skiboot 5.9.3 release notes

Signed-off-by: Stewart Smith <[email protected]>

doc/release-notes/skiboot-5.9.3.rst

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+.. _skiboot-5.9.3:
+
+=============
+skiboot-5.9.3
+=============
+
+skiboot 5.9.3 was released on Wednesday November 22nd, 2017. It replaces
+:ref:`skiboot-5.9.2` as the current stable release in the 5.9.x series.
+
+Over :ref:`skiboot-5.9.2`, we have one NPU2/NVLink2 fix that causes the
+machine to crash hard in the event of hardware error rather than crash
+mysteriously later on whenever the NVLink2 links are used.
+
+That fix is:
+
+- npu2: hw-procedures: Add check_credits procedure
+
+  As an immediate mitigator for a current hardware glitch, add a procedure
+  that can be used to validate NTL credit values. This will be called as a
+  safeguard to check that link training succeeded.
+
+  Assert that things are exactly as we expect, because if they aren't, the
+  system will experience a catastrophic failure shortly after the start of
+  link traffic.