[PATCH v3 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 v3
-------------
- 64-bit Device Tree reg Parsing: Replaced of_property_read_u32_index with
  platform_get_resource(pdev, IORESOURCE_MEM, i) to retrieve mailbox offsets,
  supporting 64-bit DT layouts (#address-cells = <2>).
- Firmware Phandle Fallback Safety: In rpi_axi_pmu__init(), if firmware node is
  missing or rpi_firmware_get() returns NULL, explicitly set MON__VPU use_mailbox_interface = false.
  config_is_valid() then returns -EINVAL for VPU events, preventing NULL pointer dereferences.
- Driver Teardown Order: In rpi_axi_pmu__exit(), unregister perf_pmu_unregister(&pmu->pmu)
  first to block new event allocations before removing CPU hotplug callbacks, cancelling hrtimer,
  and syncing workqueue.
- Probe Error Unwind Safety: Updated rpi_axi_pmu__init() error paths to call hrtimer_cancel()
  and cancel_work_sync() before returning probe failure.
- Thread-Safe enabled[idx] Setting: Set enabled[idx] = true under pmu->lock after verifying
  event generation sequence in vpu_work_handler(), eliminating TOCTOU race conditions.
- PREEMPT_RT Compatibility: Switched hrtimer setup and start to HRTIMER_MODE_REL_SOFT
  so timer callbacks run in softirq context, allowing spinlock_t acquisitions on PREEMPT_RT.
- CPU Offline Timer Migration: In rpi_axi_pmu_offline_cpu(), restart hrtimer using unpinned
  HRTIMER_MODE_REL_SOFT so perf migrates timer execution to the target CPU.
- Suppress Unsafe Sysfs Unbind: Set .suppress_bind_attrs = true in platform_driver.
- Kconfig Cleanup: Removed non-existent ARCH_BCM2712 symbol from depends on in Kconfig.
- Legacy Sysfs Alias Filtering: Updated rpi_axi_pmu_events_is_visible() to filter out all
  legacy RPi 1-4 System Monitor aliases (ccp2tx_, mphi_, h264_, v3d_, jpeg_, peripheral_,
  cpu_uc_, cpu_l2_, etc.) on BCM2712 systems.

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 | 2022 ++++++++++++++++++++++++++++++++++++
 3 files changed, 2033 insertions(+)
 create mode 100644 drivers/perf/rpi_axi_pmu.c

-- 
2.55.0.679.g6767b8d81c-goog