linux-kernel - Re: [PATCH RFCv1 3/3] iommu/arm-smmu-v3: Allow ATS to be always on

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Message-ID: <20260119200625.GP1134360@nvidia.com>
Date: Mon, 19 Jan 2026 16:06:25 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: will@...nel.org, robin.murphy@....com, bhelgaas@...gle.com,
	joro@...tes.org, praan@...gle.com, baolu.lu@...ux.intel.com,
	kevin.tian@...el.com, miko.lenczewski@....com,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH RFCv1 3/3] iommu/arm-smmu-v3: Allow ATS to be always on

On Fri, Jan 16, 2026 at 08:56:42PM -0800, Nicolin Chen wrote:
> +static int arm_smmu_master_prepare_ats(struct arm_smmu_master *master)
> +{
> +	bool s1p = master->smmu->features & ARM_SMMU_FEAT_TRANS_S1;
> +	unsigned int stu = __ffs(master->smmu->pgsize_bitmap);
> +	struct pci_dev *pdev = to_pci_dev(master->dev);
> +	int ret;
> +
> +	if (!arm_smmu_ats_supported(master))
> +		return 0;
> +
> +	if (!pci_ats_always_on(pdev))
> +		goto out_prepare;
> +
> +	/*
> +	 * S1DSS is required for ATS to be always on for identity domain cases.
> +	 * However, the S1DSS field is ignored if !IDR0_S1P or !IDR1_SSIDSIZE.
> +	 */
> +	if (!s1p || !master->smmu->ssid_bits) {
> +		dev_info_once(master->dev,
> +			      "SMMU doesn't support ATS to be always on\n");
> +		goto out_prepare;
> +	}

It looks right, IDK if Will would prefer a formal ARM_SMMU_FEAT_S1DSS
though.

Jason