linux-kernel - [RFC PATCH 21/22] thunderbolt: Update documentation with the USB4 information

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Message-Id: <20191001113830.13028-22-mika.westerberg@linux.intel.com>
Date:   Tue,  1 Oct 2019 14:38:29 +0300
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     linux-usb@...r.kernel.org
Cc:     Andreas Noever <andreas.noever@...il.com>,
        Michael Jamet <michael.jamet@...el.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Yehezkel Bernat <YehezkelShB@...il.com>,
        Rajmohan Mani <rajmohan.mani@...el.com>,
        Nicholas Johnson <nicholas.johnson-opensource@...look.com.au>,
        Lukas Wunner <lukas@...ner.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Mario.Limonciello@...l.com,
        Anthony Wong <anthony.wong@...onical.com>,
        linux-kernel@...r.kernel.org
Subject: [RFC PATCH 21/22] thunderbolt: Update documentation with the USB4 information

Update user's and administrator's guide to mention USB4, how it relates
to Thunderbolt (it is public spec of Thunderbolt 3) and and how it is
supported in Linux.

Signed-off-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
---
 Documentation/admin-guide/thunderbolt.rst | 27 ++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/thunderbolt.rst b/Documentation/admin-guide/thunderbolt.rst
index 898ad78f3cc7..4cbed319133d 100644
--- a/Documentation/admin-guide/thunderbolt.rst
+++ b/Documentation/admin-guide/thunderbolt.rst
@@ -1,6 +1,25 @@
-=============
- Thunderbolt
-=============
+======================
+ Thunderbolt and USB4
+======================
+USB4 is the public spec of Thunderbolt 3 with some differences at the
+register level among other things. There are two different
+implementations available: firmware connection manager and software
+connection manager. Typically PCs come with a firmware connection
+manager for Thunderbolt 3 and early USB4 capable systems. Apple systems
+on the other hand use software connection manager and the future USB4
+compliant PCs follow the suit.
+
+The Linux Thunderbolt driver supports both and can detect at runtime
+which connection manager implementation is to be used. To be on the safe
+side the software connection manager in Linux also advertises security
+level ``user`` which means PCIe tunneling is disabled by default. The
+documentation below applies to both implementations with the exception
+that the software connection manager only supports ``user`` security
+level and is expected to be accompanied with an IOMMU based DMA
+protection.
+
+Security levels and how to use them
+-----------------------------------
 The interface presented here is not meant for end users. Instead there
 should be a userspace tool that handles all the low-level details, keeps
 a database of the authorized devices and prompts users for new connections.
@@ -18,8 +37,6 @@ This will authorize all devices automatically when they appear. However,
 keep in mind that this bypasses the security levels and makes the system
 vulnerable to DMA attacks.
 
-Security levels and how to use them
------------------------------------
 Starting with Intel Falcon Ridge Thunderbolt controller there are 4
 security levels available. Intel Titan Ridge added one more security level
 (usbonly). The reason for these is the fact that the connected devices can
-- 
2.23.0