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

[Lukas Wunner <lukas@wunner.de>]

On Thu, Dec 12, 2024 at 08:40:07PM +0100, Niklas Schnelle 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.
> 
> Ok, gave this a test and as somewhat suspected this patch alone doesn't
> fix my boot hang nor do I get more output (also tried Lukas suggestion
> with early_printk).

Hm, that's kind of a bummer because while we know how to work around
your boot hang (by disabling bwctrl altogether), we don't really know
the root cause.

The bwctrl IRQ handler runs in hardirq context, so if it ends up in an
infinite loop for some reason or keeps waiting for a spinlock, that
might indeed cause a boot hang.  Not that I'm seeing in the code where
that might occur.  Nevertheless you can try adding "threadirqs" to the
kernel command line to force all IRQ handlers into threads.
Alternatively, enable CONFIG_PREEMPT_RT to also turn spinlocks into
sleeping locks.  Maybe this turns the boot hang into a hung task splat
and thus helps identify the root cause.

Thanks,

Lukas