linux-kernel - Re: [PATCH v15 00/15] PCI: EP: Add RC-to-EP doorbell with platform MSI controller

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Message-ID: <87senwd3mo.ffs@tglx>
Date: Sat, 01 Mar 2025 19:09:35 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Marc Zyngier <maz@...nel.org>, Frank Li <Frank.li@....com>
Cc: Kishon Vijay Abraham I <kishon@...nel.org>, "Rafael J. Wysocki"
 <rafael@...nel.org>, Anup Patel <apatel@...tanamicro.com>, Greg
 Kroah-Hartman <gregkh@...uxfoundation.org>, Danilo Krummrich
 <dakr@...nel.org>, Manivannan Sadhasivam
 <manivannan.sadhasivam@...aro.org>, Krzysztof Wilczyński
 <kw@...ux.com>,
 Bjorn Helgaas <bhelgaas@...gle.com>, Arnd Bergmann <arnd@...db.de>, Shuah
 Khan <shuah@...nel.org>, Richard Zhu <hongxing.zhu@....com>, Lucas Stach
 <l.stach@...gutronix.de>, Lorenzo
 Pieralisi <lpieralisi@...nel.org>, Rob Herring <robh@...nel.org>, Shawn Guo
 <shawnguo@...nel.org>, Sascha Hauer <s.hauer@...gutronix.de>, Pengutronix
 Kernel Team <kernel@...gutronix.de>, Fabio Estevam <festevam@...il.com>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Niklas Cassel <cassel@...nel.org>,
 dlemoal@...nel.org, jdmason@...zu.us, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-pci@...r.kernel.org,
 linux-kselftest@...r.kernel.org, imx@...ts.linux.dev,
 devicetree@...r.kernel.org
Subject: Re: [PATCH v15 00/15] PCI: EP: Add RC-to-EP doorbell with platform
 MSI controller

On Sat, Mar 01 2025 at 12:02, Marc Zyngier wrote:
> - This IMMUTABLE thing serves no purpose, because you don't randomly
>   plug this end-point block on any MSI controller. They come as part
>   of an SoC.

Yes and no. The problem is that the EP implementation is meant to be a
generic library and while GIC-ITS guarantees immutability of the
address/data pair after setup, there are architectures (x86, loongson,
riscv) where the base MSI controller does not and immutability is only
achieved when interrupt remapping is enabled. The latter can be disabled
at boot-time and then the EP implementation becomes a lottery across
affinity changes.

That was my concern about this library implementation and that's why I
asked for a mechanism to ensure that the underlying irqdomain provides a
immutable address/data pair.

So it does not matter for GIC-ITS, but in the larger picture it matters.

Thanks,

        tglx