[PATCH v8 0/2] perf: Add Raspberry Pi AXI PMU driver

This patch series adds an uncore Performance Monitoring Unit (PMU) driver
for Broadcom AXI system and VideoCore VPU performance monitors found on
Raspberry Pi SoCs (BCM2835 through BCM2712 / Raspberry Pi 1 through 5).

Motivation & Background
=======================
Currently, Linux lacks a standard perf-API compatible driver for the
Broadcom AXI performance counter blocks on Raspberry Pi platforms. Prior
out-of-tree vendor solutions relied on custom debugfs nodes and ad-hoc
kthreads, preventing integration with standard Linux perf tooling.

This driver implements standard `struct pmu` hardware uncore callbacks
under `drivers/perf/`, exposing human-readable sysfs event aliases, unit
scaling (`Bytes`), and bus filtering directly to user space.

Key Architectural Improvements & Features
=========================================
1. Standard Linux Perf Integration:
   - Exposes uncore AXI interconnect events via sysfs boundaries.
   - Supports event sampling and hardware counter accumulation,
     automatically managing 31-bit hardware counter wraparound.

2. CPU Hotplug Support (`cpuhp`):
   - Registers dynamic CPU hotplug notifiers (`CPUHP_AP_ONLINE_DYN`).
   - Automatically migrates PMU context to an online CPU core when a
     designated CPU goes offline, avoiding stale uncore state.

3. Hybrid Memory-Mapped & Mailbox Work Queue Architecture:
   - System Monitor (MMIO): Performs fast atomic-safe memory reads
     directly mapped over ARM physical memory space.
   - VPU Monitor (Mailbox IPC): For Broadcom BCM2835-BCM2711 platforms,
     VideoCore VPU monitor IPC calls are offloaded to process context via
     a dedicated workqueue (`vpu_work`) and serialized.

4. PREEMPT_RT & Safety Hardening:
   - Uses HRTIMER_MODE_REL_SOFT for timer callbacks to execute in softirq
     context, avoiding local hard interrupt nesting priority deadlocks.

5. SoC Generation Support:
   - Patch 1 adds core driver support for Broadcom BCM2835-BCM2711.
   - Patch 2 expands support for Broadcom BCM2712 (Raspberry Pi 5).

Changes in v8
=============
- Fully transitioned locking architecture from 'spinlock_t' globally to
  'raw_spinlock_t' to mathematically guarantee fastpath safety against
  "scheduling while atomic" crashes during PREEMPT_RT deployment where
  the perf core unconditionally disables hard IRQs prior to PMU callbacks.
- Re-architected 'RPI_AXI_PMU_TIMER_INTERVAL' significantly downward from
  2000ms natively to 100ms. This acts as a structural baseline compromise
  that prevents excessive VideoCore Mailbox IPC context-switch wakeovers
  on older Pi 1/2 boards, while safely minimizing uncollected residuals
  causing 0-byte dropouts during time multiplexing/undersubscribed traces.
- Enforced strict 100-column sysfs string macro breakages globally.
- Added proper .scale and .unit bytes aliases across all AXI transaction
  events.
- Fixed a fatal use-after-free initialization race condition catching cases
  where cpuhp_state instances were registered prior to the PMU backend,
  which allowed concurrent CPU hotplug migrations to panic aggressively
  against unregistered core structures.

Ian Rogers (2):
  perf: Add Raspberry Pi BCM2835 AXI PMU driver
  perf: Add Raspberry Pi 5 (BCM2712) AXI PMU support

 drivers/perf/rpi_axi_pmu.c | 2884 ++++++++++++++++++++++++++++++++++++
 1 file changed, 2884 insertions(+)
 create mode 100644 drivers/perf/rpi_axi_pmu.c

-- 
2.47.0