linux-kernel - Re: [RFC 1/2] arm64: PCI: Allow use arch-specific pci sysdata

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Message-ID: <87sg4qgdrd.wl-maz@kernel.org>
Date:   Sat, 20 Mar 2021 13:23:34 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux on Hyper-V List <linux-hyperv@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>, Clint Sbisa <csbisa@...zon.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Sunil Muthuswamy <sunilmut@...rosoft.com>
Subject: Re: [RFC 1/2] arm64: PCI: Allow use arch-specific pci sysdata

On Sat, 20 Mar 2021 13:03:13 +0000,
Arnd Bergmann <arnd@...db.de> wrote:
> 
> On Sat, Mar 20, 2021 at 1:54 PM Marc Zyngier <maz@...nel.org> wrote:
> > On Fri, 19 Mar 2021 21:12:46 +0000,
> 
> > >
> > > Ugh.  pci_root_bus_fwnode() is another callback to find the
> > > irq_domain.  Only one call, from pci_host_bridge_msi_domain(), which
> > > itself is only called from pci_set_bus_msi_domain().  This feels like
> > > another case where we could simplify things by having the host bridge
> > > driver figure out the irq_domain explicitly when it creates the
> > > pci_host_bridge.  It seems like that's where we have the most
> > > information about how to find the irq_domain.
> >
> > Urgh. This is a perfect copy paste of the x86 horror, warts and all.
> > I can't say I'm thrilled (another way to say "Gawd, Noes! Never!").
> >
> > One thing I am sure of is that I do not want to add more custom
> > indirection to build the MSI topology. We barely got rid of the
> > msi_controller structure, and this is the same thing by another
> > name. Probably worse, actually.
> >
> > In this case, I don't see the point in going via a fwnode indirection
> > given that there is no firmware tables the first place.
> >
> > As for finding the irq domain from the host bridge, that's not doable
> > in most cases on arm64, as it is pretty likely that the host bridge
> > knows nothing about MSIs when they are implemented in the GIC (see my
> > recent msi_controller removal series that has a few patches about
> > that).
> >
> > Having an optional callback to host bridges to obtain the MSI domain
> > may be possible in some cases though (there might be a chicken/egg
> > problem for some drivers though...).
> 
> I would expect that the host bridge driver can find the MSI domain
> at probe time and just add a pointer into the pci_host_bridge
> structure.

In most cases, it doesn't implement it itself, and I'd be reluctant to
duplicate information that can already be retrieved from somewhere
else in a generic way (i.e. no PCI specific).

	M.

-- 
Without deviation from the norm, progress is not possible.