extra: add RP2350 RISC-V machine timer timing provider

[tobiaguiar08]

Summary

Add aux_riscv_pico_timer, a ready-to-use timing provider library for RP2350 RISC-V that generates the kernel tick from the RISC-V machine timer. RISC-V applications no longer need to write their own timing provider. The PR also fixes problems found by following the RP2350 RISC-V getting-started guide end to end.

Problem

RISC-V applications on RP2350 have no timing provider library. The getting-started guide embeds an implementation that every integrator must copy into their project. That copy also enables the timer interrupt during timing_provider_setup() and ignores the kernel's stop request (delay_us == 0) in timing_provider_schedule().

Changes

extra: add RP2350 RISC-V machine timer timing provider

• New library aux_riscv_pico_timer (src/extra/riscv_pico_timer.c, include/extra/riscv_pico_timer.h). It only builds when CMRX_ARCH=riscv and PICO_PLATFORM=rp2350-riscv; otherwise CMake prints a status message and skips it.
  • timing_provider_setup() only stores the tick period.
  • timing_provider_schedule() disables the machine timer interrupt for delay_us == 0. Otherwise it re-arms mtimecmp from the current mtime and enables the interrupt.
  • timing_provider_delay() busy-waits on mtime, with no pico_time/hardware_timer dependency.
  • cmrx_machine_timer_handler() is called by the existing quirks/pico-sdk-riscv machine timer ISR. It re-arms the timer and calls os_sched_timing_callback().
• The getting-started guide and the library overview now use the library instead of embedding an implementation. Every target_link_libraries line in the RISC-V guide links it.

riscv: note timing provider use of HAL CSR accessors

• Comment-only update to include/cmrx/arch/riscv/hal.h. It said the CSR accessors exist only for the thread switcher, but the new library uses the mie accessors too.

docs: fix markup rendering in RP2350 RISC-V getting-started guide

• A warning in "Linking CMRX to the project" rendered with literal **, because Doxygen does not open bold emphasis directly before a code span.
• A note in the step that selects the RISC-V core rendered with literal backticks, because Doxygen does not format a code span wrapped in double quotes.

docs: clarify GDB flashing steps in RP2350 RISC-V getting-started guide

• The guide's command now names a RISC-V-capable GDB. It also lists three things a reader may see while flashing: a first-connect vMustReplyEmpty error (retry), the run confirmation prompt (answer y), and OpenOCD keep_alive() warnings during load.

Notes for review

• The tick period is fixed at setup, so the kernel's tickless mode is not used. delay_us only distinguishes stop (0) from run.
• cmrx_machine_timer_handler() is declared in the public header so its definition is checked against a prototype. It is meant to be called by the quirk ISR, not by applications.
• The header redeclares timing_provider_setup/schedule/delay without doc comments, as extra/systick.h does. Doxygen merges identically named declarations across headers, so documenting them again would leak text into the aux_systick and timing API pages. The group description documents setup instead.
• There is no host unit test for the library yet.

Testing

Host: Ubuntu 24.04, Pico SDK 2.3.1, riscv32-corev-elf-gcc 14.1, OpenOCD built from source (cd4873400), Raspberry Pi Debug Probe, Pico 2.

• Getting-started guide, end to end: I created the project from the guide's own code listings and applied its edits in order, with cmrx pointing at this branch. It builds with cmake -B build && cmake --build build.
  • I flashed it with the guide's OpenOCD and GDB target extended-remote / load / run steps, using gdb-multiarch. The LED blinks at about 0.5 s, and blink prints on UART.
  • That run used the guide text before the last commit. The last commit documents what it hit: one connect retry and the run prompt.
  • Before the guide fix in the first commit, following the guide literally failed to link, with undefined timing_provider_* symbols.
• Documentation: rendered with the repository Doxyfile (Doxygen 1.9.8).
  • The warning set is identical to master.
  • The text of the aux_systick and timing API group pages is unchanged.

Generated with Claude Code

[ventZl]

This probably isn't needed. Ignore this case silently.

[ventZl]

I'd instruct your AI to not turn this howto into tutorial of using GDB with OpenOCD :)

[ventZl]

People reading this probably don't care what it does as long as it works. Too much detail.

[ventZl]

In rare conditions, this can overflow. mtime may be shortly before overflowing and end may become number actually numerically lower than now, so the while loop below will terminate prematurely.