A4.6.6 Debug recovery

Debug recovery can be used to assist debug of external watchdog-triggered reset events.

It allows contents of the core L1 data and L2 caches that were present before the reset to be observable after the reset. The contents of the caches are retained and are not altered on the transition back to the On mode.

By default, the core invalidates its caches when transitioning from Off to On mode. If P-Channel is initialized to debug recovery, and the core is cycled through Cold or Warm reset along with system resets, then the cache invalidation is disabled. The cache contents are preserved when the core is transitioned to the On mode.

Debug recovery also supports preserving Reliability, Availability, and Serviceability (RAS) state, in addition to the cache contents. In this case, a transition to debug recovery is made from any of the current states. Once in debug recovery mode, a cluster-wide Warm reset must be applied externally. The RAS and cache state are preserved when the core is transitioned to the On mode.

Caution:

Debug recovery is strictly for debug purposes. It must not be used for functional purposes, as correct operation of the caches is not guaranteed when entering this mode.

  • This mode can occur at any time with no guarantee of the state of the core. A P-Channel request of this type is accepted immediately, therefore its effects on the core, cluster, or the wider system are unpredictable, and a wider system reset might be required. In particular, if there were outstanding memory system transactions at the time of the reset, then these might complete after the reset when the core is not expecting them and cause a system deadlock.
  • If the system sends a snoop to the cluster during this mode, then depending on the cluster state, the snoop might get a response and disturb the contents of the caches, or it might not get a response and cause a system deadlock.
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