[PATCH v11 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, safely guarding memory teardown boundaries.

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. 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 rigorously validated on real hardware:
- Raspberry Pi 400 (BCM2711): Validated System L2, ARM CPU, and VideoCore VPU
  firmware mailbox IPC performance counters organically spanning multi-slice multiplexing boundaries.
- 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 independently configuring `bcm2712_` filters.

Changes in v11 (Addressing upstream integration feedback)
==============================
- Removed the unused `num_started` state tracking variable from `struct rpi_axi_hw_events`
  and its associated start/stop increments, cleanly addressing a reviewer's note on dead code.
- Eliminated KCSAN data race warnings in `rpi_axi_pmu_read_counter()`. Annotated
  the `enabled[idx]` flag with `READ_ONCE()` during lockless VPU thread polling, and matching
  `WRITE_ONCE()` barriers during subsystem teardowns.
- Restored `flush_work()` in `rpi_axi_pmu__exit`. When the module is unloaded, 
  `perf_pmu_unregister()` asynchronously schedules `vpu_work` to gracefully
  disable the physical VideoCore IPC hardware. Using `cancel_work_sync()` was 
  blindly dropping these critical queued disables, silently leaking running hardware monitors!
  Included an explicit defensive comment mathematically documenting this dependency.
- Inverted PMU and CPU hotplug initialization sequence in `probe()` 
  to cleanly eliminate a theoretical `devm_kzalloc` memory UAF race.
- Embedded a robust boolean initialization guard physically into the 
  hotplug event handler, definitively securing offline context callbacks
  from concurrent access panics during concurrent failures.
- Architecturally radically simplified BCM2712 sysfs generation loops 
  by switching to an explicit inverse `bcm2712_` whitelist. This 
  mathematically prevents ghost suffix parameters (`.unit` and `.scale`) 
  from orphaned legacy VPU transactions bleeding into userspace dynamically.
- Explicitly appended documentation notes organically defending the inclusion 
  of the `#if LINUX_VERSION_CODE` legacy shim surrounding `hrtimer_setup` 
  for seamlessly preserving downstream compatibility with Raspberry Pi LTS
  kernel deployments natively.

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 | 2877 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 2888 insertions(+)
 create mode 100644 drivers/perf/rpi_axi_pmu.c

-- 
2.41.0