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

[Mika Westerberg]

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@xxxxxxxxxxxxxxx>
---
 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