[entropy_src] Restarting physical noise source through rng_en

[johannheyszl]

Description

We will restart the physical noise source for cases of a persistent error. This ability also helps with SP 800-90B restart testing to simplify test harnesses. An rng_en_i signal into AST is driving this off/on reset, and originates in entropy_src.

We need to make sure this rng_en is deasserted carefully and in sync with the entropy_src to maintain FIPS-compatibility. Startup testing shall be repeated upon each restart for instance.

• For example, the signal shall not be deasserted upon the FIFO full event, even though this is expected to be very rare or ‘should never happen’ (@vogelpi to pls add context here). Beware that for startup testing we might need to fill the FIFO up to capacity and certainly do not want a deassert of the signal as a result.
• Also check other conditions for deassertion.

Issue created after discussion b/w: @moidx @johannheyszl VadimS @vogelpi @zi-v @h-filali

[johannheyszl]

cc @msfschaffner @andreaskurth

[vogelpi]

We've discussed this in the Security WG meeting today. Even thought the 2 x 4-bit input FIFO should never become full during regular operation we should probably change the design to:

1. Notify software if this FIFO or any of the following ones get full (except for the Observe FIFO for which this is already correctly handled). And, because software needs to know if entropy is dropped.
2. Not lower the rng_enable_o signal if those FIFOs get full because the AST should not be disabled/enabled because of that.