linux-kernel - Re: [PATCH v2] PCI/portdrv: Disable bwctrl service if port is fixed at 2.5 GT/s

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Message-ID: <CAMuHMdVtpVTtUEX=hF+r+rt_awk=w8iYWHd5yzZYGUpKoyYcgA@mail.gmail.com>
Date: Tue, 14 Jan 2025 09:50:45 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Niklas Schnelle <niks@...nel.org>
Cc: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Lukas Wunner <lukas@...ner.de>, 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>, 
	Mika Westerberg <mika.westerberg@...ux.intel.com>, Lorenzo Pieralisi <lpieralisi@...nel.org>, 
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH v2] PCI/portdrv: Disable bwctrl service if port is fixed
 at 2.5 GT/s

On Fri, Dec 13, 2024 at 9:56 PM Niklas Schnelle <niks@...nel.org> wrote:
> Trying to enable bwctrl on an Intel JHL7540 (Titan Ridge) based
> Thunderbolt port causes a boot hang on at least 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.
>
> The PCIe r6.2 specification section 7.5.3.18 recommends to primarily
> utilize the Supported Link Speeds Vector instead of the Max Link Speeds
> field to prevent confusion if future specifications allow devices not
> to support lower speeds. This concern does not apply however when
> specifically targeting devices claiming support only for 2.5 GT/s.
>
> 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>
> Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> Reviewed-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> Signed-off-by: Niklas Schnelle <niks@...nel.org>
> ---
> Note: This issue causes a boot hang on my personal workstation.
>
> While there is an ongoing discussion about generalizing this to all
> devices with a single supported speed. It turns out however that in my
> case dev->supported_speeds incorrectly claims 2.5-8 GT/s requiring
> a seperate second fix. So in the interest of simplicity and because I'll
> be out from the 19th until January, I'd like to propose to do this simple
> fix to the boot hang now and take the time to figure out a more general
> approach afterwards.
> ---
> Changes in v2:
> - Improve commit message to mention the specific controller and
>   why using the Max Link Speeds field should be fine here.
> - Add a comment (Lukas)
> - Add R-b's (no change to logic).
> - Link to v1: https://lore.kernel.org/r/20241207-fix_bwctrl_thunderbolt-v1-1-b711f572a705@kernel.org

This is now commit e50e27a613db6f18 ("PCI/portdrv: Disable bwctrl
service if port is fixed at 2.5 GT/s") in pci/next, which conflicts
with  commit 774c71c52aa48700 ("PCI/bwctrl: Enable only if more than
one speed is supported") in v6.13-rc4.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds