126 lines
5.8 KiB
Markdown
126 lines
5.8 KiB
Markdown
# Kernel module notes
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Phase-0.5 decisions, resolved in
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`_research/pi-kernel-module-rt-gpio-2026-06-18.md` (read that for the full
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evidence and source-level analysis). This file is the short, actionable summary
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the code in `kernel/` is built on.
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## Decisions baked into the code
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| Question | Decision | Where |
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|----------|----------|-------|
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| CLK: interrupt vs. busy-poll | ATN falling-edge hardirq enters the state machine; bytes are busy-polled with `local_irq_save()` held for one byte (ninepin pattern) | `atn_isr`, `receive_byte` |
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| IRQ discipline | `local_irq_save()` around each byte only (~200 µs normal, ≤ 8 ms abort cap), **not** the whole ATN phase | `receive_byte` |
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| GPIO access (hot path) | Direct BCM register access via `ioremap` (~30–40× faster than gpiod) | `iec_lines.h` |
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| GPIO access (init/exit) | gpiod descriptor API (`gpio_to_desc`, `gpiod_to_irq`) | `iec_init`/`iec_exit` |
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| Kernel↔userspace | Character device `/dev/iec0` + `kfifo` + wait queue (IEC ≤ 1000 B/s; relayfs not justified) | `iec_read`, `emit_record` |
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| `udelay` vs. poll | Poll-with-timeout for CLK transitions; `udelay` only for fixed delays (EOI ack 80 µs, EOI detect 250 µs) | `iec_timing.h`, `wait_clk` |
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| isolcpus / nohz_full | **Not** in the Phase-1 baseline. Add `isolcpus=3 nohz_full=3 rcu_nocbs=3 irqaffinity=0-2` only if Phase-2 bit-error rate > 1% | (boot cmdline) |
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| PREEMPT_RT | Stock kernel sufficient; RT is a last resort | — |
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| Module signing | Not required on stock RPi OS Bookworm | — |
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| Peripheral base | `0x3F000000` (BCM2710A1 / Pi Zero 2 W; **not** ninepin's `0x20000000`) | `iec_lines.h` |
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## Level-shifter / DATA-sensing note (deviation from the research doc)
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The research doc (§4.3) analysed a **7406 inverting** drive + separate resistor
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divider sense, and flagged a "DATA sensing gap" (a 7406 output pin can't read the
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bus back). The **PLAN** instead specifies a **non-inverting bidirectional level
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shifter** (BSS138, sd2iec-style single DATA pin, §3.1). That choice:
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- makes the logic **non-inverting**: bus low (asserted) ⇒ Pi reads LOW;
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- **closes the sensing gap** — when the DATA pin is switched to *input* (Hi-Z),
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reading it back through the bidirectional shifter returns the real bus state
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the C64 is driving. This is exactly what `receive_byte` relies on for bit
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sampling.
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So `iec_lines.h` is non-inverting; there is no 7406 inversion to track. Confirm
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with the `IEC_IOC_SELFTEST` ioctl (drive low → read low; release → read high)
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before connecting the C64 (PLAN.md §10 risk row).
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## Build & deploy (on the Pi)
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```bash
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sudo apt install raspberrypi-kernel-headers
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cd kernel
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make # iec_listener.ko
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make overlay # dts/iec-overlay.dtbo (optional pin reservation)
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sudo insmod iec_listener.ko address=4
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ls -l /dev/iec0
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# ... talk to the C64 ...
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sudo rmmod iec_listener # releases DATA on the way out
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```
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Optional pin-reservation overlay (Bookworm paths — note the `/boot/firmware/`
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prefix; the legacy `/boot/` paths no longer apply on 64-bit Bookworm):
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```bash
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sudo cp dts/iec-overlay.dtbo /boot/firmware/overlays/
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echo "dtoverlay=iec-overlay" | sudo tee -a /boot/firmware/config.txt
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sudo reboot
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# after reboot, verify it loaded:
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dtoverlay -l
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```
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**Pin the kernel *before* the first `apt full-upgrade`** — any kernel bump
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breaks the module via a vermagic mismatch (`Invalid module format`), so hold the
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kernel packages up front rather than after the fact:
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```bash
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sudo apt-mark hold raspberrypi-kernel raspberrypi-kernel-headers raspberrypi-bootloader
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# record the pinned version here once known:
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# Pinned: raspberrypi-kernel <VERSION> (<DATE>)
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```
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## Building off the Pi (emulated arm64 Docker)
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You can compile the module on a non-Pi (x86) host with `kernel/build-in-docker.sh`
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(or `make docker-build`). It runs an **emulated arm64** Raspberry Pi OS container,
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installs the raspberrypi kernel headers via apt, and builds natively so the
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module's *vermagic* matches the Pi.
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```bash
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cd kernel
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./build-in-docker.sh # -> iec_listener.ko (arm64) in this dir
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./build-in-docker.sh clean
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```
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Configurable via env vars (kernel version is configurable as requested):
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| Var | Default | Purpose |
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|-----|---------|---------|
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| `HEADERS_PKG` | `linux-headers-rpi-v8` | headers package; use `-v7`/`-v6` for 32-bit, or `raspberrypi-kernel-headers` |
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| `KERNEL_VERSION` | *(latest)* | exact version pin, e.g. `1:6.6.51-1+rpt3` |
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| `IMAGE` | `iec-kbuild` | builder image tag |
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```bash
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KERNEL_VERSION=1:6.6.51-1+rpt3 ./build-in-docker.sh
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```
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**vermagic caveat:** the raspberrypi apt archive normally serves only the
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*latest* kernel in its pool, so pinning `KERNEL_VERSION` to an old release may
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not be downloadable. The reliable strategy is to keep the Pi current
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(`sudo apt full-upgrade`) and build with the default (latest) — then the Pi and
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the container agree. If you must target an older/specific kernel, copy the Pi's
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`/lib/modules/$(uname -r)/build` tree into the container instead of using apt.
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`uname -r` is **not** used inside the container (it reports the host kernel under
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emulation); the entrypoint derives `KDIR` from the installed headers under
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`/lib/modules/`.
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The host needs qemu binfmt for arm64; the script registers it once via
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`tonistiigi/binfmt --install arm64` (a one-time privileged container).
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## Starting timing constants
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See `kernel/iec_timing.h`. Tune in Phase 2 against a real C64 / logic analyser.
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The likely first knobs: `IEC_EOI_DETECT_US` (EOI false positives/negatives) and
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`IEC_CLK_TIMEOUT_US` (frame errors under load).
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## Open items to verify on hardware
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- GPIO IRQ latency on BCM2710A1 under representative load (target ATN ack ≪ 1 ms).
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- Direct-register read latency inside the IRQ-off loop on the A53 (budget vs. the
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20 µs C64 bit window).
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- Whether `isolcpus` is needed once Phase 2 runs under WiFi/SD load.
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- `IEC_GPIO_DATA` direction-flip latency (`GPFSEL` write) — should be tens of ns.
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