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[34.124.234.44]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2bd5cfe47c8sm190356995ad.51.2026.05.19.07.55.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 19 May 2026 07:56:00 -0700 (PDT) Date: Tue, 19 May 2026 14:55:54 +0000 From: Pranjal Shrivastava To: Jason Gunthorpe Cc: iommu@lists.linux.dev, linux-pci@vger.kernel.org, Will Deacon , Joerg Roedel , Bjorn Helgaas , Robin Murphy , Mostafa Saleh , Nicolin Chen , Samiullah Khawaja , Daniel Mentz , Pasha Tatashin , David Matlack Subject: Re: [PATCH v3 3/3] iommu/arm-smmu-v3: Fix ATS state tracking via ats_prepared gate Message-ID: References: <20260519135323.1558777-1-praan@google.com> <20260519135323.1558777-4-praan@google.com> <20260519144430.GI7702@ziepe.ca> Precedence: bulk X-Mailing-List: linux-pci@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260519144430.GI7702@ziepe.ca> On Tue, May 19, 2026 at 11:44:30AM -0300, Jason Gunthorpe wrote: > On Tue, May 19, 2026 at 01:53:22PM +0000, Pranjal Shrivastava wrote: > > @@ -4450,7 +4450,8 @@ static struct iommu_device *arm_smmu_probe_device(struct device *dev) > > if (dev_is_pci(dev)) { > > unsigned int stu = __ffs(smmu->pgsize_bitmap); > > > > - pci_prepare_ats(to_pci_dev(dev), stu); > > + if (!pci_prepare_ats(to_pci_dev(dev), stu)) > > + master->ats_prepared = true; > > } > > This should fail not keep going, it is a kernel bug if > pci_prepare_ats() fails. Are you suggesting it's a kernel bug if the STU doesn't match? IIUC, it can fail because of the following cases (including the PF STU): if (!pci_ats_supported(dev)) return -EINVAL; if (WARN_ON(dev->ats_enabled)) return -EBUSY; if (ps < PCI_ATS_MIN_STU) return -EINVAL; if (dev->is_virtfn) { struct pci_dev *pdev = pci_physfn(dev); if (pdev->ats_stu != ps) return -EINVAL; return 0; } If ats isn't supported or if there's a STU mismatch, shuoldn't we just ensure that we don't try enabling ATS for that device? Returning an error here would fail the device probe.. which seems like a big hammer for devices that can't reliably use ATS.. I see Intel and AMD IOMMU drivers don't check the retval for this pci_prepare_ats but check it for pci_enable_ats. We have a special case in SMMUv3 where we pre-allocate stuff in prepare but actually enable ATS in attach_commit, which IMO neeeds to be gated based on client device's ATS capabilities? Praan