Re: [RFC PATCH v4 03/20] PCI: Make pci_create_root_bus() declare its reliance on MSI domains

From: Sunil V L <sunilvl@ventanamicro.com>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
	linux-acpi@vger.kernel.org, linux-pci@vger.kernel.org,
	acpica-devel@lists.linux.dev,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Will Deacon" <will@kernel.org>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Rafael J . Wysocki" <rafael@kernel.org>,
	"Len Brown" <lenb@kernel.org>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Anup Patel" <anup@brainfault.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Samuel Holland" <samuel.holland@sifive.com>,
	"Robert Moore" <robert.moore@intel.com>,
	"Haibo1 Xu" <haibo1.xu@intel.com>,
	"Conor Dooley" <conor.dooley@microchip.com>,
	"Andrew Jones" <ajones@ventanamicro.com>,
	"Atish Kumar Patra" <atishp@rivosinc.com>,
	"Andrei Warkentin" <andrei.warkentin@intel.com>,
	"Marc Zyngier" <maz@kernel.org>, "Björn Töpel" <bjorn@kernel.org>
Subject: Re: [RFC PATCH v4 03/20] PCI: Make pci_create_root_bus() declare its reliance on MSI domains
Date: Thu, 18 Apr 2024 17:15:53 +0530	[thread overview]
Message-ID: <ZiEH8VF6VhCMnXz1@sunil-laptop> (raw)
In-Reply-To: <Zh/rroBTGTq/Q/FN@sunil-laptop>

Hi Bjorn,

On Wed, Apr 17, 2024 at 09:03:13PM +0530, Sunil V L wrote:
<snip>
> 
> I think fundamentally there are two issues here.
> 
> 1) MSI domain should have been setup properly when
> pci_register_host_bridge() is called. I see that pci_arch_init() which
> is supposed to get called early calls x86_create_pci_msi_domain().
> pci_register_host_bridge() also calls pci_set_bus_msi_domain() to setup
> the MSI domain which can walk up to host bridge to find. So, not sure
> why PCI_BUS_FLAGS_NO_MSI is getting set. Is there an issue in walking up
> the tree?
> 
I think I understand now why this is happening on X86. In X86, even
though x86_create_pci_msi_domain() sets up the MSI domain, it never
informs IRQ framework. Only later in pcibios_device_add(), the device's
MSI domain will be set which is too late for this bridge logic. So, when
pci_set_bus_msi_domain() is called from pci_register_host_bridge(), it
will not get it.

It works in RISC-V (I guess ARM64 as well) because, the irqchip driver
for MSI controller registers using pci_msi_register_fwnode_provider().
But, if we set bridge->msi_domain = true in X86, it will disable MSI
even though it supports MSI which is really bad.

Please correct me if I am wrong with the analysis. If correct, what do
you think the way forward if using CONFIG option is not good? I tried to
register the domain in below commit but I don't feel it is a cleaner way.
https://github.com/vlsunil/linux/commit/d6cf76f10c225c94fffb286b7a3e803a4e37df54

Thanks,
Sunil

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