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jerpelea pushed a commit to branch master
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The following commit(s) were added to refs/heads/master by this push:
new 6555e3e2832 drivers/usbdev/cdcncm: send TX immediately, not after a
tick-quantized delay
6555e3e2832 is described below
commit 6555e3e2832abd9b45c52e514902b49eda183658
Author: Ricard Rosson <[email protected]>
AuthorDate: Sat Jul 18 13:42:12 2026 +0100
drivers/usbdev/cdcncm: send TX immediately, not after a tick-quantized delay
cdcncm_send() defers each transmit with
MSEC2TICK(CDCNCM_DGRAM_COMBINE_PERIOD)
(1 ms). MSEC2TICK() rounds up to the system tick, so at the default 100 Hz
tick
the "1 ms" coalescing window becomes a full 10 ms tick (10-20 ms with
phase),
adding that latency to every single-datagram reply (ICMP echo, TCP ACK,
one-MSS
HTTP segment) and dominating the CDC-NCM round-trip time.
The window only coalesces datagrams appended within the same synchronous TX
burst (already queued before the worker runs), so an inter-burst delay adds
latency without batching benefit in the common case. Fire the transmit
worker
immediately (delay 0); within-burst coalescing is preserved.
On RP2350 (Pico 2 W) USB-NIC at 100 Hz tick: ping RTT 21.7 -> 2.8 ms, a 257
KB
HTTP download 5.79 -> 0.92 s (44.5 -> 279 KB/s).
Signed-off-by: Ricard Rosson <[email protected]>
Assisted-by: Claude (Anthropic Claude Code)
Signed-off-by: Ricard Rosson <[email protected]>
---
drivers/usbdev/cdcncm.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/drivers/usbdev/cdcncm.c b/drivers/usbdev/cdcncm.c
index d9fa738c600..7f10e5355e3 100644
--- a/drivers/usbdev/cdcncm.c
+++ b/drivers/usbdev/cdcncm.c
@@ -1331,8 +1331,13 @@ static int cdcncm_send(FAR struct netdev_lowerhalf_s
*dev, FAR netpkt_t *pkt)
}
else
{
- work_queue(ETHWORK, &self->delaywork, cdcncm_transmit_work, self,
- MSEC2TICK(CDCNCM_DGRAM_COMBINE_PERIOD));
+ /* Defer to the work thread with zero delay. A non-zero delay is
+ * rounded up to a full tick (10ms at 100Hz), which dominated the
+ * USB-NIC round-trip; delay 0 wakes the worker immediately while
+ * still coalescing datagrams appended in the same TX burst.
+ */
+
+ work_queue(ETHWORK, &self->delaywork, cdcncm_transmit_work, self, 0);
}
return OK;
