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774c71c52a
If a PCIe port only supports a single speed, enabling bandwidth control
is pointless: There's no need to monitor autonomous speed changes, nor
can the speed be changed.
Not enabling it saves a small amount of memory and compute resources,
but also fixes a boot hang reported by Niklas: It occurs when enabling
bandwidth control on Downstream Ports of Intel JHL7540 "Titan Ridge 2018"
Thunderbolt controllers. The ports only support 2.5 GT/s in accordance
with USB4 v2 sec 11.2.1, so the present commit works around the issue.
PCIe r6.2 sec 8.2.1 prescribes that:
"A device must support 2.5 GT/s and is not permitted to skip support
for any data rates between 2.5 GT/s and the highest supported rate."
Consequently, bandwidth control is currently only disabled if a port
doesn't support higher speeds than 2.5 GT/s. However the Implementation
Note in PCIe r6.2 sec 7.5.3.18 cautions:
"It is strongly encouraged that software primarily utilize the
Supported Link Speeds Vector instead of the Max Link Speed field,
so that software can determine the exact set of supported speeds on
current and future hardware. This can avoid software being confused
if a future specification defines Links that do not require support
for all slower speeds."
In other words, future revisions of the PCIe Base Spec may allow gaps
in the Supported Link Speeds Vector. To be future-proof, don't just
check whether speeds above 2.5 GT/s are supported, but rather check
whether *more than one* speed is supported.
Fixes: 665745f27487 ("PCI/bwctrl: Re-add BW notification portdrv as PCIe BW controller")
Closes: https://lore.kernel.org/r/db8e457fcd155436449b035e8791a8241b0df400.camel@kernel.org
Link: https://lore.kernel.org/r/3564908a9c99fc0d2a292473af7a94ebfc8f5820.1734428762.git.lukas@wunner.de
Reported-by: Niklas Schnelle <niks@kernel.org>
Tested-by: Niklas Schnelle <niks@kernel.org>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Jonathan Cameron <Jonthan.Cameron@huawei.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.