comodore-iec-emu/docs/kernel-notes.md

# Kernel module notes

Phase-0.5 decisions, resolved in
`_research/pi-kernel-module-rt-gpio-2026-06-18.md` (read that for the full
evidence and source-level analysis). This file is the short, actionable summary
the code in `kernel/` is built on.

## Decisions baked into the code

| Question | Decision | Where |
|----------|----------|-------|
| 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` |
| IRQ discipline | `local_irq_save()` around each byte only (~200 µs normal, ≤ 8 ms abort cap), **not** the whole ATN phase | `receive_byte` |
| GPIO access (hot path) | Direct BCM register access via `ioremap` (~30–40× faster than gpiod) | `iec_lines.h` |
| GPIO access (init/exit) | gpiod descriptor API; descriptors resolved by `(chip, hwnum)` via `gpio_device_find_by_label("pinctrl-bcm2835")` + `gpio_device_get_desc()`, **not** `gpio_to_desc()` (see "GPIO descriptor lookup" below) | `iec_init`/`iec_exit` |
| Kernel↔userspace | Character device `/dev/iec0` + `kfifo` + wait queue (IEC ≤ 1000 B/s; relayfs not justified) | `iec_read`, `emit_record` |
| `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` |
| 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) |
| PREEMPT_RT | Stock kernel sufficient; RT is a last resort | — |
| Module signing | Not required on stock RPi OS Bookworm | — |
| Peripheral base | `0x3F000000` (BCM2710A1 / Pi Zero 2 W; **not** ninepin's `0x20000000`) | `iec_lines.h` |

## Level-shifter / DATA-sensing note (deviation from the research doc)

The research doc (§4.3) analysed a **7406 inverting** drive + separate resistor
divider sense, and flagged a "DATA sensing gap" (a 7406 output pin can't read the
bus back). The **PLAN** instead specifies a **non-inverting bidirectional level
shifter** (BSS138, sd2iec-style single DATA pin, §3.1). That choice:

- makes the logic **non-inverting**: bus low (asserted) ⇒ Pi reads LOW;
- **closes the sensing gap** — when the DATA pin is switched to *input* (Hi-Z),
  reading it back through the bidirectional shifter returns the real bus state
  the C64 is driving. This is exactly what `receive_byte` relies on for bit
  sampling.

So `iec_lines.h` is non-inverting; there is no 7406 inversion to track. Confirm
with the `IEC_IOC_SELFTEST` ioctl (drive low → read low; release → read high)
before connecting the C64 (PLAN.md §10 risk row). See
[`../kernel/README.md`](../kernel/README.md#self-test) for how to run it
(`kernel/selftest.sh`), the result bitmask, and the bare-board caveat.

## GPIO descriptor lookup (do **not** use `gpio_to_desc()`)

`iec_init` resolves the four IEC line descriptors **by chip label + hardware
offset**, not by the legacy global GPIO number:

```c
iec_gdev = gpio_device_find_by_label("pinctrl-bcm2835");   /* ref held until exit */
gd_atn   = gpio_device_get_desc(iec_gdev, IEC_GPIO_ATN);   /* hwnum == BCM number */
...                                                        /* check with IS_ERR() */
```

**Why — this bit us once (`insmod: No such device` / `-ENODEV`).** On current
Raspberry Pi OS kernels (6.12 here) the BCM2835 GPIO controller no longer starts
at global number 0 — it's `gpiochip512`, **base 512**:

```
$ cat /sys/class/gpio/gpiochip*/base   # -> 512 (pinctrl-bcm2835), 566 (exp-gpio)
```

The old code used `gpio_to_desc(2/3/17/18)`, i.e. the *global* numberspace
assuming base 0. Those small numbers now fall outside the chip's range
(512–565), so every `gpio_to_desc()` returned `NULL`, the descriptor check
tripped, and init bailed with `-ENODEV`. The `(chip, hwnum)` lookup passes the
**BCM number as the chip-relative offset** (ATN=2, RESET=3, CLK=17, DATA=18),
which is base-independent and survives kernel bumps / gpiochip renumbering.

Notes:
- `gpio_device_get_desc()` returns an `ERR_PTR` on a bad offset (not `NULL`) —
  check with `IS_ERR()`, not `!desc`.
- `gpio_device_find_by_label()` takes a reference; it's released with
  `gpio_device_put(iec_gdev)` in `iec_exit` and on the init error path.
- Both symbols are `EXPORT_SYMBOL_GPL` (fine — the module is GPL) and need
  `#include <linux/gpio/driver.h>`.
- If a future kernel renames the controller, update `IEC_GPIO_CHIP_LABEL`;
  confirm the live label with `gpioinfo` / the `/sys/class/gpio/gpiochip*/label`
  files on the Pi.

## Build & deploy (on the Pi)

```bash
sudo apt install raspberrypi-kernel-headers
cd kernel
make                       # iec_listener.ko
make overlay               # dts/iec-overlay.dtbo (optional pin reservation)
sudo insmod iec_listener.ko address=4
ls -l /dev/iec0
# ... talk to the C64 ...
sudo rmmod iec_listener    # releases DATA on the way out
```

Optional pin-reservation overlay (Bookworm paths — note the `/boot/firmware/`
prefix; the legacy `/boot/` paths no longer apply on 64-bit Bookworm):

```bash
sudo cp dts/iec-overlay.dtbo /boot/firmware/overlays/
echo "dtoverlay=iec-overlay" | sudo tee -a /boot/firmware/config.txt
sudo reboot
# after reboot, verify it loaded:
dtoverlay -l
```

**Pin the kernel *before* the first `apt full-upgrade`** — any kernel bump
breaks the module via a vermagic mismatch (`Invalid module format`), so hold the
kernel packages up front rather than after the fact:

```bash
sudo apt-mark hold raspberrypi-kernel raspberrypi-kernel-headers raspberrypi-bootloader
# record the pinned version here once known:
# Pinned: raspberrypi-kernel <VERSION> (<DATE>)
```

## Verify the module before loading it (vermagic)

`insmod` refuses a module whose *vermagic* doesn't match the running kernel
(`Invalid module format` / `version magic ... should be ...` in `dmesg`). Always
check it after copying a freshly built `.ko` to the Pi:

```bash
modinfo ~/iec_listener.ko | grep vermagic
uname -r          # what the running kernel expects
```

**What correct looks like** — the kernel-version, `SMP`, `preempt`, and arch
tokens all match the running kernel (case differs: `modinfo` lowercases
`preempt`, `uname` prints `PREEMPT` — that's fine):

```
vermagic:       6.12.93+rpt-rpi-v8 SMP preempt mod_unload modversions aarch64
                ^^^^^^^^^^^^^^^^^^^ ^^^ ^^^^^^^                    ^^^^^^^^
                = uname -r          |   |                          = arch (arm64)
                                    SMP  PREEMPT
```

The leading `6.12.93+rpt-rpi-v8` **must equal `uname -r` exactly** — that token
is the only thing `insmod` hard-checks. `modversions` means symbol CRCs were
built against the matching `Module.symvers`, so symbol resolution is consistent
too. `mod_unload` just means `rmmod` is supported. All good → load it.

**What wrong looks like** — any difference in the version token, e.g.:

```
vermagic:       6.6.51+rpt-rpi-v8 SMP preempt mod_unload modversions aarch64
                ^^^^^^ kernel moved on; uname -r says 6.12.93 → insmod rejects it
```

or a wrong/blank arch (`armv7l` vs `aarch64` → you built the 32-bit `-v7`/`-v6`
flavour by mistake), or a missing `modversions` (built against the wrong headers
tree). **The fix is always the same:** rebuild against headers matching the
*current* `uname -r` (re-run the one-liner above to get `HEADERS_PKG` /
`KERNEL_VERSION`), or keep the Pi pinned so the kernel can't drift out from
under the module.

## Building off the Pi (emulated arm64 Docker)

You can compile the module on a non-Pi (x86) host with `kernel/build-in-docker.sh`
(or `make docker-build`). It runs an **emulated arm64** Raspberry Pi OS container,
installs the raspberrypi kernel headers via apt, and builds natively so the
module's *vermagic* matches the Pi.

```bash
cd kernel
./build-in-docker.sh                 # -> iec_listener.ko (arm64) in this dir
./build-in-docker.sh clean
```

Configurable via env vars (kernel version is configurable as requested):

| Var | Default | Purpose |
|-----|---------|---------|
| `HEADERS_PKG` | `linux-headers-rpi-v8` | headers package; use `-v7`/`-v6` for 32-bit, or `raspberrypi-kernel-headers` |
| `KERNEL_VERSION` | *(latest)* | exact version pin, e.g. `1:6.6.51-1+rpt3` |
| `IMAGE` | `iec-kbuild` | builder image tag |

```bash
KERNEL_VERSION=1:6.6.51-1+rpt3 ./build-in-docker.sh
```

**Find the exact values for *your* Pi** — run this on the Pi (e.g. over SSH); it
prints the two lines ready to copy into the `build-in-docker.sh` invocation:

```bash
pkg="linux-headers-$(uname -r | sed 's/.*+rpt-//')"; echo "HEADERS_PKG=$pkg KERNEL_VERSION=$(dpkg-query -W -f='${Version}' "$pkg")"
```

On the project's Pi Zero 2 W (`chris@10.1.0.41`, kernel `6.12.93+rpt-rpi-v8`)
this currently prints:

```bash
HEADERS_PKG=linux-headers-rpi-v8 KERNEL_VERSION=1:6.12.93-1+rpt1
```

**vermagic caveat:** the raspberrypi apt archive normally serves only the
*latest* kernel in its pool, so pinning `KERNEL_VERSION` to an old release may
not be downloadable. The reliable strategy is to keep the Pi current
(`sudo apt full-upgrade`) and build with the default (latest) — then the Pi and
the container agree. If you must target an older/specific kernel, copy the Pi's
`/lib/modules/$(uname -r)/build` tree into the container instead of using apt.

`uname -r` is **not** used inside the container (it reports the host kernel under
emulation); the entrypoint derives `KDIR` from the installed headers under
`/lib/modules/`.

The host needs qemu binfmt for arm64; the script registers it once via
`tonistiigi/binfmt --install arm64` (a one-time privileged container).

## Starting timing constants

See `kernel/iec_timing.h`. Tune in Phase 2 against a real C64 / logic analyser.
The likely first knobs: `IEC_EOI_DETECT_US` (EOI false positives/negatives) and
`IEC_CLK_TIMEOUT_US` (frame errors under load).

## Open items to verify on hardware

- GPIO IRQ latency on BCM2710A1 under representative load (target ATN ack ≪ 1 ms).
- Direct-register read latency inside the IRQ-off loop on the A53 (budget vs. the
  20 µs C64 bit window).
- Whether `isolcpus` is needed once Phase 2 runs under WiFi/SD load.
- `IEC_GPIO_DATA` direction-flip latency (`GPFSEL` write) — should be tens of ns.