[PATCH v11 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, 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