notes: Patch C v3 Phase 4 plan — drop sdio_rx_work, match cw1200

Supersedes v2 (PR #10).  cw1200 mining (~/src/linux-rockchip, 228
cw1200 commits) confirmed: upstream cw1200 has no sdio_rx_work
workqueue at all.  IRQ handler bumps bh_rx + wakes bh_wq; bh thread
does the SDIO read inline via cw1200_bh_rx_helper.  Single thread =
single writer for hw_bufs_used = no race by construction.  Same int
hw_bufs_used as bes2600, never atomic_t'd in 16 years upstream.

v3 brings bes2600 into that shape:

  - delete sdio_rx_work, self->rx_work, self->rx_queue,
    self->rx_queue_lock, bes2600_sdio_pipe_read
  - GPIO IRQ handler calls self->irq_handler directly (matches
    cw1200_sdio_irq_handler shape)
  - bes2600_bh_rx_helper's BES_SDIO_RX_MULTIPLE_ENABLE branch
    replaced with inline SDIO read + extract_packets + per-skb
    delivery via new bes2600_bh_handle_rx_skb()
  - GPIO wake-flag bracketing moves into bh thread

§5 shared-state delta table (the v2 lesson, applied):  zero fields
require new locking.  hw_bufs_used / hw_bufs_used_vif / wsm_tx_pending
all stay single-writer-from-bh.  v2's atomic_t prep is mooted.

§6 risk #6 is the open question for reviewer:  bes2600's
__bes2600_irq_enable(1) call is commented out in the BH-loop done:
label with an asm volatile("nop") in its place.  Either SDIO IRQ
is auto-managed (so commenting out is fine) or the current code
relies on sdio_rx_work being queued regardless of driver-side IRQ
flag.  Block Phase 6 on this audit.

Patch F (PR #4 merged) is the new baseline.  v3 will branch off
F-merged cleanups.  Phase 7 N=3 stress ramp uses wired enu1 path
(192.168.88.80) for wedge-resilient telemetry collection.

notes/patch-c-v2-phase4-plan-2026-05-07.md

@@ -0,0 +1,136 @@
+# Patch C v2 — Phase 4 Plan: atomic_t prep + direct-deliver
+
+**Author:** Claude (noether)
+**Status:** Phase 4 v2 — Phase 7 of Patch C (notes/patch-c-phase4-plan-2026-05-07.md, PR #9 merged) failed with a thread-safety race; this is the redesign.
+**Decision:** Option B from PR #3 close-out comment — `atomic_t` prep refactor first, direct-deliver on top.
+
+---
+
+## §0 What just happened (Phase 7 of Patch C)
+
+Reproduced verbatim from boot -1 of ohm 2026-05-07 20:18:10 CEST, ~13 s into a 4 MB/s nc stress:
+
+```
+WARNING: at wsm_release_tx_buffer+0x84/0xa0 [bes2600], CPU#0: kworker/0:3H/3912
+Workqueue: bes_sdio sdio_rx_work [bes2600]
+pc : wsm_release_tx_buffer+0x84/0xa0 [bes2600]
+lr : bes2600_bh_handle_rx_skb+0x134/0x370 [bes2600]
+sdio_rx_work+0x2a8/0x540 [bes2600]
+bes2600_wlan: wsm_release_tx_buffer failed: -1
+```
+
+Storm continued; chip wedged; ohm fell off the WiFi (wlan0).  Patch C module preserved at `/var/tmp/bes2600.patchC-broken.ko` for forensics.  Patch B rolled back, currently on disk on ohm.  Lesson saved as `feedback_phase6_contract_threadsafety` memory.
+
+## §1 Why it failed
+
+`wsm_release_tx_buffer()` (bh.c:222–243) does **unlocked** read–modify–write on `hw_priv->hw_bufs_used`.  Pre-Patch-C invariant was single-writer = BH thread; the lock that mattered was structural, not annotated.  Patch C's direct-deliver moved one writer (RX-confirm decrement) into `sdio_rx_work` workqueue context.  BH thread + sdio_rx_work race on the int counter; underflow below zero, WARN, return -1, bookkeeping corrupt, TX wedges.
+
+Phase 6 contract block correctly cited `wsm_handle_rx`'s sleepability and held-lock invariants — but stopped at the called function's signature.  It did not enumerate `hw_bufs_used` as shared state mutated by the callee.  That's the gap.
+
+## §2 Shared-state delta table (the thing missing from Patch C)
+
+Every field that `bes2600_bh_handle_rx_skb` mutates either directly or transitively, with current protection and required action:
+
+| field | declared at | written by (today) | written by (after Patch C v2) | current protection | action needed |
+|---|---|---|---|---|---|
+| `hw_priv->hw_bufs_used` | bes2600.h | `wsm_alloc_tx_buffer` (bh thread, TX submit), `wsm_release_tx_buffer` (bh thread, RX confirm), `main.c:543` (init) | + `wsm_release_tx_buffer` from sdio_rx_work | single-writer = BH thread (structural) | **convert to `atomic_t`** |
+| `hw_priv->hw_bufs_used_vif[i]` | bes2600.h | `wsm_release_vif_tx_buffer` (bh thread), `bh.c:1271` (vif TX submit), init | + `wsm_release_vif_tx_buffer` from sdio_rx_work | single-writer = BH thread | **convert to `atomic_t [N]`** |
+| `hw_priv->wsm_rx_seq[i]` | bes2600.h | bh thread RX | sdio_rx_work only | single-writer = BH/sdio_rx context (was BH, now is sdio_rx_work, but still **one writer**) | OK — single writer |
+| `hw_priv->wsm_tx_pending[i]` | bes2600.h | `bes2600_bh_inc_pending_count` (TX submit, BH thread), `bes2600_bh_dec_pending_count` (RX confirm) | dec moves to sdio_rx_work; inc stays BH | single-writer = BH | **also needs `atomic_t`** |
+| `hw_priv->lmac_mon_timer` / `mcu_mon_timer` | bes2600.h | mod_timer / del_timer_sync from BH | ditto from sdio_rx_work | timer API is internally locked | OK — `mod_timer` is concurrency-safe |
+| `hw_priv->wsm_cmd.lock` (taken inside wsm_handle_rx) | wsm_buf | bh thread (today) | sdio_rx_work | spinlock | OK — already protected |
+| `hw_priv->vif_lock` (taken inside wsm_handle_rx for some paths) | per vif | bh thread today | sdio_rx_work | spinlock | OK |
+| `priv->bh_evt_wq` wake-up | bes2600.h | wsm_release_tx_buffer when count hits 0 | ditto from sdio_rx_work | wake_up is concurrency-safe | OK |
+| `bes2600_pwr_clear_busy_event` (called inside release) | bes_pwr | bh thread | sdio_rx_work | internal locking via `bes_power.lock` | OK |
+| `hw_priv->buf_released` | bes2600.h | only `wsm_release_buffer_to_fw` (MCAST_FWDING ifdef, AP-only) | unchanged — BH only | single-writer = BH | OK — not on Patch C v2 hot path |
+
+**Three fields require atomic_t conversion:** `hw_bufs_used`, `hw_bufs_used_vif[]`, `wsm_tx_pending[]`.  Everything else is already concurrency-safe or moves cleanly to single-writer-in-sdio_rx_work.
+
+## §3 Read-site survey (the rest of the work — atomic_read swaps)
+
+`grep -hE "hw_bufs_used\b|hw_bufs_used_vif\b" *.c *.h | wc -l` = **57 references** across the source tree:
+
+- 5 writers (above)
+- 52 readers — converted mechanically to `atomic_read()`.  Distribution:
+  - `bh.c`: 22 read sites (most in the bh main loop, BUG_ON gates, idle / suspend predicates)
+  - `sta.c`: 3 sites (PM idle check at sta.c:1231–1253)
+  - `bes2600_sdio.c`: 1 site (PM idle check at line 958)
+  - `main.c`: 2 sites (init zero, teardown wait)
+  - `debug.c`: 1 site (debugfs stats)
+  - `itp.c`: 1 site (test mode)
+
+`wsm_tx_pending[i]` site count is smaller — ~6 references, all in bh.c and the timer monitors.  Same mechanical conversion.
+
+## §4 Plan v2 — two-step
+
+**Patch C-prep** (NFC, lands first):
+
+- Convert `hw_bufs_used` from `int` → `atomic_t`.
+- Convert `hw_bufs_used_vif[CW12XX_MAX_VIFS]` from `int[]` → `atomic_t[]`.
+- Convert `wsm_tx_pending[2]` from `int[]` → `atomic_t[]`.
+- Update writers:
+  - `wsm_alloc_tx_buffer`: `atomic_inc(&hw_priv->hw_bufs_used)`.
+  - `wsm_release_tx_buffer`: rewrite with `atomic_fetch_sub_release(count, &hw_priv->hw_bufs_used)` — returns prior value.  Re-derive the "tx restart" predicate (`prior >= numInpChBufs - 1`) and the "wake bh_evt_wq + clear busy" predicate (`prior - count == 0`) from that.  WARN if `prior - count < 0`.
+  - `wsm_release_vif_tx_buffer`: same pattern on the array element.
+  - `bes2600_bh_inc/dec_pending_count`: use `atomic_inc` and `atomic_dec_return` (need post-decrement value to decide whether to del_timer).
+- Update all 52+6 read sites: mechanical `atomic_read()` swap.
+- `main.c:543` init: `atomic_set(&hw_priv->hw_bufs_used_vif[i], 0)`.
+
+**Patch C-prep does NOT change behaviour.**  Same atomic ordering (`_release` / `_acquire` chosen to match the implicit memory ordering the BH-only path had).  Phase 7 of C-prep alone should show **identical** numbers to pre-patch baseline (`run-20260507-patchC-preflight`): 1.36 MB/s, 86.4 sdio_rx_work/sec, 90.3 dispatches per 1000 RX pkts, 0 bh_work redispatches.  If Phase 7 of C-prep shows a delta, the atomic ordering is wrong and we loop back here, not to C v2.
+
+**Patch C v2** (the actual structural change, lands on top of C-prep):
+
+- Identical to Patch C as merged in PR #3 (since closed): direct-deliver from `bes2600_sdio_extract_packets` into `bes2600_bh_handle_rx_skb`, no `rx_queue` indirection, no bh wake-up for RX.
+- The contract block in `bh.c::bes2600_bh_handle_rx_skb` is **expanded** to include the shared-state delta table from §2 of this plan, with explicit citations.
+- Same minimum-diff scope as Patch C: keep `rx_queue`, `pipe_read`, `bh_rx_helper` for clean bisection; remove in a follow-up hygiene patch.
+
+## §5 What will NOT be touched (deferred or out of scope)
+
+- mac80211-side `ieee80211_rx_irqsafe` → `ieee80211_rx_list` migration: that's Patch C2, gated on Task #19 kerneldoc verification.
+- The `#if 0` graveyard in bh.c, the `asm volatile("nop")` placeholder, the BUG_ON in steady-state hot path: still symptom-shaped per `feedback_dont_patch_downstream_artifacts`.  Re-evaluate at Task #24 after C v2 / D / E land.
+- `ba_lock` (Patch D) and `ps_state_lock` (Patch E): independent.
+
+## §6 Risk list (per Phase 6 contract-thread-safety memory)
+
+1. **C-prep memory ordering**: I've chosen `atomic_fetch_sub_release` for `wsm_release_tx_buffer` to mirror the implicit BH-thread ordering (release before subsequent atomic ops on `bh_evt_wq` / `bes_power`).  If the BH thread or other readers expect `_acquire` semantics on the value, we get reordering bugs that are hard to reproduce.  **Mitigation:** pair with `_acquire` reads where the read-then-decision pattern is critical (e.g., the bh main loop's `if (!hw_priv->hw_bufs_used)` idle predicate).  Cite the kerneldoc reference for `atomic_fetch_sub_release` in the commit message.
+
+2. **`wsm_tx_pending[]` decrement-side timer interaction**: `bes2600_bh_dec_pending_count` does `if (--hw_priv->wsm_tx_pending[idx] == 0) del_timer_sync(timer); else mod_timer(timer, ...)`.  After atomic_t conversion: `if (atomic_dec_return(&hw_priv->wsm_tx_pending[idx]) == 0) ...`.  But *another* thread could `atomic_inc` between our dec and the timer call, racing the del_timer.  `del_timer_sync` is internally safe (it can be called concurrently with `mod_timer`), but the **decision** "whether to delete vs mod" is racy.  **Mitigation:** even after atomic conversion, this function still needs to be called from a single context.  Verify `inc/dec_pending_count` callers — if both sides only fire from BH and sdio_rx_work and never overlap on the same idx, we're fine; if not, this needs a lock.
+
+3. **`hw_bufs_used_vif[]` array vs `wsm_alloc_tx_buffer`**: vif counter increment lives at bh.c:1271, called from bh thread TX-submit path.  Decrement (`wsm_release_vif_tx_buffer`) called from RX-confirm.  After Patch C v2 the decrement is in sdio_rx_work — same race shape as the global counter.  Already covered by the atomic_t array conversion.
+
+4. **PM idle predicate at sta.c:1239**: reads `hw_priv->hw_bufs_used_vif[priv->if_id]` to decide can-sleep.  Currently racy (was already reading BH-mutated state from a non-BH PM context).  Atomic conversion makes the read coherent.  PM context's read-then-decide is still fundamentally a snapshot — no change in semantics, just no torn-read.
+
+5. **Reboot / module-unload teardown** (`main.c:840`): `wait_event_timeout(... !hw_priv->hw_bufs_used ...)`.  Becomes `... !atomic_read(...)`.  No semantic change — the wait_event macro re-evaluates the predicate on each wake.
+
+6. **Phase 7 rig: Patch C v2 still wedges chip if I missed anything**: now mitigated by ohm's new wired interface (enu1, 192.168.88.80) — survives bes2600 wedges, lets us collect dmesg / ftrace / journalctl from a wedged ohm without reboot.  See `reference_ohm_wired_iface` memory.
+
+## §7 Phase 5 review handover
+
+PR on git.reauktion.de/marfrit/besser, this file as the artifact (per `feedback_phase5_surface_is_pr`).  Specifically request reviewer focus on §2 shared-state delta table — that's the part that should have caught Patch C's bug.  Don't curate.
+
+## §8 Phase 6 implementation order
+
+1. Branch off `cleanups` on bes2600-dkms-mobian: `bes2600/atomic-tx-buf-counters` (= Patch C-prep).
+2. Mechanical refactor: `int hw_bufs_used` → `atomic_t hw_bufs_used`, all reads → `atomic_read`, all writes → atomic ops.  Same for vif array and tx_pending array.  No other changes.
+3. Build, install, smoke-test.  Phase 7 of C-prep.  Should be a no-op delta.
+4. PR + Phase 5 review + merge.
+5. Branch off C-prep: `bes2600/sdio-rx-direct-deliver-v2` (= Patch C v2).
+6. Re-apply the Patch C delta (3 files: bh.h, bh.c, bes2600_sdio.c — same edits as PR #3).
+7. Build, install, Phase 7 N=3 stress ramp.
+8. PR + Phase 5 review + merge.
+
+## §9 Phase 7 v2 protocol (per `feedback_phase7_stress_ramp` + wired-rig)
+
+1. Pre-C-prep baseline rep N=3 (re-anchor, since current N=1 baseline is from `run-20260507-patchC-preflight`).
+2. Apply C-prep, N=3.  Compare to pre.  Expect: zero meaningful delta.  If non-zero → memory-ordering bug, loop back to §4 atomic-ordering choice.
+3. Apply C v2, N=3.  Compare to C-prep baseline.  Expect: §4.5 of original Patch C plan's predicted delta (rx_queue lock acquires → 0, observed RX KB/s lifts toward ≥1 MB/s sustained @ 4MB/s).
+4. **All Phase 7 stress runs use the wired path (`ssh mfritsche@192.168.88.80`) for telemetry collection.**  When the chip wedges (it shouldn't this time, but planning for it), wlan0 stops responding but enu1 stays alive.  Collect dmesg / ftrace / journalctl over enu1 BEFORE rebooting.  This is the data we lost in Patch C boot -1 because wlan0 was the only path.
+5. N=3 reps per phase per `feedback_phase7_stress_ramp`.  Don't accept N=1 as verification.
+
+## §10 Closeout
+
+If C-prep + C v2 both pass Phase 7: proceed to D (ba_lock atomicization), E (ps_state_lock skip).  Markus's "we're not on the clock" applies — sequencing per bisection clarity, not delivery deadline.
+
+---
+
+*Plan written 2026-05-07 by Claude (noether), in response to Patch C Phase 7 failure.  Phase 5 review = PR comments on this artifact at git.reauktion.de/marfrit/besser.  Don't curate the shared-state delta table for the reviewer — that's the part the previous round's reviewer should have caught me on.*

notes/patch-c-v3-phase4-plan-2026-05-07.md

@@ -0,0 +1,127 @@
+# Patch C v3 — Phase 4 Plan: drop sdio_rx_work, match cw1200 architecture
+
+**Author:** Claude (noether)
+**Status:** Phase 4 v3 — supersedes v2 (PR #10) after cw1200 mainline survey showed the race-free path is structural, not lock-based.
+**Decision:** drop the `sdio_rx_work` workqueue entirely; SDIO IRQ wakes `bh_wq`; bh thread does the SDIO read inline.  Restores single-writer-from-bh invariant on `hw_bufs_used` *by construction*.  No `atomic_t` prep needed.
+
+---
+
+## §0 Why v3 supersedes v2
+
+PR #10's plan was: convert `hw_bufs_used` etc. to `atomic_t` (prep), then direct-deliver from `sdio_rx_work` (structural).  That was a workaround for the race that *only existed because of the relay*.
+
+The cw1200 mining (`~/src/linux-rockchip`, 228 cw1200 commits) showed the upstream answer: there is no relay.  cw1200's IRQ handler bumps `bh_rx` and wakes the bh thread; the bh thread does the SDIO read itself inside `cw1200_bh_rx_helper` (`drivers/net/wireless/st/cw1200/bh.c:233`).  Single thread = single writer for `hw_bufs_used` = no race.  Same `int hw_bufs_used` as bes2600, never atomic_t'd in 16 years upstream because it never needed to be.
+
+Patch C v3 brings bes2600 into that shape.  The structural simplification is bigger than v2's diff but lands the right architecture in one move.
+
+## §1 Goal
+
+Same as Patch C v2 §1: ≥ 1 MB/s sustained receive @ 4 MB/s sender, < 15 % `_raw_spin_unlock_irqrestore` CPU%, no 30-min cascade to link-death.  Stretch toward Phase 1's full 2 MB/s once Patch C2 (rx_list batch) lands separately.
+
+## §2 Situation
+
+- Cleanups branch is at Patch F merged (commit `b717251`).  All Phase 5 reviews of the F series merged via PR #4.
+- ohm rebooted with F module live (srcversion `A9438692D6A8698F92AEEA1`) — F is the new baseline for Patch C v3 Phase 7 comparison.
+- Wired path `enu1` at `192.168.88.80` survives bes2600 wedges; lmcp `ohm` still goes through wlan0.  Phase 7 telemetry collection over enu1.
+- Reboot-permission override active (ohm dev-allocated; I can `sudo reboot` directly — `feedback_user_pushes_reboot_button` override clause).
+
+## §3 Baseline measurements
+
+Carry forward from `run-20260507-patchC-preflight/baseline.tsv` (N=1, F-less Patch B module):
+
+| metric | value |
+|---|---|
+| observed receive @ 4 MB/s | 1.362 MB/s |
+| sdio_rx_work dispatches | 86.4/s = 90.3 per 1000 RX packets |
+| sdio_tx_work dispatches | 276.1/s |
+| bes2600_bh_work redispatches | 0 (single long-lived) |
+
+**Phase 6 prereq:** capture an N=3 baseline ON THE F MODULE before Patch C v3 code lands.  Same instrumentation, same stress ramp.  This is the post-F / pre-v3 reference.  Without it, Phase 7's delta is C+F vs B+nothing — confounded.
+
+## §4 Plan v3
+
+### §4.1 What gets eliminated
+
+- **`sdio_rx_work` (bes2600_sdio.c:829)** — function deleted.  No longer queued, no longer runs.
+- **`self->rx_work` work_struct** — field deleted from `struct sbus_priv`.  `INIT_WORK` removed.
+- **`self->rx_queue` + `self->rx_queue_lock`** — fields deleted.  `skb_queue_head_init` removed.  No SKB ever queued there.
+- **`bes2600_sdio_pipe_read`** — function deleted.  No callers after this patch.
+- **`sbus_ops->pipe_read`** — sbus op slot deleted (or kept and stubbed; tx_loop.c also implements it for the test-loop bus, has to stay if test-loop is preserved).
+- **`queue_work(self->sdio_wq, &self->rx_work)`** at the 3 call sites in `bes2600_sdio.c` (lines 416, 941, 1199) — removed.
+
+### §4.2 What gets added
+
+- **A new `bes2600_bh_handle_rx_skb()`** in bh.c (same shape as Patch C added, same contract block; no longer needs to also wake the bh thread because we ARE the bh thread).
+- **A new helper `bes2600_sdio_read_rx_batch()`** in bes2600_sdio.c, exported, that does what `sdio_rx_work` used to do MINUS the queuing: lock → read ctrl_reg → memcpy_fromio → packets_check → for-each-frame extract+deliver.  Called from bh.
+
+### §4.3 What gets rewired
+
+- **`bes2600_gpio_irq_handler`** in bes2600_sdio.c:413 (the GPIO-IRQ path used when CONFIG_BES2600_USE_GPIO_IRQ is set):  drop `queue_work(self->sdio_wq, &self->rx_work)`; instead call `self->irq_handler(self->irq_priv)` directly (which is `bes2600_irq_handler` in bh.c, bumps `bh_rx` + wakes `bh_wq`).  Matches cw1200_sdio_irq_handler shape.
+- **`bes2600_bh_rx_helper`** (bh.c:961, BES_SDIO_RX_MULTIPLE_ENABLE branch): instead of `pipe_read`-ing one SKB from the (now-gone) rx_queue, call the new `bes2600_sdio_read_rx_batch()` which does the SDIO read AND delivers each frame inline via `bes2600_bh_handle_rx_skb()`.  Returns count delivered, or negative on error.
+- **`bes2600_bh()` outer loop**:  after a successful rx_batch read, the helper signals whether to continue draining (more frames pending) — same shape as today's `BH_RX_CONT_LIMIT=3` outer loop.
+- **`bes2600_gpio_wakeup_mcu(SDIO_RX)`** + **`bes2600_gpio_allow_mcu_sleep(SDIO_RX)`** brackets:  currently called inside sdio_rx_work.  Move into bh thread around the `bes2600_sdio_read_rx_batch()` call.  Same wake-flag bracketing, just from a different thread.
+- **`sdio_wq` workqueue**:  keeps `tx_work` and (briefly) `scan_work`.  Renamed or kept — cosmetic.  Don't touch in this patch.
+
+### §4.4 What stays untouched
+
+- TX path (`sdio_tx_work`, `bes2600_bh_tx_helper`, `wsm_alloc_tx_buffer`).  Independent.
+- WSM protocol layer (`wsm.c`, `wsm_handle_rx`).  Same callees, just from bh thread now.
+- mac80211 RX delivery (`ieee80211_rx_irqsafe`).  That's Patch C2.
+- `BES2600_RX_IN_BH` ifdef gate.  Stays defined; the gated branch is now the only RX path.
+- Symptom-shaped artifacts (asm nop, BUG_ON in hot path) — still deferred, see task #24 post-cleanup.
+
+## §5 Shared-state delta table (the v2 lesson, applied)
+
+Every field `bes2600_bh_handle_rx_skb` mutates directly or transitively, with the v3 protection:
+
+| field | written by (today) | written by (after v3) | concurrency | required action |
+|---|---|---|---|---|
+| `hw_priv->hw_bufs_used` | bh thread (TX submit + RX confirm), main.c init | **bh thread only** (RX moves into bh) | single-writer | none — `int` is fine, race-free by construction |
+| `hw_priv->hw_bufs_used_vif[i]` | bh thread (TX vif submit + RX vif confirm), main.c init | **bh thread only** | single-writer | none |
+| `hw_priv->wsm_rx_seq[i]` | sdio_rx_work today | bh thread | single-writer | none — moves cleanly between contexts |
+| `hw_priv->wsm_tx_pending[i]` | bh thread (inc on TX submit), bh+sdio_rx_work (dec on RX confirm) | **bh thread only** | single-writer | none |
+| `hw_priv->lmac_mon_timer` / `mcu_mon_timer` | mod_timer / del_timer_sync from bh + sdio_rx_work | bh thread only | timer API safe anyway | none |
+| `hw_priv->wsm_cmd.lock` | spinlock taken inside wsm_handle_rx | same | already protected | none |
+| `priv->bh_evt_wq` wake-up | wsm_release_tx_buffer when count→0 | same | wake_up is concurrency-safe | none |
+| `bes_pwr.lock` (inside bes2600_pwr_clear_busy_event) | bh thread (today) | bh thread | already protected | none |
+| `self->rx_data_cnt` etc. (sbus_priv stats) | sdio_rx_work | bh thread | single-writer | none |
+
+**Zero fields require new locking.** The architectural pivot eliminates the race v2's atomic_t was working around.
+
+## §6 Risks
+
+1. **bh thread now holds the SDIO bus mutex during read** (currently held by sdio_rx_work).  TX work in the same bh thread is unaffected (sdio_tx_work runs on a separate workqueue and shares the same mutex anyway).  The sdio_lock contention pattern doesn't change.
+2. **Loss of "parallelism" between sdio_rx_work and bh TX**:  sdio_rx_work and bh thread *appeared* to run in parallel today, but both serialize through `bes2600_sdio_lock(self)` for the actual bus operations.  The parallelism was illusory.  Net throughput should not regress.
+3. **bh thread CPU-busy-time per RX batch increases**:  inline SDIO read is the same cost, just charged to bh instead of sdio_wq's worker.  Mitigation: the per-IRQ workqueue dispatch cost (~86/s) is what we trade for it.  Net: -86 dispatches/s, +0 µs per frame.
+4. **Multi-RX coalescing (BES_SDIO_RX_MULTIPLE_NUM=16)** stays.  bes2600_sdio_extract_packets parses the multi-frame buffer same as before, just inline now.  No functional change to chip-side behaviour.
+5. **GPIO wake-flag bracketing**:  `bes2600_gpio_wakeup_mcu(SDIO_RX)` and `bes2600_gpio_allow_mcu_sleep(SDIO_RX)` currently bracket sdio_rx_work.  Move them to bracket the new bh-side read.  If the wake-flag accounting is sub-system-scoped (it is — flag bits per subsystem), this is a clean move.
+6. **IRQ re-enable in bh thread**:  cw1200's bh re-enables IRQ via `__cw1200_irq_enable(priv, 1)` after each round.  bes2600 has the analogous `__bes2600_irq_enable(0/1)` (commented out as the `asm volatile("nop")` symptom in `bh.c:1518-1520`).  This patch does NOT re-engage the commented-out re-enable — that's still task #24's call.  But if the IRQ stays disabled across rounds, we'd never receive the next IRQ.  **Investigate before Phase 6 lands**: where does IRQ re-enable happen in the current bes2600 hot path?  The sdio_func IRQ may be auto-managed by sdio core differently.  Block Phase 6 on this audit.
+7. **Phase 7 wedge resilience**:  if v3 has a different bug shape than v2's race (which it shouldn't, since the race is gone by construction), the wired path lets us collect telemetry from a wedged ohm.
+
+## §7 Phase 5 / 6 / 7
+
+- **Phase 5**: PR on `git.reauktion.de/marfrit/besser` with this artifact.  Specifically request reviewer focus on §6 risk #6 (IRQ re-enable mechanism).
+- **Phase 6**: branch off cleanups (post-F): `bes2600/sdio-rx-no-relay`.  Implement the file changes per §4.  Build, install, smoke-test.
+- **Phase 7**: 
+  - First: N=3 stress-ramp **on F module** (post-F pre-v3 baseline).  10 min @ 1, 30 min @ 2, 30 min @ 4 MB/s.  Use wired path for telemetry.
+  - Then: install v3 module, identical N=3 ramp.  Compare deltas.
+  - Predicted: sdio_rx_work dispatch rate → 0/s (was 86/s).  observed receive lifts toward ≥ 1.0 MB/s sustained.  `_raw_spin_unlock_irqrestore` drops by the rx_queue lock contribution (was 1914/s acquires).
+
+## §8 What gets dropped from v2 plan
+
+- atomic_t prep refactor (`hw_bufs_used` → `atomic_t`): not needed.  Single-writer invariant preserved structurally.  Still a defensible standalone hardening patch *if mainlining bes2600 ever requires defense-in-depth*, but not on the Bug-#5 critical path.
+- `wsm_tx_pending[]` decrement-decision race (v2 risk #2): also moots.  Both sides single-thread under v3.
+- v2 Phase 7's "C-prep should show zero delta" gate: replaced by "v3 should match cw1200's structural shape" gate.
+
+## §9 Open question for reviewer
+
+The big one is §6 risk #6 — IRQ re-enable. cw1200 explicitly does `__cw1200_irq_enable(priv, 1)` from bh after each round; bes2600 has the call **commented out** with an `asm volatile("nop")` placeholder.  Either:
+
+(a) bes2600's SDIO IRQ is level-triggered + auto-acked by SDIO core, so re-enable isn't needed (that would explain the nop).
+(b) The current code happens to work because sdio_rx_work is queued by the IRQ regardless of whether IRQ is "enabled" by the driver-side flag.  After v3 we have to manually re-enable like cw1200 does.
+
+Need to confirm (a) vs (b) before Phase 6 lands.  Plan to grep for `__bes2600_irq_enable` callsites and trace back to whether it's load-bearing.
+
+---
+
+*Plan written 2026-05-07 by Claude (noether), after Patch F merged and Patch C v2 (PR #10) was superseded by the cw1200 architectural mining finding.  Phase 5 review on PR.  Don't curate.*