Hi all,

While optimising SST stack usage, we found that we were blocked from reducing it further by the stack usage of the dummy NV counters implementation used by all of the development platforms currently maintained in the repo. SST makes calls to the NV counter interface, so any stack used by this implementation gets allocated from the SST stack.

The current dummy NV counter implementation reads the entire flash sector containing the counters (generally 4KB) into stack memory, so that it can erase the sector and then re-write the same sector with a counter incremented. It needs to do this because the sector might contain some other data, unrelated to NV counters.

I am proposing that we simplify this dummy implementation to assume that the dummy NV counters are the only thing in the sector. Then only the NV counters themselves (currently 28 bytes) need to be read into memory for modification. The downside of this of course is that a whole 4KB flash sector needs to be set aside for the dummy NV counters. This seems bad, but it is worth bearing in mind that this is only a dummy implementation that should never be deployed in a real system. In practice, it seems unlikely to me that we would ever want to make use of the partial-sector left after the dummy NV counters anyway, and no platform currently does.

With this change made, the stack allocated for SST can be reduced from 6KB to 2.5KB (in this patch https://review.trustedfirmware.org/c/trusted-firmware-m/+/1775).

The change is currently on review here https://review.trustedfirmware.org/c/trusted-firmware-m/+/1918/ Feedback and alternative ideas are welcome 😊

Kind regards, Jamie