[PATCH net] thunderbolt: Use needed_headroom for the transport header

[<brianrobertjames@gmail.com>]

alloc_etherdev() initializes hard_header_len to ETH_HLEN. tbnet_probe()
then adds the 12-byte Thunderbolt transport header, although that header
is not part of the Ethernet MAC header handed to eth_type_trans()/GRO.

This makes gro_list_prepare() include 12 bytes of the IP header in its
nonstandard link-layer comparison. Changing IP lengths, IDs or checksums
can prevent recognition of packets belonging to the same TCP flow,
allowing later packets to bypass earlier data held by GRO. On the tested
direct USB4 link this causes TCP reordering and excess retransmissions.

Reserve the transport header in needed_headroom instead. Keep the
Ethernet hard_header_len at 14, with 12 bytes of needed_headroom. This
preserves the transport space requirement without changing wire format,
offload features, or RX/TX credit allocation.

Shun Li published the identical correction on October 9, 2026, in
riverscn/thunderbolt-net-dkms (commit b18a196bb5ca). Credit for that prior
published correction belongs to Shun Li. This submission adds our
independent AMD/Proxmox hardware validation and receive-order evidence.

Link: https://github.com/riverscn/thunderbolt-net-dkms/commit/b18a196bb5ca
Link: https://bugzilla.proxmox.com/show_bug.cgi?id=8306
Signed-off-by: Brian James <brianrobertjames@gmail.com>
---
Follow-up to my November 8 report in this thread. Fabian Grunbichler at
Proxmox requested that the actual patch be submitted upstream. The
one-line diff is included inline below for review.

Test environment:
- Two AMD x86-64 hosts, direct native USB4/thunderbolt0 traffic.
- Receiver: Proxmox 7.0.14-28-pve; historical tests also used 7.0.2-6-pve.
- MTU 9000, TSO/GSO/GRO enabled, gso_max_size 65536.
- Unpaced single-stream IPv4 TCP iperf3, 20 seconds per arm.
- Stock thunderbolt controller unchanged; only the receiver's network
  driver's header/headroom line changed. Cable, ports, MTU, offloads and
  CPU/IRQ placement were held constant within each comparison.

Receiver-only original -> corrected -> original comparison:
  Original:           30.599221 Gb/s, 620 TCP retransmissions
  GRO corrected:      29.779541 Gb/s,   0 TCP retransmissions
  Original restored:  29.994158 Gb/s, 540 TCP retransmissions

Endpoint interface error/drop increments and receiver FQ drops were zero
in all three arms. The unchanged sender in this isolation test ran
7.0.2-6-pve with TX E2E enabled; receiver RX flags stayed FRAME|E2E (6).
This comparison isolates the receiver fix with that sender configuration.
It does not establish that GRO alone resolves every retransmission mode
when both peers run 7.0.14-28-pve.

Historical 7.0.2 receiver-only comparisons on each host, 20 seconds/arm:
  Receiver A retransmissions: 536 -> 0 -> 132
  Receiver B retransmissions: 564 -> 0 -> 769

Historical paired receive hooks covered identical TCP payload bytes
(18,871,120,863) and packet counts (303,590) before GRO and at TCP ingress,
on the same CPU. Forward sequence gaps were 0 before GRO and 1,219 at TCP
ingress, with no packet-length parsing mismatches. A corrected capture
had equal byte coverage at both hooks and zero forward gaps. Overlapping
and retransmitted sequence ranges were handled separately. This supports
an ordering defect in the receive software path for this failure mode.
Runtime tracing confirmed header/headroom 26/0 before and 14/12 after.

The measurements above came from actual Proxmox hosts. The current
mainline source was inspected but has not been runtime-tested on this
hardware. The diff context was checked against mainline commit
7475ea8b58616b33c04afb976cf0fbaad9dd63fb. The separate TX-credit diagnostic
is excluded from this patch. No Ceph qualification is claimed.

AI assistance disclosure: source review, diagnostic automation, analysis
and preparation of this email were AI-assisted using OpenAI Codex
(GPT-6 model family; a more specific serving-model identifier was not
provided). The measurements and traces are not simulated results.

 drivers/net/thunderbolt/main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/thunderbolt/main.c b/drivers/net/thunderbolt/main.c
--- a/drivers/net/thunderbolt/main.c
+++ b/drivers/net/thunderbolt/main.c
@@ -1410,7 +1410,7 @@ static int tbnet_probe(struct tb_service *svc)
 	dev->hw_features = NETIF_F_SG | NETIF_F_ALL_TSO | NETIF_F_GRO |
 			   NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM;
 	dev->features = dev->hw_features | NETIF_F_HIGHDMA;
-	dev->hard_header_len += sizeof(struct thunderbolt_ip_frame_header);
+	dev->needed_headroom = sizeof(struct thunderbolt_ip_frame_header);
 
 	netif_napi_add(dev, &net->napi, tbnet_poll);