linux-kernel - Re: [[RFC PATCH v1] 0/1] Add pci=nobbn to ignore ACPI _BBN method to override host bridge bus window

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Date:   Mon, 9 Dec 2019 21:22:49 +0000
From:   Nicholas Johnson <nicholas.johnson-opensource@...look.com.au>
To:     "bjorn@...gaas.com" <bjorn@...gaas.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>
Subject: Re: [[RFC PATCH v1] 0/1] Add pci=nobbn to ignore ACPI _BBN method to
 override host bridge bus window

On Mon, Dec 09, 2019 at 10:10:55AM -0600, Bjorn Helgaas wrote:
> On Thu, Nov 21, 2019 at 9:02 PM Nicholas Johnson
> <nicholas.johnson-opensource@...look.com.au> wrote:
> >
> > Hi all,
> >
> > I want to be able to override the bus resource from ACPI, but nocrs does
> > not do it. I am putting this out here to get a feel for the sentiment
> > for doing something like this.
> 
> This should be cc'd to linus-pci and linux-acpi (added).  I only
> noticed this message by accident.  And I don't see the patch at all.
I thought I had you cc'd into this - you were in the get_maintainer.pl 
output.

Looks like I posted the cover letter twice instead of the patch. I will 
re-post shortly.

> 
> > What is my motivation for doing this?
> >
> > I have a Gigabyte Z170X Designare motherboard which only gives resource
> > [bus 00-7e]. I want the full [bus 00-ff] because I am trying to add as
> > many Thunderbolt 3 ports with add-in cards as possible. Thunderbolt
> > consumes bus numbers quickly. An Intel Ice Lake implementation (ideal)
> > consumes 42 busses per port, but prior solutions consume 50 busses per
> > port and have additional busses required for the NHI and USB
> > controllers, as well as the bridges from the root port.
> >
> > Why not change nocrs to do this? Why the new kernel parameter?
> >
> > I imagine that on systems with multiple PCI root complexes, things will
> > get hairy if we do this, if they are not placed on separate segments /
> > domains by the firmware. I do not own such a beast, but from what I
> > understand, the firmware normally places them on the same segment /
> > domain with non-overlapping bus numbers. But we may still want to use
> > nocrs for other reasons. I need to use nocrs to allow Linux to allocate
> > vast amounts of MMIO and MMIO_PREF under the Thunderbolt root ports
> > without the BIOS support for Thunderbolt. Hence, they should be kept
> > separate.
> >
> > Why do this in general?
> >
> > The bus resource is still a resource which is specified from ACPI, just
> > like those overridden by nocrs. Even if we do not use pci=nocrs to
> > override it, it should be possible to override it, just as it is
> > possible to override _CRS.
> 
> pci=nocrs is for working around defects in firmware or Linux.  The
> firmware knows more about the platform than Linux, and in general we
> have to trust it.  We probably should taint the kernel when we use it.
> 
> Any parameter like this should work the same on all ACPI systems,
> including ia64 and arm64, and should probably also taint the kernel.
> 
> I can't see the patch itself, but I'm a little confused because we
> normally get the bus number range from _CRS in acpi_pci_root_add() and
> your patch doesn't appear to touch that.
Because pci=nocrs is x86-only, I was making pci=nobbn x86-only. Which 
means I only touched the areas in arch/x86 where pci=nocrs is handled.

If you want, I can change nocrs to work on any arch, although I will 
need somebody who owns ACPI systems of different architectures to test.

The only such system I know of (that is not server hardware) is the 
Microsoft Surface Pro X, which is an ARM64 system with ACPI and at least 
three PCIe root complexes on PCI domains/segments 0, 2, 3. Mainline 
Linux does not run properly even on X86 Surface devices, so I imagine it 
will be an absolute nightmare getting Linux to run on this. I also have 
no intention of buying anything without Thunderbolt / USB4.

Kind regards,
Nicholas

> 
> > Nicholas Johnson (1):
> >   PCI: Add pci=nobbn to ignore ACPI _BBN method to override host bridge
> >     bus window
> >
> >  Documentation/admin-guide/kernel-parameters.txt |  2 ++
> >  arch/x86/include/asm/pci_x86.h                  |  1 +
> >  arch/x86/pci/acpi.c                             | 11 +++++++++++
> >  arch/x86/pci/common.c                           |  3 +++
> >  4 files changed, 17 insertions(+)
> >
> > --
> > 2.24.0
> >