s2idle reaches S0ix (~96% of suspend); note which residency counter to trust

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

NEON-CUSTOMIZATIONS.md

@@ -120,10 +120,16 @@ Tested 2026-09-27. The firmware offers both `s2idle` (default) and `deep`
   meant for root on USB storage) was already on and didn't save it.
   There's no dependable fix, so **never set `mem_sleep_default=deep`**
   while neon's root is on the microSD slot.
-- `s2idle` keeps USB powered and has worked for months. Whether it
-  reaches S0ix (the package's deepest idle) is shown by
-  `/sys/devices/system/cpu/cpuidle/low_power_idle_system_residency_us`
-  growing across a suspend.
+- `s2idle` keeps USB powered and has worked for months. **It does
+  reach S0ix**: in a 44s lid-close test (2026-09-28),
+  `/sys/kernel/debug/pmc_core/slp_s0_residency_usec` (needs root)
+  grew by ~42.2s, and `package_cstate_show` showed Package C10 ~42.0s.
+  So ~96% of the suspend was in the deepest idle state, and drain should
+  be close to S3's. Root stayed `rw` with no I/O errors.
+- Use the `pmc_core` counter above, not
+  `/sys/devices/system/cpu/cpuidle/low_power_idle_system_residency_us`.
+  That one comes from ACPI LPIT, which isn't wired up properly under
+  coreboot here: it read 270µs for the same suspend.
 
 ## Networking