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

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Message-ID: <8634g9sthr.wl-maz@kernel.org>
Date: Wed, 19 Feb 2025 18:02:24 +0000
From: Marc Zyngier <maz@...nel.org>
To: Manivannan Sadhasivam <mani@...nel.org>
Cc: Brian Norris <briannorris@...omium.org>,	Manivannan Sadhasivam
 <manivannan.sadhasivam@...aro.org>,	Tsai Sung-Fu <danielsftsai@...gle.com>,
	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,	Andrew Chant <achant@...gle.com>,	Sajid Dalvi
 <sdalvi@...gle.com>
Subject: Re: [PATCH] PCI: dwc: Separate MSI out to different controller

On Wed, 19 Feb 2025 17:51:19 +0000,
Manivannan Sadhasivam <mani@...nel.org> wrote:
> 
> On Fri, Feb 14, 2025 at 11:54:52AM -0800, Brian Norris wrote:
> > On Fri, Feb 14, 2025 at 12:45:52PM +0530, Manivannan Sadhasivam wrote:
> > > 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/
> > 
> > It's hard to meaningfully talk about a patch that hasn't been posted
> > yet, but the implementation we have at least attempts to make *some*
> > kind of resolution to those ABI questions. For one, it rejects affinity
> > changes that are incompatible (by some definition) with affinities
> > requested by other virqs shared on the same parent line. It also updates
> > their effective affinities upon changes.
> > 
> > Those replies seem to over-focus on dynamic, user-space initiated
> > changes too. But how about for "managed-affinity" interrupts? Those are
> > requested by drivers internally to the kernel (a la
> > pci_alloc_irq_vectors_affinity()), and can't be changed by user space
> > afterward. It seems like there'd be room for supporting that, provided
> > we don't allow conflicting/non-overlapping configurations.
> > 
> > I do see that Marc sketched out a complex sysfs/hierarchy API in some of
> > his replies. I'm not sure that would provide too much value to the
> > managed-affinity cases we're interested in, but I get the appeal for
> > user-managed affinity.
> > 
> 
> Whatever your proposal is, please get it reviewed by Marc.

Please see b673fe1a6229a and avoid dragging me into discussion I have
purposely removed myself from. I'd also appreciate if you didn't
volunteer me for review tasks I have no intention to perform (this is
the second time you are doing it, and that's not on).

	M.

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