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

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Message-ID: <aXewprV/zHKQ+ra+@Asurada-Nvidia>
Date: Mon, 26 Jan 2026 10:21:26 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Will Deacon <will@...nel.org>
CC: <jgg@...dia.com>, <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 Mon, Jan 26, 2026 at 12:39:50PM +0000, Will Deacon wrote:
> On Fri, Jan 16, 2026 at 08:56:42PM -0800, Nicolin Chen wrote:
> > @@ -1422,7 +1422,7 @@ void arm_smmu_clear_cd(struct arm_smmu_master *master, ioasid_t ssid)
> >  	if (!arm_smmu_cdtab_allocated(&master->cd_table))
> >  		return;
> >  	cdptr = arm_smmu_get_cd_ptr(master, ssid);
> > -	if (WARN_ON(!cdptr))
> > +	if (!cdptr)
> >  		return;
> 
> Should we still warn if !master->ats_always_on?

Hmm, yes. I'll fix this.

> > @@ -3205,7 +3222,7 @@ static void arm_smmu_attach_dev_ste(struct iommu_domain *domain,
> >  				    struct iommu_domain *old_domain,
> >  				    struct device *dev,
> >  				    struct arm_smmu_ste *ste,
> > -				    unsigned int s1dss)
> > +				    unsigned int s1dss, bool ats_always_on)
> >  {
> >  	struct arm_smmu_master *master = dev_iommu_priv_get(dev);
> 
> Can we avoid the 'bool' parameter if possible, please? They tend to make the
> callsites pretty horrible to read and you're already passing the 'struct
> device *' so you should have the master in hand?

Trying to set ats_always_on=false for blocked domain here:

@@ -3260,7 +3277,8 @@ static int arm_smmu_attach_dev_identity(struct iommu_domain *domain,
	arm_smmu_attach_dev_ste(domain, old_domain, dev, &ste,
				STRTAB_STE_1_S1DSS_BYPASS,
				master->ats_always_on);
@@ -3283,7 +3301,7 @@ static int arm_smmu_attach_dev_blocked(struct iommu_domain *domain,
	arm_smmu_attach_dev_ste(domain, old_domain, dev, &ste,
				STRTAB_STE_1_S1DSS_TERMINATE, false);

But I think we could do that by combining master->ats_always_on
with the s1dss. I will drop the "bool".

Thanks
Nicolin