linux-kernel - Re: [PATCH 8/8] iommu/arm-smmu-v3: Add support for PCI PASID

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Message-ID: <20190919151002.GF1013538@lophozonia>
Date:   Thu, 19 Sep 2019 17:10:02 +0200
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Auger Eric <eric.auger@...hat.com>
Cc:     Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        will.deacon@....com, mark.rutland@....com,
        devicetree@...r.kernel.org, jacob.jun.pan@...ux.intel.com,
        joro@...tes.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, robh+dt@...nel.org,
        robin.murphy@....com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 8/8] iommu/arm-smmu-v3: Add support for PCI PASID

On Mon, Jul 08, 2019 at 09:58:16AM +0200, Auger Eric wrote:
> > +	ret = pci_enable_pasid(pdev, features);
> > +	if (!ret)
> > +		master->ssid_bits = min_t(u8, ilog2(num_pasids),
> > +					  master->smmu->ssid_bits);
> I don't really get why this setting is conditional to the success of
> pci_enabled_pasid and not num_pasids > 0.

num_pasids only contains the value of the PCIe PASID capability. If
pci_enable_pasid() fails then we want to leave master->ssid_bits to 0 so
that we report to users that SVA and AUXD aren't supported.

> If it fails the ssid_bits is set to min(smmu->ssid_bits,
> fwspec->num_pasid_bits) anyway.
>
> > +	return ret;
> > +}
> > +
> > +static void arm_smmu_disable_pasid(struct arm_smmu_master *master)
> > +{
> > +	struct pci_dev *pdev;
> > +
> > +	if (!dev_is_pci(master->dev))
> > +		return;
> > +
> > +	pdev = to_pci_dev(master->dev);
> > +
> > +	if (!pdev->pasid_enabled)
> > +		return;
> > +
> > +	pci_disable_pasid(pdev);
> > +	master->ssid_bits = 0;
> in case of a platform device you leave the ssid_bits to a value != 0. Is
> that what you want?

Yes, this is only for PCI devices, there is no standard way of disabling
PASID in platform devices. We just take whatever the firmware gives us.

> > +}
> > +
> >  static void arm_smmu_detach_dev(struct arm_smmu_master *master)
> >  {
> >  	unsigned long flags;
> > @@ -2413,6 +2456,9 @@ static int arm_smmu_add_device(struct device *dev)
> >  
> >  	master->ssid_bits = min(smmu->ssid_bits, fwspec->num_pasid_bits);
> >  
> > +	/* Note that PASID must be enabled before, and disabled after ATS */
> > +	arm_smmu_enable_pasid(master);
> In case the call fails, don't you want to handle the error and reset the
> ssid_bits?

This function fails if the device doesn't support PASID, and we leave
ssid_bits to 0. That said, I think it would be nicer to move the above
line (that deals with fwspec) into arm_smmu_enable_pasid()

Thanks,
Jean