_phys from __arm_v7s_free_table
>    __arm_v7s_free_table from arm_v7s_free_pgtable
>    arm_v7s_free_pgtable from msm_iommu_identity_attach
>    msm_iommu_identity_attach from __iommu_attach_device
>    __iommu_attach_device from iommu_detach_device
>    iommu_detach_device from arm_iommu_detach_device
>    arm_iommu_detach_device from arch_teardown_dma_ops
>    arch_teardown_dma_ops from device_unbind_cleanup
> 
> This is reached on apq8064 (ifc6410) when the adreno GPU probe fails and
> its DMA ops are torn down.
> 
> Reset the DMA ops before iommu_detach_device() so page-table teardown
> runs with the same direct ops that mapped them at attach time.
> 
> Fixes: 78fc30b4bb35 ("iommu/msm: Implement an IDENTITY domain")
> Signed-off-by: Dmitry Baryshkov <[email protected]>
> Assisted-by: Claude:claude-opus-5
> ---
>   arch/arm/mm/dma-mapping.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index f9bc53b60f99..a467e0d01a5b 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -1642,10 +1642,13 @@ void arm_iommu_detach_device(struct device *dev)
>   		return;
>   	}
>   
> +	/* Reset DMA ops before detach so page-table teardown uses the same
> +	 * direct ops that mapped them at attach time.
> +	 */
> +	set_dma_ops(dev, NULL);
>   	iommu_detach_device(mapping->domain, dev);
>   	kref_put(&mapping->kref, release_iommu_mapping);
>   	to_dma_iommu_mapping(dev) = NULL;
> -	set_dma_ops(dev, NULL);
>   
>   	pr_debug("Detached IOMMU controller from %s device.\n", dev_name(dev));
>   }
>