linux-kernel - Re: [PATCH 5/5] powerpc/pci: Add config option for using all 256 PCI buses

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Message-ID: <20220726111001.a2upqf5m5welcla6@pali>
Date:   Tue, 26 Jul 2022 13:10:01 +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>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Cédric Le Goater <clg@...d.org>,
        Nick Child <nick.child@....com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] powerpc/pci: Add config option for using all 256 PCI
 buses

On Tuesday 26 July 2022 21:02:22 Michael Ellerman wrote:
> Pali Rohár <pali@...nel.org> writes:
> > On Wednesday 06 July 2022 12:43:08 Pali Rohár wrote:
> >> By default on PPC32 are PCI bus numbers unique across all PCI domains.
> >> So system could have only 256 PCI buses independently of available
> >> PCI domains.
> >>
> >> This is due to filling DT property pci-OF-bus-map which does not reflect
> >> multi-domain setup.
> >>
> >> On all powerpc platforms except chrp and powermac there is no DT property
> >> pci-OF-bus-map anymore and therefore it is possible on non-chrp/powermac
> >> platforms to avoid this limitation of maximal number of 256 PCI buses in
> >> system even on multi-domain setup.
> >>
> >> But avoiding this limitation would mean that all PCI and PCIe devices would
> >> be present on completely different BDF addresses as every PCI domain starts
> >> numbering PCI bueses from zero (instead of the last bus number of previous
> >> enumerated PCI domain). Such change could break existing software which
> >> expects fixed PCI bus numbers.
> >>
> >> So add a new config option CONFIG_PPC_PCI_BUS_NUM_DOMAIN_DEPENDENT which
> >> enables this change. By default it is disabled. It cause that initial value
> >> of hose->first_busno is zero.
> >>
> >> Signed-off-by: Pali Rohár <pali@...nel.org>
> >> ---
> >>  arch/powerpc/Kconfig         | 11 +++++++++++
> >>  arch/powerpc/kernel/pci_32.c |  6 ++++++
> >>  2 files changed, 17 insertions(+)
> >>
> >> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> >> index be68c1f02b79..f66084bc1dfe 100644
> >> --- a/arch/powerpc/Kconfig
> >> +++ b/arch/powerpc/Kconfig
> >> @@ -370,6 +370,17 @@ config PPC_DCR
> >>  	depends on PPC_DCR_NATIVE || PPC_DCR_MMIO
> >>  	default y
> >>
> >> +config PPC_PCI_BUS_NUM_DOMAIN_DEPENDENT
> >> +	depends on PPC32
> >> +	depends on !PPC_PMAC && !PPC_CHRP
> >> +	bool "Assign PCI bus numbers from zero individually for each PCI domain"
> >> +	help
> >> +	  By default on PPC32 were PCI bus numbers unique across all PCI domains.
> >> +	  So system could have only 256 PCI buses independently of available
> >> +	  PCI domains. When this option is enabled then PCI bus numbers are
> >> +	  PCI domain dependent and each PCI controller on own domain can have
> >> +	  256 PCI buses, like it is on other Linux architectures.
> >> +
> >
> > What do you think, would it be possible to set default value of this
> > option to enabled?
> 
> My preference would be to not have the option at all, just make it the
> default behaviour. Every new CONFIG option adds more combinations that
> need testing, or more likely don't get well tested.
> 
> But I don't have a good feel for what could break if we turn it on, so
> honestly I don't really know.
> 
> Also I do most of my 32-bit testing on pmacs, which are not affected by
> the change.

It is because this change is incompatible with deprecated pci-OF-bus-map
which pmac uses. I do not have any pmac box for testing or development,
so I let this part as is.

If one day pci-OF-bus-map would be possible to disable on pmac then this
pci bus number change can be enabled also for pmac.

> So I'll probably take the series as-is, and then we can do some more
> widespread testing and possibly flip the default to enabled, and then
> maybe remove the option entirely in future.
> 
> cheers

I have tested it on P2020 board with 3 PCIe devices, each on own bus
where each bus is connected to different PCIe controller / domain and it
works correctly. Every PCIe device is on bus 1 bus but on different
domains.