linux-kernel - Re: [RESEND PATCH v1] PCI: pcie_bus

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Message-ID: <20200910133155.GA782074@bjorn-Precision-5520>
Date:   Thu, 10 Sep 2020 08:31:55 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Jim Quinlan <james.quinlan@...adcom.com>
Cc:     "open list:PCI NATIVE HOST BRIDGE AND ENDPOINT DRIVERS" 
        <linux-pci@...r.kernel.org>,
        "maintainer:BROADCOM BCM7XXX ARM ARCHITECTURE" 
        <bcm-kernel-feedback-list@...adcom.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND PATCH v1] PCI: pcie_bus_config can be set at build time

On Thu, Sep 10, 2020 at 08:57:10AM -0400, Jim Quinlan wrote:
> Hi Bjorn,
> 
> On Wed, Sep 9, 2020 at 10:25 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
> >
> > On Tue, Sep 08, 2020 at 12:32:48PM -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.
> >
> > I guess...  I really hate these build-time config settings for both
> > ASPM and MPS/MRRS.  But Linux just isn't smart or flexible enough to
> > do the right thing at run-time, so I guess we're kind of stuck.
> >
> > I guess you have systems where you need a different default?
>
> Yes, we've been shipping our kernel with the DEFAULT and since we do
> not have FW it is not configured optimally.  Some customers have
> noticed and I tell them to put 'pci=pcie_bus_safe' on their bootline
> but I'd rather have this setting work for all customers as it yields
> the option we want.

I'm guessing you probably don't have any hotplug slots.  Seems like we
ought to be able to recognize that and pick pcie_bus_safe
automatically.  Someday.

Maybe that's part of the description: if you have a closed system with
no possibility of adding new devices, we can use the largest MPS
that's supported by all devices, i.e., pcie_bus_safe.

> > It'd be nice if we could put a little more detail in the Kconfig to
> > help users choose the correct one.  "Ensure MPS matches upstream
> > bridge" is *accurate*, but it doesn't really tell me why I would
> > choose this rather than a different one.
>
> IIRC I just copied the comments that were in the bootline settings.
> I'm concerned about there being the same comment in two places; sooner
> or later someone will update one place and not the other.

True.  It'd be nice if we at least had *one* place with a useful
description.  I don't think we have any today.

Bjorn