ricardgb opened a new pull request, #19574:
URL: https://github.com/apache/nuttx/pull/19574
## Summary
`rp2040_update_buffer_control()` / `rp23xx_update_buffer_control()` re-arm an
endpoint buffer by setting the `AVAILABLE` bit in the buffer-control word,
which lives in **DPSRAM**. When a buffer is re-armed from the completion path
(`rp2040_usbintr_buffstat` / `rp23xx_usbintr_buffstat`), that runs right
after
the handler clears the endpoint's bit in `BUFF_STATUS`, which lives in the
**USB controller register block** — a different peripheral region.
The bus fabric may reorder those two stores. If the controller observes the
`AVAILABLE` re-arm before the `BUFF_STATUS` clear lands, it can transmit the
next IN packet and latch its completion in `BUFF_STATUS` *before* the clear
arrives; the late clear then wipes that just-set completion bit. The lost
completion edge stops all further buffer interrupts for the endpoint, so the
class driver's write-complete callback never runs and TX wedges permanently.
The fix adds a `UP_DMB()` at the top of the `AVAILABLE` re-arm so the
preceding
`BUFF_STATUS` clear is ordered ahead of it. (Non-SMP builds reduce
`spin_lock_irqsave` to a barrier-free `up_irq_save`, so nothing else orders
these two stores.)
This is the hardware-ordering issue tracked in #19472. The recently merged
#19470 fixed the `cdcncm`-layer write-buffer handling; this is the companion
controller-level fix it referenced, needed for full TX stability on RP2350
under maximal IN density.
## Impact
- Most visible on RP2350 (Cortex-M33) under dense/bursty IN traffic — e.g.
CDC-NCM with `CONFIG_NET_TCP_WRITE_BUFFERS=y` — where TX would wedge after
a
handful of large transfers.
- One `UP_DMB()` per chip on the re-arm path; no API or behavior change
otherwise. The barrier is a single `dmb` instruction.
## Testing
- **Silicon:** RP2350 (Raspberry Pi Pico 2 W class board) running CDC-NCM
over
USB with TCP write buffers. Before: TX wedges after ~5 large transfers
(endpoint stops raising buffer interrupts). After: a 144 dense/concurrent
HTTP download soak completes with 0 wedges (~486 KB/s sustained).
- **Build:** `raspberrypi-pico-2:usbnsh` (compiles `rp23xx_usbdev.c`) and
`raspberrypi-pico:usbnsh` (compiles `rp2040_usbdev.c`) both build and link
clean.
- `tools/checkpatch.sh -c -u -m -g` passes.
---
*Disclosure: this change was developed with the help of an AI agent and
reviewed by a human before submission.*
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