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Fan Ye 1881f2efbf Revert "net: thunderbolt: Enable end-to-end flow control also in transmit"
This reverts commit a8065af334.

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: a8065af334 ("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>
Link: https://patch.msgid.link/20260727123002.25225-1-fy15309206903@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
History 2026-07-30 16:10:24 -07:00
arch powerpc fixes for 7.2 #3 2026-07-30 08:24:12 -07:00
block block: fix aligning of bounced dio read bios 2026-07-16 06:11:37 -06:00
certs treewide: fix indentation and whitespace in Kconfig files 2026-05-28 21:24:42 -07:00
crypto crypto: aes - Fix conditions for selecting MAC dependencies 2026-07-09 11:06:22 -04:00
Documentation This is again larger than usual: the backlog accumulated in the past weeks 2026-07-30 09:18:49 -07:00
drivers Revert "net: thunderbolt: Enable end-to-end flow control also in transmit" 2026-07-30 16:10:24 -07:00
fs for-7.2-rc5-tag 2026-07-28 08:13:45 -07:00
include This is again larger than usual: the backlog accumulated in the past weeks 2026-07-30 09:18:49 -07:00
init rust: allow suspicious_runtime_symbol_definitions lint for Rust >= 1.98 2026-06-29 13:02:15 +02:00
io_uring io_uring-7.2-20260724 2026-07-24 19:58:03 -07:00
ipc kernel-7.2-rc1.misc 2026-06-15 03:05:50 +05:30
kernel fprobe: Fix module reference count leak on error in register_fprobe() 2026-07-29 22:50:02 +09:00
lib hardening fixes for v7.2-rc6 2026-07-28 13:36:01 -07:00
LICENSES LICENSES: Add modern form of the LGPL-2.1 tags to the usage guide section 2025-10-22 07:58:19 +02:00
mm 13 hotfixes. All are cc:stable. 11 are for MM. All are singletons - 2026-07-27 14:36:26 -07:00
net This is again larger than usual: the backlog accumulated in the past weeks 2026-07-30 09:18:49 -07:00
rust rust: time: fix as_micros_ceil() to round correctly for negative Delta 2026-07-18 19:07:35 +02:00
samples tracing fixes for 7.2: 2026-07-12 09:46:37 -07:00
scripts scripts/sorttable: Handle RISC-V patchable ftrace entries 2026-06-25 13:15:03 -06:00
security keys: make keyring key-chunk byte order agree with keyring_diff_objects() 2026-07-23 18:23:39 +03:00
sound ALSA: hda/realtek - Add quirk for Dell Pro QC1255 2026-07-21 09:55:17 +02:00
tools This is again larger than usual: the backlog accumulated in the past weeks 2026-07-30 09:18:49 -07:00
usr kbuild: uapi: also test UAPI headers against C++ compilers 2026-03-25 13:24:42 +01:00
virt KVM/arm64 fixes for 7.2, take #2 2026-07-15 12:13:30 +02:00
.clang-format clang-format: fix formatting of guard() and scoped_guard() statements 2026-05-28 21:24:41 -07:00
.clippy.toml rust: bump Clippy's MSRV and clean incompatible_msrv allows 2026-04-07 09:51:39 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig editorconfig: add rst extension 2026-01-26 19:07:09 -08:00
.get_maintainer.ignore .get_maintainer.ignore: add myself 2026-04-02 16:48:25 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore SPDX patches for 7.2-rc1 2026-06-22 12:06:22 -07:00
.mailmap This is again larger than usual: the backlog accumulated in the past weeks 2026-07-30 09:18:49 -07:00
.pylintrc docs: Move the python libraries to tools/lib/python 2025-11-18 09:22:40 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS CREDITS: Add Wolfram Sang 2026-07-14 16:34:04 +02:00
Kbuild checksyscalls: move instance functionality into generic code 2026-04-05 09:21:32 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS This is again larger than usual: the backlog accumulated in the past weeks 2026-07-30 09:18:49 -07:00
Makefile Linux 7.2-rc5 2026-07-26 14:45:48 -07:00
README docs: add AI Coding Assistants documentation 2026-01-06 14:55:06 -07:00

Linux kernel
============

The Linux kernel is the core of any Linux operating system. It manages hardware,
system resources, and provides the fundamental services for all other software.

Quick Start
-----------

* Report a bug: See Documentation/admin-guide/reporting-issues.rst
* Get the latest kernel: https://kernel.org
* Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst
* Join the community: https://lore.kernel.org/

Essential Documentation
-----------------------

All users should be familiar with:

* Building requirements: Documentation/process/changes.rst
* Code of Conduct: Documentation/process/code-of-conduct.rst
* License: See COPYING

Documentation can be built with make htmldocs or viewed online at:
https://www.kernel.org/doc/html/latest/


Who Are You?
============

Find your role below:

* New Kernel Developer - Getting started with kernel development
* Academic Researcher - Studying kernel internals and architecture
* Security Expert - Hardening and vulnerability analysis
* Backport/Maintenance Engineer - Maintaining stable kernels
* System Administrator - Configuring and troubleshooting
* Maintainer - Leading subsystems and reviewing patches
* Hardware Vendor - Writing drivers for new hardware
* Distribution Maintainer - Packaging kernels for distros
* AI Coding Assistant - LLMs and AI-powered development tools


For Specific Users
==================

New Kernel Developer
--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
* Your First Patch: Documentation/process/submitting-patches.rst
* Coding Style: Documentation/process/coding-style.rst
* Build System: Documentation/kbuild/index.rst
* Development Tools: Documentation/dev-tools/index.rst
* Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst
* Core APIs: Documentation/core-api/index.rst

Academic Researcher
-------------------

Explore the kernel's architecture and internals:

* Researcher Guidelines: Documentation/process/researcher-guidelines.rst
* Memory Management: Documentation/mm/index.rst
* Scheduler: Documentation/scheduler/index.rst
* Networking Stack: Documentation/networking/index.rst
* Filesystems: Documentation/filesystems/index.rst
* RCU (Read-Copy Update): Documentation/RCU/index.rst
* Locking Primitives: Documentation/locking/index.rst
* Power Management: Documentation/power/index.rst

Security Expert
---------------

Security documentation and hardening guides:

* Security Documentation: Documentation/security/index.rst
* LSM Development: Documentation/security/lsm-development.rst
* Self Protection: Documentation/security/self-protection.rst
* Reporting Vulnerabilities: Documentation/process/security-bugs.rst
* CVE Procedures: Documentation/process/cve.rst
* Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst
* Security Features: Documentation/userspace-api/seccomp_filter.rst

Backport/Maintenance Engineer
-----------------------------

Maintain and stabilize kernel versions:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* Backporting Guide: Documentation/process/backporting.rst
* Applying Patches: Documentation/process/applying-patches.rst
* Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git for Maintainers: Documentation/maintainer/configure-git.rst

System Administrator
--------------------

Configure, tune, and troubleshoot Linux systems:

* Admin Guide: Documentation/admin-guide/index.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst
* Tracing/Debugging: Documentation/trace/index.rst
* Performance Security: Documentation/admin-guide/perf-security.rst
* Hardware Monitoring: Documentation/hwmon/index.rst

Maintainer
----------

Lead kernel subsystems and manage contributions:

* Maintainer Handbook: Documentation/maintainer/index.rst
* Pull Requests: Documentation/maintainer/pull-requests.rst
* Managing Patches: Documentation/maintainer/modifying-patches.rst
* Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst
* Development Process: Documentation/process/maintainer-handbooks.rst
* Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git Configuration: Documentation/maintainer/configure-git.rst

Hardware Vendor
---------------

Write drivers and support new hardware:

* Driver API Guide: Documentation/driver-api/index.rst
* Driver Model: Documentation/driver-api/driver-model/driver.rst
* Device Drivers: Documentation/driver-api/infrastructure.rst
* Bus Types: Documentation/driver-api/driver-model/bus.rst
* Device Tree Bindings: Documentation/devicetree/bindings/
* Power Management: Documentation/driver-api/pm/index.rst
* DMA API: Documentation/core-api/dma-api.rst

Distribution Maintainer
-----------------------

Package and distribute the kernel:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* ABI Documentation: Documentation/ABI/README
* Kernel Configuration: Documentation/kbuild/kconfig.rst
* Module Signing: Documentation/admin-guide/module-signing.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst

AI Coding Assistant
-------------------

CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and
follow the AI coding assistants documentation before contributing to the Linux
kernel:

* Documentation/process/coding-assistants.rst

This documentation contains essential requirements about licensing, attribution,
and the Developer Certificate of Origin that all AI tools must comply with.


Communication and Support
=========================

* Mailing Lists: https://lore.kernel.org/
* IRC: #kernelnewbies on irc.oftc.net
* Bugzilla: https://bugzilla.kernel.org/
* MAINTAINERS file: Lists subsystem maintainers and mailing lists
* Email Clients: Documentation/process/email-clients.rst