[PATCH v4 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 (`perf stat`,
`perf list`, etc.).

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 `/sys/bus/event_source/devices/rpi_axi_pmu/`.
   - Supports event sampling and hardware counter accumulation (`local64_add`),
     automatically managing 31-bit hardware counter wraparound across high-bandwidth
     interconnect transfers.

2. CPU Hotplug Support (`cpuhp`):
   - Registers dynamic CPU hotplug notifiers (`CPUHP_AP_ONLINE_DYN`).
   - Automatically migrates PMU context (`perf_pmu_migrate_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 (~15ns)
     directly mapped over ARM physical memory space (`MON__SYSTEM`).
   - VPU Monitor (Mailbox IPC): For Broadcom BCM2835-BCM2711 platforms (RPi 1-4),
     VideoCore VPU monitor IPC calls are offloaded to process context via a dedicated
     workqueue (`vpu_work`) and serialized under `vpu_mutex`. This avoids atomic
     sleeps or blocking in timer/interrupt context.

4. PREEMPT_RT & Safety Hardening:
   - Uses HRTIMER_MODE_REL_SOFT for timer callbacks to execute in softirq context,
     ensuring spinlock acquisitions are 100% PREEMPT_RT safe.
   - Sets suppress_bind_attrs = true to prevent unsafe manual sysfs unbinding while
     active perf events exist.

5. SoC Generation Support:
   - Patch 1 adds core driver support for Broadcom BCM2835-BCM2711 (RPi 1-4).
   - Patch 2 expands support for Broadcom BCM2712 (Raspberry Pi 5), adding PCIe RP1
     Southbridge links, HEVC decoder, HVS display engine, and Cortex-A76 DSU L3
     interconnect monitoring.

Hardware Validation
-------------------
The driver has been validated on real hardware across multiple SoC generations:
- Raspberry Pi 400 (BCM2711): Validated System L2, ARM CPU, and VideoCore VPU
  firmware mailbox IPC performance counters.
- Raspberry Pi 5 (BCM2712): Validated live byte throughput across HVS display
  refresh cycles, Cortex-A76 DSU L3 interconnect memory traffic, and PCIe RP1
  Southbridge transfers.

Changes in v4
-------------
- Global Monitor Reset Protection: In rpi_axi_pmu_enable_bus_watcher(), only issue
  global monitor resets (GEN_CTL_RESET_BIT) when num_monitored == 1. For subsequent
  watchers, reset only the individual bus watcher, keeping running watchers intact.
- IPC Failure Handling: In rpi_axi_pmu_read_counter(), return 0 on rpi_firmware_property()
  failure instead of uninitialized -1, preventing 0xFFFFFFFF phantom counter spikes.
- Lockdep & MMIO Safety: In rpi_axi_pmu_vpu_work_handler(), only unlock spinlock during
  counter reads if use_mailbox_interface is true, keeping MMIO reads lock-protected.
- Hardirq IPI Deadlock Fix: In rpi_axi_pmu_del(), use hrtimer_try_to_cancel() when in_hardirq()
  to avoid infinite spinning if an SMP cross-CPU IPI interrupts a running softirq timer callback.
- Timer Handler Early Return: In rpi_axi_pmu_timer_handler(), return HRTIMER_NORESTART early
  if active_events == 0.
- CPU Hotplug Preemption Race Fix: Initialize pmu->cpu = cpumask_first(cpu_online_mask) in
  rpi_axi_pmu__init() to guarantee assignment to a valid online CPU core.
- Type-Safe Driver Data Extraction: In rpi_axi_pmu_events_is_visible() and cpumask_show(),
  retrieve struct pmu * from dev_get_drvdata(dev) and convert via pmu_to_rpi_axi_pmu().
- Backward Compatibility: Added LINUX_VERSION_CODE check for hrtimer_setup fallback wrapper,
  allowing out-of-tree builds to compile cleanly on Linux 6.12 LTS kernels.

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

 drivers/perf/Kconfig       |   10 +
 drivers/perf/Makefile      |    1 +
 drivers/perf/rpi_axi_pmu.c | 2049 ++++++++++++++++++++++++++++++++++++
 3 files changed, 2060 insertions(+)
 create mode 100644 drivers/perf/rpi_axi_pmu.c

-- 
2.55.0.679.g6767b8d81c-goog