docs/open-questions.md

# Open questions

The genuinely open, thinking-worthy problems — nothing here is resolved. Analyses and settled facts live in the device docs; each entry points at the doc that holds its detail. Read `state.md` first for placement context.

## 1. ECD run disturbance

The BCM ECD run blips the link (PMA 1.1 latch-low catches a drop even with the break-link bit clear). Until the disturbance is characterized, length measurement is a between-runs operation, not a during-run one ([modules/fs/](modules/fs/README.md)).

## 2. Smear window sizing

`smearWindow` (4 buckets = 64 ms) is a chosen default, not a measured one. Measure the host read-stall distribution under load on the X520 datapath, then size the window comfortably past the worst stall — it is also the display latency, so no larger than needed ([nics/x520/](nics/x520/README.md)).

## 3. Pre-FEC correlation run

The register question is answered (post-FEC vs corrected-by-iteration histogram located); the graded-noise correlation run that verifies the expected ordering has not happened. Design and instrument: [modules/fibergaga/](modules/fibergaga/README.md).

## 4. Wiitek VCT — pursue or leave dead?

No confirmed-safe path exists (every candidate lands in the µC danger window). The open decision is whether the capability is worth the NDA route or a sacrificial unit — the product doesn't need it for length ([modules/wiitek/](modules/wiitek/README.md), [modules/README.md](modules/README.md)).
Add open-questions doc for pre-hardware deep-dive 2026-08-10 11:31:19 -07:00			`# Open questions`

Dissolve resolved analyses into standalone device docs; open-questions.md keeps only the four live problems; discard NIC-timestamp length path (BCM ECD is proven) and Aquantia length trivia (30m part, oracle-only) 2026-08-12 21:30:16 -07:00			The genuinely open, thinking-worthy problems — nothing here is resolved. Analyses and settled facts live in the device docs; each entry points at the doc that holds its detail. Read `state.md` first for placement context.
Add open-questions doc for pre-hardware deep-dive 2026-08-10 11:31:19 -07:00
X520 live as the product NIC: rate buckets re-keyed to read time (one shared clock, one commit per drained batch) with the backward smear as a settled window — each completed bucket enters once, donates excess to the nearest earlier deficits once, locks for display when nothing later can refill it (per-sample recompute double-counted excess and read >20G on a 20G wire); timestamp machinery deleted (ts.go, SO_TIMESTAMPING, rx_filter=ALL check) after the audit confirmed the buckets were its sole consumer; testDriver ixgbe; RollBall mailbox hardened against orphaned-command completions (never sample status before a poll gap, value from the same block read, devad/reg echo required — a killed session's 7.60 read answered a PHY ID as 0x0006); full hardware pass: 20.00G flat both directions, zero errors, ECD 42m, SNR +7.4dB 2026-08-17 09:35:58 -07:00			`## 1. ECD run disturbance`
Add open-questions doc for pre-hardware deep-dive 2026-08-10 11:31:19 -07:00
X520 live as the product NIC: rate buckets re-keyed to read time (one shared clock, one commit per drained batch) with the backward smear as a settled window — each completed bucket enters once, donates excess to the nearest earlier deficits once, locks for display when nothing later can refill it (per-sample recompute double-counted excess and read >20G on a 20G wire); timestamp machinery deleted (ts.go, SO_TIMESTAMPING, rx_filter=ALL check) after the audit confirmed the buckets were its sole consumer; testDriver ixgbe; RollBall mailbox hardened against orphaned-command completions (never sample status before a poll gap, value from the same block read, devad/reg echo required — a killed session's 7.60 read answered a PHY ID as 0x0006); full hardware pass: 20.00G flat both directions, zero errors, ECD 42m, SNR +7.4dB 2026-08-17 09:35:58 -07:00			`The BCM ECD run blips the link (PMA 1.1 latch-low catches a drop even with the break-link bit clear). Until the disturbance is characterized, length measurement is a between-runs operation, not a during-run one ([modules/fs/](modules/fs/README.md)).`
Answer open questions from desk analysis: ETQF steering, E820 is post-FEC, compound I2C op, forced co-location 2026-08-10 11:51:39 -07:00
X520 live as the product NIC: rate buckets re-keyed to read time (one shared clock, one commit per drained batch) with the backward smear as a settled window — each completed bucket enters once, donates excess to the nearest earlier deficits once, locks for display when nothing later can refill it (per-sample recompute double-counted excess and read >20G on a 20G wire); timestamp machinery deleted (ts.go, SO_TIMESTAMPING, rx_filter=ALL check) after the audit confirmed the buckets were its sole consumer; testDriver ixgbe; RollBall mailbox hardened against orphaned-command completions (never sample status before a poll gap, value from the same block read, devad/reg echo required — a killed session's 7.60 read answered a PHY ID as 0x0006); full hardware pass: 20.00G flat both directions, zero errors, ECD 42m, SNR +7.4dB 2026-08-17 09:35:58 -07:00			`## 2. Smear window sizing`
Add open-questions doc for pre-hardware deep-dive 2026-08-10 11:31:19 -07:00
X520 live as the product NIC: rate buckets re-keyed to read time (one shared clock, one commit per drained batch) with the backward smear as a settled window — each completed bucket enters once, donates excess to the nearest earlier deficits once, locks for display when nothing later can refill it (per-sample recompute double-counted excess and read >20G on a 20G wire); timestamp machinery deleted (ts.go, SO_TIMESTAMPING, rx_filter=ALL check) after the audit confirmed the buckets were its sole consumer; testDriver ixgbe; RollBall mailbox hardened against orphaned-command completions (never sample status before a poll gap, value from the same block read, devad/reg echo required — a killed session's 7.60 read answered a PHY ID as 0x0006); full hardware pass: 20.00G flat both directions, zero errors, ECD 42m, SNR +7.4dB 2026-08-17 09:35:58 -07:00			`smearWindow` (4 buckets = 64 ms) is a chosen default, not a measured one. Measure the host read-stall distribution under load on the X520 datapath, then size the window comfortably past the worst stall — it is also the display latency, so no larger than needed ([nics/x520/](nics/x520/README.md)).
Answer open questions from desk analysis: ETQF steering, E820 is post-FEC, compound I2C op, forced co-location 2026-08-10 11:51:39 -07:00
Dissolve resolved analyses into standalone device docs; open-questions.md keeps only the four live problems; discard NIC-timestamp length path (BCM ECD is proven) and Aquantia length trivia (30m part, oracle-only) 2026-08-12 21:30:16 -07:00			`## 3. Pre-FEC correlation run`
Vendor AQR datasheet, correct timestamp model (hw rx stamps are a hard requirement), order ConnectX-5, map 82599 bucket-timestamp routes 2026-08-10 12:50:57 -07:00
Dissolve resolved analyses into standalone device docs; open-questions.md keeps only the four live problems; discard NIC-timestamp length path (BCM ECD is proven) and Aquantia length trivia (30m part, oracle-only) 2026-08-12 21:30:16 -07:00			`The register question is answered (post-FEC vs corrected-by-iteration histogram located); the graded-noise correlation run that verifies the expected ordering has not happened. Design and instrument: [modules/fibergaga/](modules/fibergaga/README.md).`
Vendor AQR datasheet, correct timestamp model (hw rx stamps are a hard requirement), order ConnectX-5, map 82599 bucket-timestamp routes 2026-08-10 12:50:57 -07:00
Dissolve resolved analyses into standalone device docs; open-questions.md keeps only the four live problems; discard NIC-timestamp length path (BCM ECD is proven) and Aquantia length trivia (30m part, oracle-only) 2026-08-12 21:30:16 -07:00			`## 4. Wiitek VCT — pursue or leave dead?`
Vendor AQR datasheet, correct timestamp model (hw rx stamps are a hard requirement), order ConnectX-5, map 82599 bucket-timestamp routes 2026-08-10 12:50:57 -07:00
Dissolve resolved analyses into standalone device docs; open-questions.md keeps only the four live problems; discard NIC-timestamp length path (BCM ECD is proven) and Aquantia length trivia (30m part, oracle-only) 2026-08-12 21:30:16 -07:00			`No confirmed-safe path exists (every candidate lands in the µC danger window). The open decision is whether the capability is worth the NDA route or a sacrificial unit — the product doesn't need it for length ([modules/wiitek/](modules/wiitek/README.md), [modules/README.md](modules/README.md)).`