[PATCH v2 0/2] irqchip/riscv-imsic: Synchronize remote state with IRQ work

[Troy Mitchell]

This was found while bringing up the SoC timer on SpacemiT K3 as a tick
broadcast device.

Without a real broadcast device, the tick code falls back to the hrtimer
based broadcast, which keeps one CPU's local timer running. Once the SoC
timer is registered it replaces the hrtimer broadcast, and every CPU may
stop its local timer in idle.

On K3 the SoC timer interrupt goes through APLIC in MSI mode to IMSIC.
When the CPU owning it goes offline, the interrupt is moved to a CPU
that may already be idle with its local timer stopped. IMSIC leaves the
enable on the new target to a timer on that CPU, and that timer can only
fire through the broadcast interrupt, which is not enabled yet. The
system stalls.

Patch 1 makes remote synchronization use hard IRQ work, which fixes the
stall.

A vector move can still take long to finish. The CPU entering idle
takes over the broadcast interrupt and syncs the new vector before the
old CPU has released the old one, so it arms a retry timer on itself
with its tick already stopped. The move then completes only on its next
interrupt, up to 0.5s later on an idle K3, and affinity changes of the
IRQ return -EBUSY until then. Patch 2 lets the old CPU kick the new CPU
instead and removes the retry timer.

---
Changes in v2:
- Patch 1 unchanged.
- New patch 2: once the old CPU has released the old vector it queues the
  sync work on the new CPU, which then completes the move from its own
  IRQ work. The per-CPU retry timer is dropped, since a move that has to
  wait for the old CPU could only complete on an interrupt driven by a
  timer that may itself be stopped.
- Link to v1: https://patch.msgid.link/20261103-imsic-remote-sync-irqwork-v1-1-8b5c951fc62a@linux.dev

To: Thomas Gleixner <tglx at kernel.org>
To: Radu Rendec <radu at rendec.net>
To: Anup Patel <anup at brainfault.org>
To: Paul Walmsley <pjw at kernel.org>
To: Palmer Dabbelt <palmer at dabbelt.com>
To: Albert Ou <aou at eecs.berkeley.edu>
To: Alexandre Ghiti <alex at ghiti.fr>
To: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
To: Clark Williams <clrkwllms at kernel.org>
To: Steven Rostedt <rostedt at goodmis.org>
To: Björn Töpel <bjorn at rivosinc.com>
Cc: Anup Patel <apatel at ventanamicro.com>
Cc: linux-kernel at vger.kernel.org
Cc: linux-riscv at lists.infradead.org
Cc: linux-rt-devel at lists.linux.dev

---
Troy Mitchell (2):
      irqchip/riscv-imsic: Synchronize remote state with IRQ work
      irqchip/riscv-imsic: Complete vector moves without a timer

 drivers/irqchip/Kconfig                 |  1 +
 drivers/irqchip/irq-riscv-imsic-early.c |  5 +-
 drivers/irqchip/irq-riscv-imsic-state.c | 87 ++++++++++++++-------------------
 drivers/irqchip/irq-riscv-imsic-state.h |  6 +--
 4 files changed, 42 insertions(+), 57 deletions(-)
---
base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
change-id: 20261103-imsic-remote-sync-irqwork-9cfec6c410de

Best regards,
--  
Troy Mitchell <troy.mitchell at linux.dev>