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From:   Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
To:     Thomas Gleixner <tglx@linutronix.de>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Bjorn Helgaas <bhelgaas@google.com>
Subject: Re: [[RFC PATCH v1] 1/1] PCI: Add pci=nobbn to ignore ACPI _BBN
 method to override host bridge bus window
Thread-Topic: [[RFC PATCH v1] 1/1] PCI: Add pci=nobbn to ignore ACPI _BBN
 method to override host bridge bus window
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Date:   Fri, 17 Jan 2020 15:00:11 +0000
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On Thu, Jan 16, 2020 at 11:13:13PM +0100, Thomas Gleixner wrote:
> Nicholas,
> 
> Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au> writes:
> 
> > Add pci=nobbn kernel parameter.
> >
> > Override the host bridge bus resource to [bus 00-ff] when specified.
> 
> Fine, but you completely fail to explain why this is useful and why
> someone would utilize this command line parameter.
There are motherboards with single PCIe root complex which give 
significantly less than [bus 00-ff] via CRS. I own one with [bus 00-7f] 
and have seen some with significantly less.

A user who wants to use more busses than the motherboard advertises will 
want to use this kernel parameter, for instance if they have a lot of 
PCIe switches or Thunderbolt 3 devices.

This is similar to how we have pci=nocrs to override motherboards with 
issues. The bus resource is not overridden by pci=nocrs, even though it 
will usually come from the same method. However, I believe it would be 
unwise to change pci=nocrs to include bus resource, as detailed in my 
original RFC.

> 
> Thanks,
> 
>         tglx

Thanks,
Nicholas