Re: [PATCH v2 RESEND 1/1] PCI: pcie_bus_config can be set at build time

[Jim Quinlan]

On Mon, Mar 2, 2026 at 7:03 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>
> On Tue, Feb 24, 2026 at 04:10:17PM -0500, Jim Quinlan wrote:
> > On Fri, Feb 20, 2026 at 5:11 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> > > On Mon, Sep 28, 2020 at 03:46:51PM -0400, Jim Quinlan wrote:
> > > > The Kconfig is modified so that the pcie_bus_config setting can be done at
> > > > build time in the same manner as the CONFIG_PCIEASPM_XXXX choice.  The
> > > > pci_bus_config setting may still be overridden by the bootline param.
> > > >
> > > > Signed-off-by: Jim Quinlan <james.quinlan@xxxxxxxxxxxx>
> > >
> > > We merged this as b0e85c3c8554 ("PCI: Add Kconfig options for MPS/MRRS
> > > strategy"), which appeared in v5.10.
> > >
> > > In retrospect, I think this might have been a mistake because it
> > > forces a build-time configuration for something that may not be known
> > > at build time and can be set via command-line parameter.
> > ...
>
> > > But I can't find any discussion about it.  Did you have a use case
> > > where command line parameters weren't usable?
> >
> > Yes.  We have a Cable Modem (CM) customer who can update their Linux
> > version but cannot update their boot loader or its default bootline.
> > FWIW, they only want the option to set pcie_bus_config to
> > PCIE_BUS_SAFE via the .config.
> > ...
>
> > It's a CM product which uses a particular Wifi chip.  Recognizing this
> > scenario would have the RC trying to grab the EP's vendor-id (and
> > possibly more info), and that seems awkward at best.
>
> This is actually kind of weird.  I don't know why a particular device
> would have any special MPS or MRRS requirements.
>
> Do you have any more details about this?  Does the WiFi chip actually
> not *work* with other MPS settings?  Or is this a performance thing
> where other settings don't give acceptable performance?

Hi Bjorn,
I looked up the original request mail thread on this -- it appears
that the device defaulted to an MPS value of 128, and that value is
what it was assigned. Unfortunately, this device has an occasional  HW
bug when MPS=128.  Rather than fix the HW bug, they wanted a SW change
in Linux,  and somehow that request morphed into a request for
PCIE_BUS_XXX config settings.  So the reason for the original
submission is weak and we should have done something different, but
here we are.

>
> I assume there's no hotplug in this setup, so we don't't have to worry
> about adding new devices at runtime.  Do you know if there's any
> P2PDMA usage?

No hotplug, no P2P.

Regards,
Jim Quinlan
Broadcom STB/CM

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