linux-kernel - Re: [PATCH v7 3/3] iommu/tegra-smmu: Add PCI support

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Date:   Fri, 16 Oct 2020 18:56:08 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     thierry.reding@...il.com, joro@...tes.org, digetx@...il.com,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, jonathanh@...dia.com
Subject: Re: [PATCH v7 3/3] iommu/tegra-smmu: Add PCI support

On Fri, Oct 16, 2020 at 03:10:26PM +0100, Robin Murphy wrote:
> On 2020-10-16 04:53, Nicolin Chen wrote:
> > On Thu, Oct 15, 2020 at 10:55:52AM +0100, Robin Murphy wrote:
> > > On 2020-10-15 05:13, Nicolin Chen wrote:
> > > > On Wed, Oct 14, 2020 at 06:42:36PM +0100, Robin Murphy wrote:
> > > > > On 2020-10-09 17:19, Nicolin Chen wrote:
> > > > > > This patch simply adds support for PCI devices.
> > > > > > 
> > > > > > Reviewed-by: Dmitry Osipenko <digetx@...il.com>
> > > > > > Tested-by: Dmitry Osipenko <digetx@...il.com>
> > > > > > Signed-off-by: Nicolin Chen <nicoleotsuka@...il.com>
> > > > > > ---
> > > > > > 
> > > > > > Changelog
> > > > > > v6->v7
> > > > > >     * Renamed goto labels, suggested by Thierry.
> > > > > > v5->v6
> > > > > >     * Added Dmitry's Reviewed-by and Tested-by.
> > > > > > v4->v5
> > > > > >     * Added Dmitry's Reviewed-by
> > > > > > v3->v4
> > > > > >     * Dropped !iommu_present() check
> > > > > >     * Added CONFIG_PCI check in the exit path
> > > > > > v2->v3
> > > > > >     * Replaced ternary conditional operator with if-else in .device_group()
> > > > > >     * Dropped change in tegra_smmu_remove()
> > > > > > v1->v2
> > > > > >     * Added error-out labels in tegra_smmu_probe()
> > > > > >     * Dropped pci_request_acs() since IOMMU core would call it.
> > > > > > 
> > > > > >     drivers/iommu/tegra-smmu.c | 35 +++++++++++++++++++++++++----------
> > > > > >     1 file changed, 25 insertions(+), 10 deletions(-)
> > > > > > 
> > > > > > diff --git a/drivers/iommu/tegra-smmu.c b/drivers/iommu/tegra-smmu.c
> > > > > > index be29f5977145..2941d6459076 100644
> > > > > > --- a/drivers/iommu/tegra-smmu.c
> > > > > > +++ b/drivers/iommu/tegra-smmu.c
> > > > > > @@ -10,6 +10,7 @@
> > > > > >     #include <linux/kernel.h>
> > > > > >     #include <linux/of.h>
> > > > > >     #include <linux/of_device.h>
> > > > > > +#include <linux/pci.h>
> > > > > >     #include <linux/platform_device.h>
> > > > > >     #include <linux/slab.h>
> > > > > >     #include <linux/spinlock.h>
> > > > > > @@ -865,7 +866,11 @@ static struct iommu_group *tegra_smmu_device_group(struct device *dev)
> > > > > >     	group->smmu = smmu;
> > > > > >     	group->soc = soc;
> > > > > > -	group->group = iommu_group_alloc();
> > > > > > +	if (dev_is_pci(dev))
> > > > > > +		group->group = pci_device_group(dev);
> > > > > 
> > > > > Just to check, is it OK to have two or more swgroups "owning" the same
> > > > > iommu_group if an existing one gets returned here? It looks like that might
> > > > > not play nice with the use of iommu_group_set_iommudata().
> > > > 
> > > > Do you mean by "gets returned here" the "IS_ERR" check below?
> > > 
> > > I mean that unlike iommu_group_alloc()/generic_device_group(),
> > > pci_device_group() may give you back a group that already contains another
> > > device and has already been set up from that device's perspective. This can
> > > happen for topological reasons like requester ID aliasing through a PCI-PCIe
> > > bridge or lack of isolation between functions.
> > 
> > Okay..but we don't really have two swgroups owning the same groups
> > in case of PCI devices. For Tegra210, all PCI devices inherit the
> > same swgroup from the PCI controller. And I'd think previous chips
> > do the same. The only use case currently of 2+ swgroups owning the
> > same iommu_group is for display controller.
> > 
> > Or do you suggest we need an additional check for pci_device_group?
> 
> Ah, OK - I still don't have the best comprehension of what exactly swgroups

The "swgroup" stands for "software group", which associates with
an ASID (Address Space Identifier) for SMMU to determine whether
this client is going through SMMU translation or not.

> are, and the path through .of_xlate looked like you might be using the PCI
> requester ID as the swgroup identifier, but I guess that ultimately depends
> on what your "iommu-map" is supposed to look like. If pci_device_group()

This is copied from pcie node in our downstream dtsi file:

		iommus = <&mc TEGRA_SWGROUP_AFI>;
		iommu-map = <0x0 &mc TEGRA_SWGROUP_AFI 0x1000>;
		iommu-map-mask = <0x0>;

> will effectively only ever get called once regardless of how many endpoints
> exist, then indeed this won't be a concern (although if that's *guaranteed*
> to be the case then you may as well just stick with calling
> iommu_group_alloc() directly). Thanks for clarifying.

All PCI devices are supposed to get this swgroup of the pcie node
above. So the function will return the existing group of the pcie
controller, before giving a chance to call iommu_group_alloc().

Thanks for the review and input.