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

From: Arnd Bergmann <arnd@arndb.de>
To: Marc Zyngier <maz@kernel.org>
Cc: Bjorn Helgaas <helgaas@kernel.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux on Hyper-V List <linux-hyperv@vger.kernel.org>,
	linux-pci <linux-pci@vger.kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Wei Liu <wei.liu@kernel.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Rob Herring <robh@kernel.org>, Clint Sbisa <csbisa@amazon.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Sunil Muthuswamy <sunilmut@microsoft.com>
Subject: Re: [RFC 1/2] arm64: PCI: Allow use arch-specific pci sysdata
Date: Sat, 20 Mar 2021 14:03:13 +0100	[thread overview]
Message-ID: <CAK8P3a1OGZsGmwGTHaVWBjpr_G4aDvO1mfUGU3o8XyLLgHqXpw@mail.gmail.com> (raw)
In-Reply-To: <87tup6gf3m.wl-maz@kernel.org>

On Sat, Mar 20, 2021 at 1:54 PM Marc Zyngier <maz@kernel.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.

        Arnd