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thunderbolt: Update documentation with the USB4 information

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Update user's and administrator's guide to mention USB4, how it relates
to Thunderbolt and and how it is supported in Linux.

While there add the missing SPDX identifier to the document.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Link: https://lore.kernel.org/r/20191217123345.31850-10-mika.westerberg@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Mika Westerberg 2019-12-17 15:33:45 +03:00 committed by Greg Kroah-Hartman
parent e6f8185857
commit ea81896dc9
1 changed files with 25 additions and 5 deletions
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30
Documentation/admin-guide/thunderbolt.rst
View File

@ -1,6 +1,28 @@
=============
Thunderbolt
=============
.. SPDX-License-Identifier: GPL-2.0
======================
USB4 and Thunderbolt
======================
USB4 is the public specification based on Thunderbolt 3 protocol with
some differences at the register level among other things. Connection
manager is an entity running on the host router (host controller)
responsible for enumerating routers and establishing tunnels. A
connection manager can be implemented either in firmware or software.
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 later USB4 compliant devices 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 +40,6 @@ 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