[PATCH v4 0/2] perf: Add Raspberry Pi AXI PMU driver

[Ian Rogers <irogers@google.com>]

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