[PATCH v5 0/2] perf: Add Raspberry Pi AXI PMU driver

[Ian Rogers]

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 v5
-------------
- Teardown UAF/Orphaning Fix: Reversed module exit logic, calling cpuhp_state_remove_instance()
  *before* perf_pmu_unregister() to prevent accessing an unregistered PMU in the offline
  CPU handler during device removal, and to prevent event orphaning races during hotplug.
- VPU Hardware Watcher Leak Fix: In rpi_axi_pmu_del(), hardware disablement operations for VPU 
  bus watchers are now properly deferred to the vpu_work workqueue to avoid silently leaking 
  running hardware counters when the VPU monitor is no longer in use.
- Grouped Event Start Bug Fix: Deprecated reliance on num_monitored == 1 to initialize
  the global bus watcher. Instead, use a dedicated monitor_running flag correctly to
  start the global monitor when the first bus watcher gets enabled, mitigating bugs when
  adding grouped PMU events.
- IPC Phantom Spikes Zero Fallback Fix: Modified rpi_axi_pmu_read_counter() to return U32_MAX
  on IPC or MMIO hardware read failures, gracefully ignoring the delta calculation returning.
  This fixes unsigned 31-bit huge spikes where delta (0 - old_count) & 0x7FFFFFFF occurred.
- Code Comments: Added doc block comments inside rpi_axi_pmu_read(), rpi_axi_pmu_vpu_work_handler(),
  and everywhere U32_MAX returns occur to document the explicit justification for why U32_MAX indicates
  failure (valid counters are 31-bit) and why the driver drops the sample to avoid artificial spikes.
- VPU Data Race Lockdep Fix: Rewrote locking hierarchy inside vpu_work_handler(). PMU spinlock
  is now correctly held across rpi_axi_pmu_enable_bus_watcher() unconditionally since the function
  touches active monitor registers directly via MMIO, dropping it purely for mailbox IPC reads.
- Hrtimer Add/Del Forward Concurrency Warning Fix: rpi_axi_pmu_timer_handler() now executes
  hrtimer_forward_now() *before* releasing the PMU spinlock, fully mitigating timer enqueue races
  resulting from a simultaneous parallel pmu->add()/pmu->del().
- VPU Start Asynchronous Latency: Added explicit schedule_work(&pmu->vpu_work) call inside
  rpi_axi_pmu_start() when starting VPU-tracked events so they begin immediately instead of
  waiting for a 2-second timer interrupt period for short workloads.
- Event Stop Read-Modify-Write Spinlock Protection: Moved event->hw.state modifications inside
  rpi_axi_pmu_stop() under the local CPU PMU irqsave spinlock to safeguard atomic bitwise flags
  against the timer handler or workqueue updates.
- BCM2712 Pre-probe Initializer UAF race: Shifted perf_pmu_register() to the very end of
  rpi_axi_pmu__init() to ensure all hardware structures and CPU HP bindings securely exist prior to
  exposing the PMU to userspace tools preventing devres unregistration memory faults.
- BCM2712 ARM CPU L2 Event Correction: Corrected the bus index for the Cortex-A76 Cores PMU event
  aliases to bus=14 (BCM2712_SB__CPU_L2) to match architectural routing specifications on the RPi 5.
- BCM2712 JPEG Bus Monitor Visibility: Stopped improperly hiding the legacy JPEG bus aliases in the
  is_visible() callback on BCM2712 chips.
- BCM2712 Custom Event Replacements: Created specific BCM2712 equivalents for peripheral_rtrans/wtrans
  and cpu_uc_rtrans/wtrans since raw bus mappings shifted structurally on VideoCore VII.

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 | 2078 ++++++++++++++++++++++++++++++++++++
 3 files changed, 2089 insertions(+)
 create mode 100644 drivers/perf/rpi_axi_pmu.c

-- 
2.55.0.679.g6767b8d81c-goog