[PATCH v5 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 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 | 2124 ++++++++++++++++++++++++++++++++++++
 3 files changed, 2135 insertions(+)
 create mode 100644 drivers/perf/rpi_axi_pmu.c

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2.55.0.691.gc56d675ccc-goog