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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, zhangjianrong Subject: Re: [PATCH net v2] Revert "net: thunderbolt: Enable end-to-end flow control also in transmit" Message-ID: <20260728043219.GN2365036@black.igk.intel.com> References: <20260727123002.25225-1-fy15309206903@gmail.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20260727123002.25225-1-fy15309206903@gmail.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 > Signed-off-by: Fan Ye Acked-by: Mika Westerberg