[PATCH v6 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 rigorously validated on physical hardware across multiple 
SoC generations:
- Raspberry Pi 400 (BCM2711): Validated System MMIO and VideoCore VPU firmware
  mailbox IPC performance counters. Explicitly mechanically verified precisely zero 
  lockup behaviors across 100Hz aggressive asynchronous timer interrupts. Profiling 
  the `h264_v4l2m2m` pipeline hardware isolated >11.1MB in pure AXI VPU hardware matrix reads.
- 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 v6
-------------
- Reorganized commit structure rigorously into two cleanly bisectable patches
  (BCM2835 core in Patch 1, BCM2712 additions in Patch 2).
- Resolved unmet Kconfig dependency warnings when MAILBOX=n by cleanly relying
  on 'depends on RASPBERRYPI_FIRMWARE || (COMPILE_TEST && !RASPBERRYPI_FIRMWARE)'
  rather than forcefully using 'select' (Patch 1).
- Resolved catastrophic PREEMPT_RT/softirq deadlocks associated with atomic
  spinlocks globally preempting 'hrtimer_cancel()' routines by permitting the
  timer loop to detect 0 active events and cleanly self-terminate (Patch 1).
- Purged unsafe 'spin_unlock_irqrestore' lock dropping maneuvers occurring
  abruptly inside 'rpi_axi_pmu_read()', ensuring atomic contexts and missing
  flags no longer inadvertently trigger kernel panics (Patch 1).
- Eliminated silent ABA VPU slot data-corruption and event hardware leaks by
  decoupling 'vpu_work_handler' loops safely onto a 'vpu_disable_pending' matrix.
  Deleted VPU watchers are safely disabled exactly prior to newly allocated slots 
  executing initialization within the same asynchronous worker loop (Patch 1).
- Resolved lockless data races across VPU mailbox hardware mutation checks by fully
  protecting 'monitor_running' and 'enabled' state modifications structurally inside 
  the 'rpi_axi_pmu_start' and 'rpi_axi_pmu_vpu_work_handler' core locks.
- Disentangled rigid 'cancel_work_sync' usages back to safe 'flush_work' variants in
  teardown unloading contexts so dynamic Mailbox IPC teardown flows successfully 
  execute without aborting directly in mid-flight (Patch 2).
- Sharpened the sysfs 'is_visible' string-masking logic to properly accommodate
  Raspberry Pi 5 aliases, resolving an overzealous 'strstr' bug directly hiding
  "bcm2712_peripheral_" aliases on RPi 5 hardware (Patch 2).
- Rewrote component destruction sequencing logic completely onto safe CPU hotplug 
  '_nocalls' handlers to strictly avoid User-After-Free events from persisting 
  against unregistered/defunct driver models during 'perf_pmu_unregister' (Patch 1).
- Eradicated cross-execution bus tracker leaks across failed/zombied workflows where
  'monitored' mask properties remained structurally trapped inside invalid state
  loops by surgically injecting manual 'enabled = false' boolean teardowns into 'del()'.
- Inserted explicit comments explaining the critical 31-bit counter-masking logic
  regarding standard U32_MAX hardware exception signaling (Patch 1).
- Scrubbed all erroneous 'TAG=agy' trailing metadata from commit descriptions.

Ian Rogers (2):
  perf: Add Raspberry Pi BCM2835 AXI PMU driver
  perf: Add Raspberry Pi 5 (BCM2712) AXI PMU support

 drivers/perf/Kconfig       |    9 +
 drivers/perf/Makefile      |    1 +
 drivers/perf/rpi_axi_pmu.c | 2119 ++++++++++++++++++++++++++++++++++++
 3 files changed, 2129 insertions(+)
 create mode 100644 drivers/perf/rpi_axi_pmu.c

-- 
2.47.0