linux-kernel - RE: [PATCH v3 3/5] iommu: Add iommu_get_domain_for_dev

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Message-ID: <BN9PR11MB5276262E1925E2CD8AE037EF8C32A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Thu, 21 Aug 2025 08:11:05 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Nicolin Chen <nicolinc@...dia.com>, Jason Gunthorpe <jgg@...dia.com>
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Subject: RE: [PATCH v3 3/5] iommu: Add iommu_get_domain_for_dev_locked()
 helper

> From: Nicolin Chen <nicolinc@...dia.com>
> Sent: Tuesday, August 19, 2025 1:23 AM
> 
> ... I found that in SMMUv3 driver, iommu_get_domain_for_dev() is
> used to get the RID domain for an SVA domain:
>     arm_smmu_set_pasid()
>     arm_smmu_blocking_set_dev_pasid()
> 
> These two are already given an "old" (SVA) domain pointer, FWIW.
> 
> So, we may change to passing in the old domain as you suggested,
> yet we still have to fix the iommu_get_domain_for_dev() in order
> to reflect the RID domain correctly for the driver that calls it
> (or even potentially) in some group->mutex locked context where
> the RID domain might not be naturally passed in.
> 

Out of curiosity.

arm_smmu_blocking_set_dev_pasid()

	/*                              
	 * When the last user of the CD table goes away downgrade the STE back
	 * to a non-cd_table one.
	 */
	if (!arm_smmu_ssids_in_use(&master->cd_table)) {
		struct iommu_domain *sid_domain =
			iommu_get_domain_for_dev(master->dev);

		if (sid_domain->type == IOMMU_DOMAIN_IDENTITY ||
		   sid_domain->type == IOMMU_DOMAIN_BLOCKED)
			sid_domain->ops->attach_dev(sid_domain, dev); 
	}

why cannot downgrade apply to the case where the RID is attached to
a DMA domain?