of, numa: Validate some distance map rules

of, numa: Validate some distance map rules

Currently the NUMA distance map parsing does not validate the distance
table for the distance-matrix rules 1-2 in [1].

However the arch NUMA code may enforce some of these rules, but not all.
Such is the case for the arm64 port, which does not enforce the rule that
the distance between separates nodes cannot equal LOCAL_DISTANCE.

The patch adds the following rules validation:
- distance of node to self equals LOCAL_DISTANCE
- distance of separate nodes > LOCAL_DISTANCE

This change avoids a yet-unresolved crash reported in [2].

A note on dealing with symmetrical distances between nodes:

Validating symmetrical distances between nodes is difficult. If it were
mandated in the bindings that every distance must be recorded in the
table, then it would be easy. However, it isn't.

In addition to this, it is also possible to record [b, a] distance only
(and not [a, b]). So, when processing the table for [b, a], we cannot
assert that current distance of [a, b] != [b, a] as invalid, as [a, b]
distance may not be present in the table and current distance would be
default at REMOTE_DISTANCE.

As such, we maintain the policy that we overwrite distance [a, b] = [b, a]
for b > a. This policy is different to kernel ACPI SLIT validation, which
allows non-symmetrical distances (ACPI spec SLIT rules allow it). However,
the distance debug message is dropped as it may be misleading (for a distance
which is later overwritten).

Some final notes on semantics:

- It is implied that it is the responsibility of the arch NUMA code to
  reset the NUMA distance map for an error in distance map parsing.

- It is the responsibility of the FW NUMA topology parsing (whether OF or
  ACPI) to enforce NUMA distance rules, and not arch NUMA code.

[1] Documents/devicetree/bindings/numa.txt
[2] https://www.spinics.net/lists/arm-kernel/msg683304.html

Cc: stable@vger.kernel.org # 4.7
Signed-off-by: John Garry <john.garry@huawei.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>