An AOSP vendor daemon that reads a physical potentiometer via SPI and controls Android media volume in real time. Built entirely inside vendor/myoem/ with no modifications to frameworks/ or device/.
Potentiometer (wiper) → MCP3008 CH0 → /dev/spidev0.0 → potvolumed
↓
/dev/uinput
↓
Android InputReader
↓
PhoneWindowManager
↓
AudioManager (STREAM_MUSIC)
Components:
- Potentiometer (10kΩ linear) wired to MCP3008 analog channel 0
- MCP3008 10-bit SPI ADC connected to RPi5 GPIO header (SPI0, CE0)
/dev/spidev0.0— SPI character device (enabled viadtparam=spi=onin/boot/config.txt)
The daemon runs a 20 Hz poll loop:
- SpiReader — reads a 10-bit ADC sample (0–1023) from the MCP3008 via a 3-byte SPI transaction
- AdcFilter — applies a two-stage filter to eliminate noise:
- Exponential Moving Average (α = 0.2) smooths rapid fluctuations
- Dead zone (±8 counts) suppresses jitter when the knob is stationary
- VolumeMapper — linearly maps the filtered ADC value to an
AUDIO_STREAM_MUSICindex (0–15) - VolumeController — injects
KEY_VOLUMEUP/KEY_VOLUMEDOWNevents via auinputvirtual input device to reach the target index
libaudioclient has no image:vendor variant in AOSP 15 — it is a system-only library that vendor binaries cannot link against. The uinput approach requires only kernel headers and liblog, avoids VNDK/LLNDK issues entirely, and routes through Android's standard input pipeline exactly as physical volume buttons do.
potvolumed/
├── Android.bp — Soong build (cc_binary, vendor: true)
├── potvolumed.rc — init.rc service definition + spidev/uinput permissions
├── sepolicy/private/
│ ├── potvolumed.te — SELinux domain + spidev_device type
│ ├── file_contexts — labels /vendor/bin/potvolumed and /dev/spidev*
│ └── service_contexts — empty (no Binder service registered)
└── src/
├── main.cpp — entry point, poll loop, signal handling
├── SpiReader.h / SpiReader.cpp — spidev ioctl wrapper for MCP3008
├── AdcFilter.h / AdcFilter.cpp — EMA + dead zone noise filter
├── VolumeMapper.h / VolumeMapper.cpp — ADC-to-volume-index linear mapping
└── VolumeController.h / VolumeController.cpp — uinput virtual device + key injection
lunch myoem_rpi5-trunk_staging-userdebug
m potvolumedAdd to myoem_base.mk:
PRODUCT_PACKAGES += potvolumed
PRODUCT_SOONG_NAMESPACES += vendor/myoem/services/potvolumed
PRODUCT_PRIVATE_SEPOLICY_DIRS += vendor/myoem/services/potvolumed/sepolicy/private| Topic | Detail |
|---|---|
| SPI config | Mode 0 (CPOL=0, CPHA=0), 1 MHz, 8 bits/word — within MCP3008's 3.6 MHz max |
| MCP3008 protocol | 3-byte transaction: 0x01, 0x80|(ch<<4), 0x00; result in rx[1][1:0] + rx[2] |
| ADC range | 0–1023 (10-bit, 0V–3.3V) |
| Volume steps | 16 steps (AUDIO_STREAM_MUSIC index 0–15); each step ≈ 64 ADC counts |
| uinput device name | PotVolume Knob (visible in /proc/bus/input/devices) |
| Boot behaviour | First reading sets the reference index silently — no volume jump at startup |
| SELinux | /dev/uinput uses uhid_device label in AOSP 15 (not uinput_device); /dev/spidev* uses a custom spidev_device type declared in vendor policy |
| spidev permissions | Kernel creates as root:root 0600; RC file chown root system + chmod 0660 at on boot |
| Item | Value |
|---|---|
| Device | Raspberry Pi 5 |
| AOSP branch | android-15.0.0_r14 |
| Build system | Soong (Android.bp) |
| Language | C++17 |
| Dependencies | liblog, Linux kernel headers only |