linux-kernel - Re: [PATCH] PCI/portdrv: Disable bwctrl service if port is fixed at 2.5 GT/s

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Message-ID: <20241209154210.GL4955@black.fi.intel.com>
Date: Mon, 9 Dec 2024 17:42:10 +0200
From: Mika Westerberg <mika.westerberg@...ux.intel.com>
To: Niklas Schnelle <niks@...nel.org>
Cc: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
	Rob Herring <robh@...nel.org>, Krzysztof Wilczy??ski <kw@...ux.com>,
	"Maciej W . Rozycki" <macro@...am.me.uk>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	Alexandru Gagniuc <mr.nuke.me@...il.com>,
	Krishna chaitanya chundru <quic_krichai@...cinc.com>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	"Rafael J . Wysocki" <rafael@...nel.org>, linux-pm@...r.kernel.org,
	Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>,
	linux-kernel@...r.kernel.org,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Amit Kucheria <amitk@...nel.org>, Zhang Rui <rui.zhang@...el.com>,
	Christophe JAILLET <christophe.jaillet@...adoo.fr>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>
Subject: Re: [PATCH] PCI/portdrv: Disable bwctrl service if port is fixed at
 2.5 GT/s

On Sat, Dec 07, 2024 at 07:44:09PM +0100, Niklas Schnelle wrote:
> Trying to enable bwctrl on a Thunderbolt port causes a boot hang on some
> systems though the exact reason is not yet understood. As per the spec
> Thunderbolt PCIe Downstream Ports have a fake Max Link Speed of 2.5 GT/s
> (USB4 v2 sec 11.2.1):
> 
>    "Max Link Speed field in the Link Capabilities Register set to 0001b
>     (data rate of 2.5 GT/s only).
>     Note: These settings do not represent actual throughput.
>     Throughput is implementation specific and based on the USB4 Fabric
>     performance."
> 
> More generally if 2.5 GT/s is the only supported link speed there is no
> point in throtteling as this is already the lowest possible PCIe speed
> so don't advertise the capability stopping bwctrl from being probed on
> these ports.
> 
> Link: https://lore.kernel.org/linux-pci/Z1R4VNwCOlh9Sg9n@wunner.de/
> Fixes: 665745f27487 ("PCI/bwctrl: Re-add BW notification portdrv as PCIe BW controller")
> Tested-by: Niklas Schnelle <niks@...nel.org>
> Signed-off-by: Niklas Schnelle <niks@...nel.org>

Makes sense to me,

Reviewed-by: Mika Westerberg <mika.westerberg@...ux.intel.com>