Re: [PATCH v1 0/2] PCI: Add pci=nobwctrl to disable the PCIe bandwidth controller

在 2026/7/29 15:01, Lukas Wunner 写道:
> On Wed, Jul 29, 2026 at 11:45:03AM +0800, Guixin Liu wrote:
>> Enabling the PCIe bandwidth controller sets the Link Bandwidth Management
>> Interrupt Enable (LBMIE) and Link Autonomous Bandwidth Interrupt Enable
>> (LABIE) bits.  On some (typically old) platforms this has been observed to
>> cause boot hangs; commit 46a9f70e93ef ("PCI/bwctrl: Disable BW controller
>> on Intel P45 using a quirk") works around one such platform with a
>> per-device quirk.
>>
>> A per-device quirk only helps once the offending device is identified and
>> a quirk has shipped, which is awkward for a boot hang since the machine
>> cannot be booted to report the culprit.  The only existing escape hatch,
>> pcie_ports=compat, is a blunt instrument that also disables AER, PME,
>> hotplug and DPC.
> Ideally we want to identify all devices that have problems if the
> bandwidth controller is enabled and add quirks for every one of them.
>
> In a sense, a command line option disincentivizes reporting and
> fixing affected devices because users can just work around the problem.
>
> Additionally, there is a general dislike in the community for
> adding new command line options.
>
> Are you seeing issues on different devices than the one that's
> already been quirked?  If so, a patch to add quirks for them
> would be welcome.
>
> Thanks,
>
> Lukas
Thanks Lukas, that's fair - I fully agree that per-device quirks are
the right long-term fix, and I'll gladly send quirks for any device I
can reproduce this on.

I'd argue this option isn't really an alternative to quirks, though.
It's what lets you *get to* a quirk in the first place, and it closes
a gap that is specific to bwctrl:

- To write a bwctrl quirk you first have to prove bwctrl is the
   culprit. pcie_ports=compat can get a hung machine booted, but it
   tears down *all* port services at once, so it can't tell you whether
   the offender was bwctrl, AER, PME or hotplug. pci=nobwctrl isolates
   exactly one variable, which is precisely the information a quirk
   needs. In that sense it feeds the quirk pipeline rather than
   discouraging reports.

- compat is also too blunt as a workaround on modern systems: losing
   native AER/DPC/hotplug just to silence bwctrl is a large regression
   in error handling. A surgical switch keeps everything else native.

- bwctrl is currently the only PCIe port service with neither a
   native/_OSC gate nor a disable of its own. hotplug, AER and PME are
   all gated on pcie_ports_native || host->native_*, and pci=noaer
   already exists as precedent. This mostly brings bwctrl up to parity
   rather than inventing a new class of knob.

If the command-line parameter itself is the sticking point, I'm happy
to implement this in whatever form you prefer - e.g. gating bwctrl
registration the same way the other services are gated, or a per-port
sysfs runtime control. My goal is to close the gap; the mechanism is
negotiable.

Best Regards,
Guixin Liu