Re: [PATCH net v2] Revert "net: thunderbolt: Enable end-to-end flow control also in transmit"

[Mika Westerberg <mika.westerberg@linux.intel.com>]

On Mon, Jul 27, 2026 at 12:29:48PM +0000, Fan Ye wrote:
> This reverts commit a8065af3346ebd7c76ebc113451fb3ba94cf7769.
> 
> Per the USB4 spec, a Transmit Descriptor Ring with E2E flow control
> disabled does not require any credits to be available before the Host
> Interface Adapter Layer transmits a tunneled packet from it. Once E2E is
> enabled on that ring the controller must first obtain end-to-end
> credits.
> 
> The ASMedia ASM4242 USB4 host router (PCI 1b21:2425) never delivers
> those credits. The controller does accept the configuration: reading the
> ring OPTIONS register back right after tb_ring_start() returns exactly
> what was written, including RING_FLAG_E2E_FLOW_CONTROL (bit 28) and the
> E2E HopID field. No credit ever arrives though, so the Tx ring's
> hardware consumer index never advances and the link carries no traffic
> at all.
> 
> Measured on two hosts connected point to point, onboard ASM4242 on MSI
> X870E and X870, v6.17, stock drivers/net/thunderbolt/main.c with only
> this revert applied on top:
> 
>   before: 100% packet loss to the peer; thunderbolt0 is up and the
>           XDomain handshake completes ("new host found"), but iperf3
>           fails with "No route to host" once the neighbour entry
>           expires
>   after:  0% packet loss, 0.28 ms RTT; iperf3 4.21 Gb/s one way and
>           5.17 Gb/s the other (5 runs each, stddev <= 0.02), 1
>           retransmit in 10 s
> 
> An instrumented build additionally showed a frozen-Tx-consumer watchdog
> firing ~30k times in a 10 s window before this change.
> 
> Rx-side E2E is not touched by this revert, so peers that do return
> credits keep receive-side flow control.
> 
> ASMedia does not look like an isolated case. The out-of-tree
> thunderbolt-ibverbs project disables native E2E on AMD NHI by default,
> noting that "Strix Halo has reproduced TX completion wedges with
> multiple native E2E rings active" -- the same failure mode, on a
> different vendor. Since the driver has no way to tell in advance which
> host router returns the credits, going back to the previous behaviour
> looks safer than adding a quirk per affected part; Tx-side E2E can be
> reintroduced as an opt-in for controllers that are known to implement
> the credit return.
> 
> Note that the reverted commit was not fixing a reported problem, it was
> derived from the spec wording alone, so this revert is not expected to
> regress a known workload. Cc'ing the original author in case there was
> one.
> 
> Fixes: a8065af3346e ("net: thunderbolt: Enable end-to-end flow control also in transmit")
> Cc: zhangjianrong <zhangjianrong5@huawei.com>
> Signed-off-by: Fan Ye <fy15309206903@gmail.com>

Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>