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

[Jim Quinlan]

On Tue, Feb 24, 2026 at 6:05 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.
> >
> > I don't see much difference between setting the default at build time
> > versus setting it via the boot command line --  both methods are
> > performed before runtime and make assumptions.
>
> True.  I don't like baking unnecessary decisions into the kernel
> binary.  This one seems useful for a point product but at the cost of
> a small burden on everybody who builds a kernel.
>
> > > 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.
> >
> > > If there's a platform that requires one of these settings, maybe
> > > there's a way to do that programmatically in the host bridge driver
> > > rather than using a Kconfig symbol.
> >
> > 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.
>
> I assume a closed product like a CM doesn't run a plain vanilla
> upstream kernel, so removing the Kconfig option would mean carrying a
> one-line patch to set pcie_bus_config.  There's some trade-off between
> carrying a source patch and tracking Kconfig requirements, but I don't
> know where that would be in your case.

Our goal is to run a plain vanilla kernel with -- if needed -- BRCM
modules, but we've got a ways to go.
>
>
> Could this be decided in pcie-brcmstb.c based on DT or something
> similar?  I guess you said it only applies to CMs with a particular
> WiFi chip, so maybe that's not visible in DT.  But I wonder why it's
> specific to the WiFi -- I would think the important thing is that it's
> a closed system and every closed system would want PCIE_BUS_SAFE.
We only needed it for a specific Wifi chip with a CM board.  All other
boards were fine with the default action.

Going forward, there are a number of ways for them to handle this if
the CONFIG_PCIE_BUS_*  choices are dropped.  One is to use the DT
bootargs with "pci=pcie_bus_safe".  Another is to use
CONFIG_CMDLINE/CMDLINE_FORCE to carry the same param.  And also what
you have suggested above.

If you are just going to modify upstream only, there should be no problem.
If you are going to call this a "Fix" and backport to all of the past
stable versions down to 5.10, there may be some havoc when this
particular product pulls in the fixes.  I've given a heads-up to my CM
colleague, but this info may fall through the cracks.

Regards,
Jim Quinlan
Broadcom CM/STB

>
>
> > > > ---
> > > >  drivers/pci/Kconfig | 56 +++++++++++++++++++++++++++++++++++++++++++++
> > > >  drivers/pci/pci.c   | 12 ++++++++++
> > > >  2 files changed, 68 insertions(+)
> > > >
> > > > diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
> > > > index 4bef5c2bae9f..15ce948858fb 100644
> > > > --- a/drivers/pci/Kconfig
> > > > +++ b/drivers/pci/Kconfig
> > > > @@ -187,6 +187,62 @@ config PCI_HYPERV
> > > >         The PCI device frontend driver allows the kernel to import arbitrary
> > > >         PCI devices from a PCI backend to support PCI driver domains.
> > > >
> > > > +choice
> > > > +     prompt "PCIE default bus config setting"
> > > > +     default PCIE_BUS_DEFAULT
> > > > +     depends on PCI
> > > > +     help
> > > > +       One of the following choices will set the pci_bus_config at
> > > > +       compile time.  The choices offered are the same as those offered
> > > > +       for the bootline parameter 'pci'; i.e. 'pci=pcie_bus_tune_off',
> > > > +       'pci=pcie_bus_safe', 'pci=pcie_bus_perf', and 'pci=pcie_bus_peer2peer'.
> > > > +       This is a compile-time setting and is still be overridden by the
> > > > +       above bootline parameters, if present.  If unsure, chose PCIE_BUS_DEFAULT.
> > > > +
> > > > +config PCIE_BUS_TUNE_OFF
> > > > +     bool "Tune Off"
> > > > +     depends on PCI
> > > > +     help
> > > > +       Use the BIOS defaults; doesn't touch MPS at all.  This is the same
> > > > +       as booting with 'pci=pcie_bus_tune_off'.
> > > > +
> > > > +config PCIE_BUS_DEFAULT
> > > > +     bool "Default"
> > > > +     depends on PCI
> > > > +     help
> > > > +       Default choice; ensures that the MPS matches upstream bridge.
> > > > +
> > > > +config PCIE_BUS_SAFE
> > > > +     bool "Safe"
> > > > +     depends on PCI
> > > > +     help
> > > > +       Use largest MPS that boot-time devices support.  If you have a
> > > > +       closed system with no possibility of adding new devices,
> > > > +       this will use the largest MPS that's supported by all devices.
> > > > +       This is the same as booting with 'pci=pcie_bus_safe'.
> > > > +
> > > > +config PCIE_BUS_PERFORMANCE
> > > > +     bool "Performance"
> > > > +     depends on PCI
> > > > +     help
> > > > +       Use MPS and MRRS for best performance.  This setting ensures
> > > > +       that a given device's MPS is no larger than its parent MPS,
> > > > +       which allows us to keep all switches/bridges to the max MPS supported
> > > > +       by their parent and eventually the PHB.  This is the same as
> > > > +       booting with 'pci=pcie_bus_perf'.
> > > > +
> > > > +config PCIE_BUS_PEER2PEER
> > > > +     bool "Peer2peer"
> > > > +     depends on PCI
> > > > +     help
> > > > +       Set MPS = 128 for all devices.  MPS configuration effected by
> > > > +       the other options could cause the MPS on one root port to be
> > > > +       different than that of the MPS on another.  Simply making the system
> > > > +       wide MPS be set to the smallest possible value (128B) solves
> > > > +       this issue.  This is the same as booting with 'pci=pcie_bus_peer2peer'.
> > > > +
> > > > +endchoice
> > > > +
> > > >  source "drivers/pci/hotplug/Kconfig"
> > > >  source "drivers/pci/controller/Kconfig"
> > > >  source "drivers/pci/endpoint/Kconfig"
> > > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > > index e39c5499770f..dfb52ed4a931 100644
> > > > --- a/drivers/pci/pci.c
> > > > +++ b/drivers/pci/pci.c
> > > > @@ -101,7 +101,19 @@ unsigned long pci_hotplug_mmio_pref_size = DEFAULT_HOTPLUG_MMIO_PREF_SIZE;
> > > >  #define DEFAULT_HOTPLUG_BUS_SIZE     1
> > > >  unsigned long pci_hotplug_bus_size = DEFAULT_HOTPLUG_BUS_SIZE;
> > > >
> > > > +
> > > > +/* PCIE bus config, can be overridden by bootline param */
> > > > +#ifdef CONFIG_PCIE_BUS_TUNE_OFF
> > > > +enum pcie_bus_config_types pcie_bus_config = PCIE_BUS_TUNE_OFF;
> > > > +#elif defined CONFIG_PCIE_BUS_SAFE
> > > > +enum pcie_bus_config_types pcie_bus_config = PCIE_BUS_SAFE;
> > > > +#elif defined CONFIG_PCIE_BUS_PERFORMANCE
> > > > +enum pcie_bus_config_types pcie_bus_config = PCIE_BUS_PERFORMANCE;
> > > > +#elif defined CONFIG_PCIE_BUS_PEER2PEER
> > > > +enum pcie_bus_config_types pcie_bus_config = PCIE_BUS_PEER2PEER;
> > > > +#else
> > > >  enum pcie_bus_config_types pcie_bus_config = PCIE_BUS_DEFAULT;
> > > > +#endif
> > > >
> > > >  /*
> > > >   * The default CLS is used if arch didn't set CLS explicitly and not
> > > > --
> > > > 2.17.1
> > > >
> > >
> > >
>
>

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