linux-kernel - Re: [PATCH] PCI: dwc: Separate MSI out to different controller

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Message-ID: <20250214071552.l4fufap6q5latcit@thinkpad>
Date: Fri, 14 Feb 2025 12:45:52 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Tsai Sung-Fu <danielsftsai@...gle.com>
Cc: Jingoo Han <jingoohan1@...il.com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	Brian Norris <briannorris@...gle.com>,
	Andrew Chant <achant@...gle.com>, Sajid Dalvi <sdalvi@...gle.com>
Subject: Re: [PATCH] PCI: dwc: Separate MSI out to different controller

On Tue, Feb 11, 2025 at 04:23:53PM +0800, Tsai Sung-Fu wrote:
> >Because you cannot set affinity for chained MSIs as these MSIs are muxed to
> >another parent interrupt. Since the IRQ affinity is all about changing which CPU
> >gets the IRQ, affinity setting is only possible for the MSI parent.
> 
> So if we can find the MSI parent by making use of chained
> relationships (32 MSI vectors muxed to 1 parent),
> is it possible that we can add that implementation back ?
> We have another patch that would like to add the
> dw_pci_msi_set_affinity feature.
> Would it be a possible try from your perspective ?
> 

This question was brought up plenty of times and the concern from the irqchip
maintainer Marc was that if you change the affinity of the parent when the child
MSI affinity changes, it tends to break the userspace ABI of the parent.

See below links:

https://lore.kernel.org/all/87mtg0i8m8.wl-maz@kernel.org/
https://lore.kernel.org/all/874k0bf7f7.wl-maz@kernel.org/

- Mani

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