Incomplete ITRNG health test documentation

[akash-singh-NV]

Current spec does not document which ITRNG health tests are enabled by Caliptra ROM in version and how are relevant thresholds calculated and passed from SOC integrator. It is important to document these because any inconsistencies would make Caliptra unusable due to ROM dependencies.

[varuns-nvidia]

@mhatrevi @jhand2 could you help with documenting the TRNG health tests in the ROM spec?

[jhand2]

Health test thresholds are set here but I don't know how they were chosen. @korran do you know?

The health tests are are done by the IP are briefly described here.

It looks like we support 2 health checks (the two that are supported by the OpenTitan IP we use):

• Repetition Count Test
• Adaptive Proportion Test

The documentation for that IP lists two other checks ("Bucket and Markov tests"), but I don't see anything in ROM that configures them. Is anyone else on the thread able to confirm that these are not enabled?

[jhand2]

It looks like Kor has also created this PR with some guidance chipsalliance/caliptra-rtl#569. Does this cover what you're looking for?

[akash-singh-NV]

Kor's PR covers this to an extent. We also need to specify -

1. How ITRNG interprets 4 bit raw noise data - binary symbols vs 4-bit symbols ?
2. RCT implementation in Caliptra 1.x ITRNG + ROM combination is slightly different from expected. Ideally a programmed threshold of N should result in failure when ITRNG sees N identical bits (assuming binary noise source) or N identical 4 bit symbols (assuming non binary 4 bit noise source). However, Caliptra 1.x implementation will flag a failure if ITRNG sees N repetitions on any of the 4 bit lines.

[varuns-nvidia]

@howardtr for the above questions