linux-kernel - [PATCH 0/3] Enable MSI affinity support for dwc PCI

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Message-ID: <20251003160421.951448-1-rrendec@redhat.com>
Date: Fri,  3 Oct 2025 12:04:18 -0400
From: Radu Rendec <rrendec@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>,
	Manivannan Sadhasivam <mani@...nel.org>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Wilczyński <kwilczynski@...nel.org>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Jingoo Han <jingoohan1@...il.com>,
	Brian Masney <bmasney@...hat.com>,
	Eric Chanudet <echanude@...hat.com>,
	Alessandro Carminati <acarmina@...hat.com>,
	Jared Kangas <jkangas@...hat.com>,
	linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 0/3] Enable MSI affinity support for dwc PCI

Various attempts have been made so far to support CPU affinity control
for (de)multiplexed interrupts. Some examples are [1] and [2]. That work
was centered around the idea to control the parent interrupt's CPU
affinity, since the child interrupt handler runs in the context of the
parent interrupt handler, on whatever CPU it was triggered.

This is a new attempt based on a different approach. Instead of touching
the parent interrupt's CPU affinity, the child interrupt is allowed to
freely change its affinity setting, independently of the parent. If the
interrupt handler happens to be triggered on an "incompatible" CPU (a
CPU that's not part of the child interrupt's affinity mask), the handler
is redirected and runs in IRQ work context on a "compatible" CPU. This
is a direct follow up to the (unsubmitted) patches that Thomas Gleixner
proposed in [3].

The first patch adds support for interrupt redirection to the IRQ core,
without making any functional change to irqchip drivers. The other two
patches modify the dwc PCI core driver to enable interrupt redirection
using the new infrastructure added in the first patch.

Thomas, however, I made a small design change to your original patches.
Instead of keeping track of the parent interrupt's affinity setting (or
rather the first CPU in its affinity mask) and attempting to pick the
same CPU for the child (as the target CPU) if possible, I just check if
the child handler fires on a CPU that's part of its affinity mask (which
is already stored anyway). As an optimization for the case when the
current CPU is *not* part of the mask and the handler needs to be
redirected, I pre-calculate and store the first CPU in the mask, at the
time when the child affinity is set. In my opinion, this is simpler and
cleaner, at the expense of a cpumask_test_cpu() call on the fast path,
because:
- It no longer needs to keep track of the parent interrupt's affinity
  setting.
- If the parent interrupt can run on more than one CPU, the child can
  also run on any of those CPUs without being redirected (in case the
  child's affinity mask is the same as the parent's or a superset).

Last but not least, since most of the code in these patches is your
code, I took the liberty to add your From and Signed-off-by tags to
properly attribute authorship. I hope that's all right, and if for any
reason you don't want that, then please accept my apologies and I will
remove them in a future version. Of course, you can always remove them
yourself if you want (assuming the patches are merged at some point),
since you are the maintainer :)

[1] https://lore.kernel.org/all/20220502102137.764606ee@thinkpad/
[2] https://lore.kernel.org/all/20230530214550.864894-1-rrendec@redhat.com/
[3] https://lore.kernel.org/linux-pci/878qpg4o4t.ffs@tglx/

Radu Rendec (3):
  genirq: Add interrupt redirection infrastructure
  PCI: dwc: Code cleanup
  PCI: dwc: Enable MSI affinity support

 .../pci/controller/dwc/pcie-designware-host.c | 123 ++++++++----------
 drivers/pci/controller/dwc/pcie-designware.h  |   7 +-
 include/linux/irq.h                           |   6 +
 include/linux/irqdesc.h                       |  11 +-
 kernel/irq/chip.c                             |  20 +++
 kernel/irq/irqdesc.c                          |  51 +++++++-
 kernel/irq/manage.c                           |  16 ++-
 7 files changed, 154 insertions(+), 80 deletions(-)

-- 
2.51.0