linux-kernel - Re: [PATCH] powerpc/pci: Add config option for using OF 'reg' for PCI domain

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Date:   Wed, 6 Jul 2022 12:23:43 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/pci: Add config option for using OF 'reg' for
 PCI domain

On Friday 10 June 2022 17:33:32 Michael Ellerman wrote:
> Pali Rohár <pali@...nel.org> writes:
> > Since commit 63a72284b159 ("powerpc/pci: Assign fixed PHB number based on
> > device-tree properties"), powerpc kernel always fallback to PCI domain
> > assignment from OF / Device Tree 'reg' property of the PCI controller.
> >
> > PCI code for other Linux architectures use increasing assignment of the PCI
> > domain for individual controllers (assign the first free number), like it
> > was also for powerpc prior mentioned commit.
> >
> > Upgrading powerpc kernels from LTS 4.4 version (which does not contain
> > mentioned commit) to new LTS versions brings a regression in domain
> > assignment.
> 
> I'm sorry this broke your system. But I don't really consider it a
> regression, the kernel provides no guarantee about the PCI domain
> numbering across LTS releases.
> 
> Prior to the change the numbering was just based on the order the PHBs
> were discovered in the device tree, which is not robust. A cosmetic
> refactor of the device tree source could cause PHBs to be discovered in
> a different order.
> 
> Similarly a change in firmware PCI discovery or device tree generation
> could cause the numbering to change.
> 
> If you have scripts that are looking for certain devices they can use
> the vendor/device fields in sysfs to find the actual devices they want,
> not just whatever happens to be at 0000:01:00.0.
> 
> > Fix this issue by introducing a new option CONFIG_PPC_PCI_DOMAIN_FROM_OF_REG
> > When this options is disabled then powerpc kernel would assign PCI domains
> > in the similar way like it is doing kernel for other architectures and also
> > how it was done prior that commit.
> 
> I really don't want a config option for that.
> 
> There is a device tree property "linux,pci-domain", described in
> Documentation/devicetree/bindings/pci/pci.txt.
> 
> Can you try adding that to your device tree and updating
> get_phb_number() to look for it?
> 
> cheers

I sent another proposal in V2, now also with "linux,pci-domain" support.