fix: turn off SPI and the serial login console for the DPi bus

.github/workflows/validate.yml

@@ -53,9 +53,13 @@ jobs:
               dpkg -S /usr/sbin/avahi-daemon 2>/dev/null | grep -q '^avahi-dpea:' \
                 && echo "OK   /usr/sbin/avahi-daemon owned by avahi-dpea" \
                 || { echo "FAIL avahi-daemon binary not replaced by avahi-dpea"; fail=1; }
-              for line in "dtparam=i2c_arm=on" "dtparam=spi=on" "enable_uart=1" "dtoverlay=disable-bt"; do
+              for line in "dtparam=i2c_arm=on" "enable_uart=1" "dtoverlay=disable-bt"; do
                 grep -qx "$line" /boot/firmware/config.txt && echo "OK   config: $line" || { echo "FAIL config: $line"; fail=1; }
               done
+              # SPI must NOT be enabled: it would claim GPIO7 (SPI0 CE1), a DPi computer input.
+              grep -qx "dtparam=spi=on" /boot/firmware/config.txt && { echo "FAIL config: spi should be off"; fail=1; } || echo "OK   config: spi not enabled"
+              # serial login console masked so the DPi RS485 bus owns /dev/serial0.
+              test -L /etc/systemd/system/serial-getty@ttyAMA0.service && echo "OK   serial getty masked" || { echo "FAIL serial getty not masked"; fail=1; }
               test -f /etc/NetworkManager/system-connections/dpea-eth0.nmconnection \
                 && echo "OK   eth0 profile present" || { echo "FAIL eth0 profile"; fail=1; }
               # best-effort: mDNS binds 5358 (a container may not let the daemon start; verify on hardware)

README.md

@@ -8,8 +8,11 @@ dependencies and installs them with `uv`.
 ## What the image bakes in
 
 - **OS**: Raspberry Pi OS Trixie 64-bit (arm64), Desktop
-- **Hardware**: I2C, SPI, and UART enabled. `dtoverlay=disable-bt` so
-  `/dev/serial0` is the PL011 UART for the SlushEngine / DPi bus. `i2c-dev` loaded.
+- **Hardware**: I2C and UART enabled, SPI deliberately OFF. `dtoverlay=disable-bt`
+  so `/dev/serial0` is the PL011 UART for the SlushEngine / DPi bus, and the serial
+  login console is disabled so a getty does not fight that bus on the same port.
+  `i2c-dev` loaded. SPI stays off because the DPi computer uses GPIO7 (SPI0 CE1) as
+  a digital input, which an SPI overlay would claim ("GPIO busy").
 - **Networking (`eth0`)**: DHCP for internet, plus an always-on static
   `172.17.21.2/22` for a direct laptop-to-Pi cable.
 - **mDNS**: the patched avahi that serves mDNS on port **5358**.
@@ -21,7 +24,7 @@ dependencies and installs them with `uv`.
 Current contents:
 
 - core: `git`, `curl`, `ca-certificates`
-- I2C/SPI: `i2c-tools`
+- I2C: `i2c-tools`
 - kivy runtime: `libsdl2-2.0-0`, `libsdl2-image-2.0-0`, `libsdl2-mixer-2.0-0`, `libsdl2-ttf-2.0-0`, `libmtdev1`, `libgl1-mesa-dri`
 - mDNS resolution: `libnss-mdns`
 
@@ -66,20 +69,21 @@ repo is public, so no GitHub account is needed to download the image.
 
 Runs `shellcheck`, then runs `customize.sh` inside an arm64 Trixie container
 and asserts what can be checked without hardware: `uv` present, the 5358 avahi
-`.deb` installed, the I2C/SPI/UART lines written, and the eth0 NetworkManager
-profile present. This gates merges to `main`.
+`.deb` installed, the I2C/UART config lines written (and SPI *not* enabled), the
+serial getty masked, and the eth0 NetworkManager profile present. This gates
+merges to `main`.
 
 Container checks cannot exercise real hardware or multicast, so they do not cover
-actual I2C/SPI/UART function, live `.local` resolution, or the eth0 address
-coming up. Those need a flashed Pi.
+actual I2C/UART function, live `.local` resolution, or the eth0 address coming up.
+Those need a flashed Pi.
 
 ### On real hardware (`hardware-smoke-test.sh`)
 
 After flashing a Pi, run `sudo ./hardware-smoke-test.sh` on it. It checks: `uv`
-present, `/dev/i2c-*` and `/dev/spidev*` nodes, `/dev/serial0 -> ttyAMA0`, avahi
-listening on 5358 (and not 5353), and eth0 holding `172.17.21.2`. For the
-direct-cable path, set another machine's ethernet to `172.17.21.1` and confirm
-you can reach `172.17.21.2`.
+present, `/dev/i2c-*` present and no `/dev/spidev*` (SPI off), `/dev/serial0 ->
+ttyAMA0` with the serial login console off, avahi listening on 5358 (and not
+5353), and eth0 holding `172.17.21.2`. For the direct-cable path, set another
+machine's ethernet to `172.17.21.1` and confirm you can reach `172.17.21.2`.
 
 ## Files
 

customize.sh

@@ -16,14 +16,28 @@ apt-get install -y --no-install-recommends \
 	$(sed -e 's/#.*//' -e '/^[[:space:]]*$/d' /tmp/packages.txt)
 
 # --- hardware interfaces (edit config.txt directly) ---
+# SPI is deliberately NOT enabled: the DPi computer board uses GPIO7 (SPI0 CE1)
+# as a digital input (_IN_3_PIN), so an enabled SPI overlay claims that pin and
+# the board's GPIO.setup fails with "GPIO busy".
 CONFIG=/boot/firmware/config.txt
 [ -f "$CONFIG" ] || CONFIG=/boot/config.txt
 grep -q '^dtparam=i2c_arm=on'   "$CONFIG" || echo 'dtparam=i2c_arm=on'   >> "$CONFIG"
-grep -q '^dtparam=spi=on'       "$CONFIG" || echo 'dtparam=spi=on'       >> "$CONFIG"
 grep -q '^enable_uart=1'        "$CONFIG" || echo 'enable_uart=1'        >> "$CONFIG"
 grep -q '^dtoverlay=disable-bt' "$CONFIG" || echo 'dtoverlay=disable-bt' >> "$CONFIG"
 grep -q '^i2c-dev' /etc/modules || echo 'i2c-dev' >> /etc/modules
 
+# --- serial login console OFF (the UART hardware stays on via enable_uart above) ---
+# Raspberry Pi OS runs a getty on the serial console by default; it holds
+# /dev/serial0 open and fights the DPi RS485 bus (serial.Serial on /dev/serial0),
+# giving "device reports readiness to read but returned no data (multiple access
+# on port?)". Disable it two ways: drop the console= token from cmdline.txt, and
+# mask the serial getty. enable_uart=1 above keeps the UART itself available.
+CMDLINE=/boot/firmware/cmdline.txt
+[ -f "$CMDLINE" ] || CMDLINE=/boot/cmdline.txt
+[ -f "$CMDLINE" ] && sed -i -E 's/ ?console=(serial0|ttyAMA0|ttyS0),[0-9]+//g' "$CMDLINE"
+install -d /etc/systemd/system
+ln -sf /dev/null /etc/systemd/system/serial-getty@ttyAMA0.service
+
 # --- patched avahi (mDNS on 5358) from the prebuilt arm64 .deb (avahi_0.8 CI) ---
 AVAHI_DEB_URL="${AVAHI_DEB_URL:-https://github.com/dpengineering/avahi_0.8/releases/latest/download/avahi-dpea_0.8_arm64.deb}"
 if curl -fLsS "$AVAHI_DEB_URL" -o /tmp/avahi.deb; then

hardware-smoke-test.sh

@@ -11,8 +11,9 @@ skip(){ echo "SKIP  $1 ($2)"; skipped=$((skipped+1)); }
 
 chk "uv installed"                "command -v uv"
 chk "I2C device node present"     "ls /dev/i2c-*"
-chk "SPI device node present"     "ls /dev/spidev*"
+chk "SPI off (no spidev; GPIO7 free for DPi)" "! ls /dev/spidev*"
 chk "/dev/serial0 -> ttyAMA0"     "[ \"\$(readlink -f /dev/serial0)\" = /dev/ttyAMA0 ]"
+chk "serial login console off"    "! systemctl is-active --quiet serial-getty@ttyAMA0.service"
 chk "avahi listening on 5358"     "ss -lun | grep -q ':5358'"
 chk "avahi NOT on 5353"           "! ss -lun | grep -q ':5353'"
 # The eth0 static (172.17.21.2) only activates when the link has carrier. Running