POC import

config.json.example

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+{
+	"control": {
+		"interval": 5, // [s] delay between rounds
+		"verbose": true
+	},
+	"sensor": {
+		"source": "/sys/class/thermal/thermal_zone0/temp",
+		"weight": 0.1 // [0.0-1.0] sample contribution weight
+		              //           lower value -> slower response
+	},
+	"pwm": {
+		"chip": 1,
+		"channel": 0,
+		"period": 100000 // [ns]
+	},
+	"map": [
+		// each tuple is [m°C, percentage of period]
+		[25000, 0],
+		[35000, 50],
+		[45000, 75],
+		[50000, 90],
+		[55000, 100]
+	]
+}

fand.py

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+#! /usr/bin/env python3
+# encoding: utf-8
+
+"""
+Fan control service.
+
+Measures temperatures and sets fan PWM according to a configured curve.
+
+See /etc/fand/config.json.example for more.
+"""
+
+import dec8
+import os
+import sys
+import time
+
+class Sensor:
+	def __init__(self, source, weight=0.2):
+		self.f = open(source, "rb")
+		self.weight = weight
+		self.value = None
+	
+	def measure(self):
+		self.f.seek(0)
+		raw = int(self.f.read())
+		
+		if self.value is None:
+			self.value = raw
+		else:
+			self.value = int(self.weight * raw + (1-self.weight) * self.value)
+		
+		return self.value
+
+class PWM:
+	def __init__(self, chip, ch, period):
+		self.chip_base = "/sys/class/pwm/pwmchip%d/" %(chip)
+		self.ch_base = "/sys/class/pwm/pwmchip%d/pwm%d/" %(chip, ch)
+		self.period = period
+		
+		# export the channel to sysfs if necessary
+		if not os.path.isdir(self.ch_base):
+			with open(self.chip_base + "export", "w") as f:
+				f.write(str(ch))
+		
+		# disable channel
+		self.set_enabled(False)
+		
+		# set period & polarity
+		with open(self.ch_base + "period", "w") as f:
+			f.write(str(period))
+			
+		with open(self.ch_base + "polarity", "w") as f:
+			f.write("normal")
+		
+		# acquire a duty cycle file descriptor
+		self.f_dc = open(self.ch_base + "duty_cycle", "w")
+		
+		# set to idle
+		self.set_percent(0)
+		
+		# re-enable channel
+		self.set_enabled(True)
+	
+	def set_enabled(self, enabled):
+		with open(self.ch_base + "enable", "r+") as f:
+			current = f.read().strip()
+			requested = "1" if enabled else "0"
+			
+			if current != requested:
+				f.seek(0)
+				f.write(requested)
+	
+	def set_percent(self, pc):
+		self.duty_cycle = int(self.period * pc/100)
+		self.f_dc.seek(0)
+		self.f_dc.write(str(self.duty_cycle))
+		self.f_dc.flush()
+
+def linlut(a, seq):
+	"""
+	Given a value `a` and a sorted list of tuples `seq`,
+	finds a pair in `seq` whose first elements are closest
+	to ... well you know what I mean. Read the code. It's
+	shorter than this paragraph.
+	"""
+	
+	smaller = None
+	larger = None
+	
+	for d in seq:
+		if d[0] <= a:
+			smaller = d
+		
+		elif d[0] > a:
+			larger = d
+			break
+	
+	if not smaller:
+		return seq[0][1]
+	
+	if not larger:
+		return seq[-1][1]
+	
+	ratio = (a - smaller[0]) / (larger[0] - smaller[0])
+	dy = larger[1] - smaller[1]
+	
+	return smaller[1] + ratio * dy
+
+def printf(fmt, *args, stream=sys.stderr):
+	print(fmt % args, file=stream)
+
+if __name__ == "__main__":
+	try:
+		cfg = dec8.cfg.Config.from_file("/etc/fand/config.json")
+	except FileNotFoundError:
+		printf("!!! missing /etc/fand/config.json")
+	
+	s = Sensor(cfg.sensor.source, cfg.sensor.weight)
+	p = PWM(cfg.pwm.chip, cfg.pwm.channel, cfg.pwm.period)
+	
+	while True:
+		temperature = s.measure()
+		percentage = linlut(temperature, cfg.map)
+		p.set_percent(percentage)
+		
+		if cfg.control.verbose:
+			printf("%.01f °C -> %d %% (%d/%d µs)",
+				temperature,
+				percentage,
+				p.duty_cycle // 1000,
+				p.period // 1000
+			)
+		
+		time.sleep(cfg.control.interval)