# USB 32 KiB production baseline qualification

Date: 2026-07-29
Result: qualified as the current compatibility baseline

## Candidate

- Production HPT USB file-write cap: 32,768 bytes
- Debug S-record SHA-256:
  `157DC86B781FCE23B9D8F048B3F4F8C7524FFCBF0029E001310282338BDF1DFB`
- Release S-record SHA-256:
  `BE76DF6BDC3D6F6FBCBA843D82967AE18170781FE3B9302163F376F3364428B9`
- Focused checks: passed
- Full repository check suites: 23/23 passed
- Managed Debug and Release builds: passed

The 32 KiB change was isolated to the production HPT file-write cap.
Previously qualified R5 removal/remount lifecycle behavior and the diagnostic
16/32/64/96 KiB tiers remain present.

## Qualification matrix

| Controller | USB label | Published bytes | Calls | Maximum call | USB errors | Host persistence | FAT state |
| --- | --- | ---: | ---: | ---: | ---: | --- | --- |
| A | `USB01_2G` | 282,267 | 9 | 32,768 | 0 | pass | clean |
| A | `USB02_8G` | 282,267 | 9 | 32,768 | 0 | pass | summary mismatch preserved |
| A | `USB03_32G` | 282,267 | 9 | 32,768 | 0 | pass | summary mismatch preserved |
| A | `USB04_32G` | 282,267 | 9 | 32,768 | 0 | pass | summary mismatch preserved |
| B | `USB01_2G` | 248,593 | 8 | 32,768 | 0 | pass | clean |
| B | `USB04_32G` | 248,593 | 8 | 32,768 | 0 | pass | summary mismatch preserved |

Every run:

- used the pinned candidate image and controller-specific SD CID;
- required a mounted USB drive and valid retained HPT bounds;
- completed without snapshot or requested-range gaps;
- reached the published terminal state with HPT and USB status zero;
- exercised multiple file writes with an observed maximum of exactly
  32,768 bytes;
- was independently read on the Pi through a read-only mount;
- ended with a `COMPLETE` CSV trailer matching the published record count.

Controller B exported 81 recoverable degraded-lane records in both runs.
There were no requested-range gaps or HPT/USB failures; this is retained-media
health evidence, not a USB publication failure.

## Decision

Adopt 32 KiB as the current production HPT USB write-size compatibility
baseline for the tested Renesas/USBX implementation. The evidence supports
the production path on both controller assemblies and all four available USB
specimens, with representative cross-controller coverage at 2 GB and 32 GB.

The FAT summary inconsistencies on `USB02_8G`, `USB03_32G`, and `USB04_32G`
were preserved and not repaired. Their completed exports were readable and
hashable, but those specimens do not provide clean-filesystem qualification.

## 64 KiB promotion boundary

Do not promote 64 KiB from diagnostic/durability evidence alone. Promotion
remains a separate candidate and requires:

1. an isolated production-cap change with fresh Debug and Release artifacts;
2. production HPT exports large enough to exercise repeated 64 KiB writes;
3. both controller assemblies;
4. the full specimen matrix or a reviewed equivalent coverage rationale;
5. removal/remount lifecycle regression;
6. independent host persistence and FAT checks;
7. no regression in USB errors, publication state, retained-range integrity,
   or controller responsiveness.

Until those gates pass, 32 KiB remains the production compatibility baseline.
