[PATCH v4 3/5] iommu/arm-smmu-v3: Fix ATS state tracking

[Nicolin Chen]

On Mon, May 25, 2026 at 06:43:45PM +0000, Pranjal Shrivastava wrote:
> @@ -3065,8 +3065,14 @@ static void arm_smmu_enable_ats(struct arm_smmu_master *master)
>  	 * ATC invalidation of PASID 0 causes the entire ATC to be flushed.
>  	 */
>  	arm_smmu_atc_inv_master(master, IOMMU_NO_PASID);
> -	if (pci_enable_ats(pdev, stu))
> -		dev_err(master->dev, "Failed to enable ATS (STU %zu)\n", stu);
> +
> +	/*
> +	 * Any failure at this point is a kernel bug. pci_ats_supported()
> +	 * and pci_prepare_ats() have already verified the hardware capability
> +	 * and programmed the STU. Thus, pci_enable_ats() should not fail here.

Nits:
 - WARN usually indicates a kernel bug already.
 - pci_prepare_ats() covers pci_ats_supported().

	/*
	 * As pci_prepare_ats() have already verified the hardware capability
	 * and programmed the STE, pci_enable_ats() should not fail here.
	 */

> @@ -4264,9 +4270,16 @@ static struct iommu_device *arm_smmu_probe_device(struct device *dev)
>  		master->stall_enabled = true;
>  
>  	if (dev_is_pci(dev)) {
> -		unsigned int stu = __ffs(smmu->pgsize_bitmap);
> +		struct pci_dev *pdev = to_pci_dev(dev);
>  
> -		pci_prepare_ats(to_pci_dev(dev), stu);
> +		if (pci_ats_supported(pdev)) {
> +			unsigned int stu = __ffs(smmu->pgsize_bitmap);
> +			int ret;
> +
> +			ret = pci_prepare_ats(pdev, stu);
> +			if (ret)
> +				return ERR_PTR(ret);
> +		}

Again, pci_prepare_ats() covers pci_ats_supported(). So, the check
is redundant. Instead, it should check arm_smmu_ats_supported().

By the way, this would conflict into my series:
https://lore.kernel.org/linux-iommu/18bb6f421b3be891caa8f1fb50f3a4d56b52d5be.1779392420.git.nicolinc@nvidia.com/

It would be nicer to have an arm_smmu_master_prepare_ats() so that
both series would have a common ground; mine would be just adding
some additional lines if your series goes in first.

Thanks
Nicolin