Add backlight daemon

The backlight on the CM3-PANEL cannot be controled with normal DPMS.  Instead,
the brightness is set using PWM on GPIO pin 22.  The
`thermostat.backlight` program runs as a background process to manage
the PWM duty cycle.  It exposes a D-Bus service to allow other programs
to control the backlight.  It also watches for *xscreensaver* events to
detect when the screen is blanked and disable the backlight.

.vscode/settings.json

@@ -1,3 +1,4 @@
 {
-    "python.pythonPath": ".venv/bin/python3.9"
+    "python.pythonPath": ".venv/bin/python",
+    "python.formatting.provider": "black"
 }

backlight.service

@@ -0,0 +1,8 @@
+[Unit]
+Description=CM3-Panel backlight brightness control service
+
+[Service]
+ExecStart=/usr/bin/python3 -m thermostat.backlightd
+
+[Install]
+WantedBy=default.target

poetry.lock

@@ -58,6 +58,14 @@ category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 
+[[package]]
+name = "dbussy"
+version = "1.3"
+description = "language bindings for libdbus, for Python 3.5 or later"
+category = "main"
+optional = false
+python-versions = "*"
+
 [[package]]
 name = "flake8"
 version = "3.9.1"
@@ -274,7 +282,7 @@ testing = ["pytest (>=4.6)", "pytest-checkdocs (>=1.2.3)", "pytest-flake8", "pyt
 [metadata]
 lock-version = "1.1"
 python-versions = "^3.7"
-content-hash = "b45f2d08de21121b2d91ec83f040e748438c74c4d10a1480d56504fc242568da"
+content-hash = "8ec699d6ce96b5082d8ff5f190cca908fe5043c6b74fdfbfd825a281ac230264"
 
 [metadata.files]
 appdirs = [
@@ -297,6 +305,9 @@ colorama = [
     {file = "colorama-0.4.4-py2.py3-none-any.whl", hash = "sha256:9f47eda37229f68eee03b24b9748937c7dc3868f906e8ba69fbcbdd3bc5dc3e2"},
     {file = "colorama-0.4.4.tar.gz", hash = "sha256:5941b2b48a20143d2267e95b1c2a7603ce057ee39fd88e7329b0c292aa16869b"},
 ]
+dbussy = [
+    {file = "DBussy-1.3-py35-none-any.whl", hash = "sha256:511cf4c76b9c82fa08075ebee01eb9331fe1277404fab52924a083d9ae3f62b3"},
+]
 flake8 = [
     {file = "flake8-3.9.1-py2.py3-none-any.whl", hash = "sha256:3b9f848952dddccf635be78098ca75010f073bfe14d2c6bda867154bea728d2a"},
     {file = "flake8-3.9.1.tar.gz", hash = "sha256:1aa8990be1e689d96c745c5682b687ea49f2e05a443aff1f8251092b0014e378"},

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "thermostat"
-version = "0.1.0"
+version = "0.2.0"
 description = ""
 authors = ["Dustin C. Hatch <dustin@hatch.name>"]
 
@@ -9,6 +9,7 @@ python = "^3.7"
 smbus2 = "^0.4.1"
 "RPi.bme280" = "^0.2.3"
 paho-mqtt = "^1.5.1"
+DBussy = "^1.3"
 
 [tool.poetry.dev-dependencies]
 black = "^21.4b2"

src/thermostat/backlight.py

@@ -0,0 +1,169 @@
+import asyncio
+import logging
+import os
+import signal
+import threading
+from types import TracebackType
+from typing import Optional, Type
+
+import ravel
+from RPi import GPIO
+
+
+log = logging.getLogger("backlightd")
+
+
+@ravel.interface(
+    ravel.INTERFACE.SERVER, name="me.dustinhatch.home.thermostat.Backlight"
+)
+class Backlight:
+    def __init__(self) -> None:
+        self.brightness = 100
+        self.pwm: Optional[GPIO.PWM] = None
+
+    def __enter__(self) -> None:
+        GPIO.setmode(GPIO.BCM)
+        GPIO.setup(22, GPIO.OUT)
+        self.pwm = GPIO.PWM(22, 100)
+        self.pwm.start(0)
+
+    def __exit__(
+        self,
+        exc_type: Optional[Type[Exception]],
+        exc_value: Optional[Exception],
+        tb: Optional[TracebackType],
+    ) -> None:
+        assert self.pwm
+        self.pwm.stop()
+        GPIO.cleanup()
+
+    @ravel.method(in_signature="", out_signature="i")
+    def CurrentBrightness(self) -> int:
+        return self.brightness
+
+    @ravel.method(in_signature="i", out_signature="")
+    def DecreaseBrightness(self, percent: int = 10) -> None:
+        if percent < 0 or percent > 100:
+            raise ValueError(f"Invalid percentage: {percent}")
+        log.info("Decreasing brightness by %d%%", percent)
+        self.brightness = max(self.brightness - percent, 0)
+
+    @ravel.method(in_signature="i", out_signature="")
+    def IncreaseBrightness(self, percent: int = 10) -> None:
+        if percent < 0 or percent > 100:
+            raise ValueError(f"Invalid percentage: {percent}")
+        log.info("Increasing brightness by %d%%", percent)
+        self.brightness = min(self.brightness + percent, 100)
+        self._set_brightness()
+
+    @ravel.method(in_signature="i", out_signature="")
+    def SetBrightness(self, percent: int) -> None:
+        if percent < 0 or percent > 100:
+            raise ValueError(f"Invalid percentage: {percent}")
+        log.info("Setting brigness to %d%%", percent)
+        self.brightness = percent
+        self._set_brightness()
+
+    def _set_brightness(self) -> None:
+        assert self.pwm
+        log.debug("Setting PWM duty cycle to %d", 100 - self.brightness)
+        self.pwm.ChangeDutyCycle(100 - self.brightness)
+
+
+class ScreensaverWatcher:
+    def __init__(
+        self, service: Backlight, loop: asyncio.AbstractEventLoop
+    ) -> None:
+        self.service = service
+        self.loop = loop
+
+        self.done: asyncio.Future
+
+    async def run(self) -> None:
+        assert self.loop
+        self.done = self.loop.create_future()
+        cmd = [
+            "xscreensaver-command",
+            "-watch",
+        ]
+        p = await asyncio.create_subprocess_exec(
+            *cmd,
+            stdin=asyncio.subprocess.DEVNULL,
+            stdout=asyncio.subprocess.PIPE,
+            stderr=None,
+            loop=self.loop,
+        )
+        assert p.stdout
+
+        async def process_stdout():
+            brightness = 100
+            while 1:
+                line = await p.stdout.readline()
+                if not line:
+                    break
+                event = line.split(None, 1)[0].decode()
+                if event == "BLANK":
+                    log.debug("Screensaver activated, disabling backlight")
+                    brightness = self.service.CurrentBrightness()
+                    self.service.SetBrightness(0)
+                elif event == "UNBLANK":
+                    log.debug("Screensaver deactivated, restoring backlight")
+                    self.service.SetBrightness(brightness)
+                else:
+                    log.warning("Unknown event: %s", line)
+
+        task = self.loop.create_task(process_stdout())
+        await self.done
+        p.terminate()
+        await task
+
+    def stop(self) -> None:
+        log.debug("Stopping screensaver watcher")
+        self.loop.call_soon(self.done.set_result, True)
+
+
+class Daemon:
+    def __init__(self, loop: asyncio.AbstractEventLoop) -> None:
+        self.loop = loop
+        self.bus: ravel.Connection
+        self.watcher: ScreensaverWatcher
+
+    async def run(self) -> None:
+        service = Backlight()
+        self.loop.add_signal_handler(signal.SIGINT, self.stop, signal.SIGINT)
+        self.loop.add_signal_handler(signal.SIGTERM, self.stop, signal.SIGTERM)
+        self.bus = await ravel.session_bus_async(self.loop)
+        self.bus.register(
+            "/me/dustinhatch/home/thermostat/Backlight",
+            False,
+            service,
+        )
+        await self.bus.request_name_async(
+            "me.dustinhatch.home.thermostat.Backlight",
+            ravel.DBUS.NAME_FLAG_DO_NOT_QUEUE,
+        )
+
+        self.watcher = ScreensaverWatcher(service, self.loop)
+        with service:
+            await self.watcher.run()
+
+    def stop(self, signum: int) -> None:
+        log.debug("Got signal %d", signum)
+        log.info("Shutting down backlight daemon")
+        self.watcher.stop()
+
+
+def main() -> None:
+    os.environ.setdefault("DISPLAY", ":0.0")
+    logging.basicConfig(level=logging.DEBUG)
+
+    loop = asyncio.get_event_loop()
+    daemon = Daemon(loop)
+    try:
+        loop.run_until_complete(daemon.run())
+    finally:
+        loop.close()
+
+
+if __name__ == "__main__":
+    main()