Re: [PATCH for-linus] PCI: Honor Max Link Speed when determining supported speeds

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Thu, 2024-12-12 at 10:11 -0600, Bjorn Helgaas wrote:
> On Thu, Dec 12, 2024 at 09:56:16AM +0100, Lukas Wunner wrote:
> > The Supported Link Speeds Vector in the Link Capabilities 2 Register
> > indicates the *supported* link speeds.  The Max Link Speed field in
> > the Link Capabilities Register indicates the *maximum* of those speeds.
> > 
> > Niklas reports that the Intel JHL7540 "Titan Ridge 2018" Thunderbolt
> > controller supports 2.5-8 GT/s speeds, but indicates 2.5 GT/s as maximum.
> > Ilpo recalls seeing this inconsistency on more devices.
> > 
> > pcie_get_supported_speeds() neglects to honor the Max Link Speed field
> > and will thus incorrectly deem higher speeds as supported.  Fix it.
> > 
> > Fixes: d2bd39c0456b ("PCI: Store all PCIe Supported Link Speeds")
> > Reported-by: Niklas Schnelle <niks@kernel.org>
> > Closes: https://lore.kernel.org/r/70829798889c6d779ca0f6cd3260a765780d1369.camel@kernel.org/
> > Signed-off-by: Lukas Wunner <lukas@wunner.de>
> > Cc: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
> 
> Looks like you want this in v6.13?  Can we make commit log more
> explicit as to why we need it there?  Is this change enough to resolve
> the boot hang Niklas reported?

As for if it fixes my hang I will test this later today when I come
home. But even if it is not enough on its own, I believe that this will
be needed as a prerequisite for the fix of my hang issue in that
without this patch the dev->supported_speeds incorrectly shows multiple
speeds as supported making it impossible to suppress probing of bwctrl
for devices that only support a fixed speed.