[PATCH v4 6/8] iommu/arm-smmu: Implement of_xlate() for SMMUv3

On 29/07/16 19:55, Robin Murphy wrote:

On 29/07/16 15:46, Jean-Philippe Brucker wrote:

quoted

Hi Robin,

Very sorry about the delay, I forgot about this minor comment, below

On Fri, Jul 01, 2016 at 05:50:15PM +0100, Robin Murphy wrote:

[...]

quoted

quoted

+		smmu = arm_smmu_get_by_node(fwspec->iommu_np);
+		if (!smmu)
+			return -ENODEV;
+		master = kzalloc(sizeof(*master), GFP_KERNEL);
+		if (!master)
+			return -ENOMEM;
+
+		master->smmu = smmu;
+		fwspec->iommu_priv = master;

It's probably best to initialise master->ste.bypass = true here, to
reflect the initial state of STEs. Otherwise attach_dev always calls
detach_dev first for nothing.

I'm actually now thinking that that check in attach_dev() should be for
ste->valid, rather than ste->bypass. That matches the similar check in
remove_device(), and looking at old local branches I apparently did have
it that way at some point, and I now can't quite remember why it ended
up differently. I'll have a proper dig into it next week.

[for a value of "next week" relative to "last week", at least...]

Having now remembered, the reason it is this way is a subtle concession
to the nasty PCI RID alias case. When you come to probe the second
device behind a non-transparent bridge, the first device is already
attached to the default domain so the underlying STE is no longer a
bypass entry, but we've got no way of knowing that - we can't tell we're
going to be in an alias group until we call iommu_group_get_for_dev(),
and by the time that returns it's already too late, since the attach to
the default domain (and thus the attempt to write a now-valid STE with
the same data again) will have happened.

The least complicated ways around that that I can think of are 1)
inspect the stream table itself on attach, 2) maintain a semi-redundant
list within the group of exactly which ID is attached to which domain at
any point in time, 3) treat the initial per-device state as undefined
(!valid, !bypass) so that the initial domain attach is always a safe
break-before-make on the stream table, or 4) disallow aliasing IDs
entirely and just let the BUG() in write_strtab_ent() fire if a
non-transparent bridge ever comes along.

Discounting #4 as unreasonable, #3 is by far the least complicated, so
I've kept this as it is for now. #2 might seem reasonable at a glance,
but it wouldn't be useful for anything _except_ this specific situation,
which in practice we'd expect to encounter rarely if ever.

Robin.

quoted

Apart from that, this version works fine with my twisted setup. Note
that we might have to add a master->fwspec reference in the future, to
access those SIDs from various places. If I understood correctly, it
should be fine as those objects have the same lifetime after the
add_device call.

That sounds reasonable, although I can't think offhand where we might
have a master_data without having got it via the containing device
(unless we also add some other means of keeping track of them). Since
this series doesn't need any kind of reverse-lookup capabilities I'm
just keeping things as simple as possible for the time being.

Robin.

quoted

Thanks,
Jean-Philippe


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