SCAMP: neighbouring chip link checks need a better approach #142
Description
To probe the state of its chip-to-chip links, scamp reads system controller register SC_CHIP_ID of every neighbouring chip [function init_link_en(), scamp-3.c]. This allows it to check if the neighbour is alive and if the link is behaving correctly. The returned value is ignored, only the correct completion of the read operation is checked.
- if the neighbour is alive and well then the read succeeds and the link is enabled.
- if the neighbour is truly dead then the read fails and the link is disabled.
- if the neighbour is alive but blacklisted, then the read may succeed, leaving the link enabled.
The last case can be avoided by blacklisting both ends of the link. This is possible only if the two chips are on the same board. If the two chips are on different boards then an inconsistent link state can only be avoided with a better link check.
This issue complements issues:
- BMP: blacklisting mechanism in FPGAs #143
- SpiNNakerManchester/SpiNNaker_hardware_tests#2