kobo: Add sysfs-based interface to frontlight

By default, the ioctl interface is used for accessing the
frontlight. However, the ioctl interface has two problems: the overall
brightness seems to be lower (issue #2321), and it does not support
the natural light support from the Kobo Aura One (issue #2285). This
interface is the first step to fix both these issues.

From observing Nickel's behavior, it appears the white/green/red
intensities are roughly linear dependent on the 4th root of brightness
and warmth values. This leads to a very quick increase of the
intensities for lower brightness/warmth values, which then saturate
for larger ones. The values for gain/offset were determined by a
simple linear regression, but the plan is to create a widget so that
users may adapt these values to their liking.

frontend/device/kobo/sysfs_light.lua

@@ -0,0 +1,97 @@
+-- Interface to the SysFS interface of the Kobo Aura One Frontlight.
+-- This also supports the natural light, which consists of additional
+-- red and green light LEDs.
+
+local logger = require("logger")
+local dbg = require("dbg")
+
+local KoboSysfsLight = {
+    frontlight_white = "/sys/class/backlight/lm3630a_led1b",
+    frontlight_red = "/sys/class/backlight/lm3630a_led1a",
+    frontlight_green = "/sys/class/backlight/lm3630a_ledb",
+    current_brightness = 0,
+    current_warmth = 0,
+    white_gain = 25,
+    red_gain = 24,
+    green_gain = 24,
+    white_offset = -25,
+    red_offset = 0,
+    green_offset = -65,
+}
+
+function KoboSysfsLight:setBrightness(brightness)
+    self:setNaturalBrightness(brightness, self.current_warmth)
+end
+
+dbg:guard(KoboSysfsLight, 'setBrightness',
+          function(self, brightness)
+              assert(brightness >= 0 and brightness <= 100,
+                     "Wrong brightness value given!")
+          end)
+
+function KoboSysfsLight:setWarmth(warmth)
+    self:setNaturalBrightness(self.current_brightness, warmth)
+end
+
+dbg:guard(KoboSysfsLight, 'setWarmth',
+          function(self, warmth)
+              assert(warmth >= 0 and warmth <= 100,
+                     "Wrong warmth value given!")
+          end)
+
+function KoboSysfsLight:setNaturalBrightness(brightness, warmth)
+    -- On Nickel, the values for white/red/green are roughly linearly dependent
+    -- on the 4th root of brightness and warmth.
+    local white = math.min(self.white_gain * math.pow(brightness, 0.25) *
+                       math.pow(100 - warmth, 0.25) + self.white_offset, 255)
+    local red = math.min(self.red_gain * math.pow(brightness, 0.25) *
+                     math.pow(warmth, 0.25) + self.red_offset, 255)
+    local green = math.min(self.green_gain * math.pow(brightness, 0.25) *
+                       math.pow(warmth, 0.25) + self.green_offset, 255)
+
+    white = math.max(white, 0)
+    red = math.max(red, 0)
+    green = math.max(green, 0)
+
+    self:_set_light_value(self.frontlight_white, math.floor(white))
+    self:_set_light_value(self.frontlight_green, math.floor(green))
+    self:_set_light_value(self.frontlight_red, math.floor(red))
+
+    self.current_brightness = brightness
+    self.current_warmth = warmth
+end
+
+dbg:guard(KoboSysfsLight, 'setNaturalBrightness',
+          function(self, brightness, warmth)
+              assert(brightness >= 0 and brightness <= 100,
+                     "Wrong brightness value given!")
+              assert(warmth >= 0 and warmth <= 100,
+                     "Wrong warmth value given!")
+          end)
+
+function KoboSysfsLight:_set_light_value(sysfs_directory, value)
+    -- bl_power is '31' when the light is turned on, '0' otherwise.
+    if (value > 0) then
+        self:_write_value(sysfs_directory .. "/bl_power", 31)
+    else
+        self:_write_value(sysfs_directory .. "/bl_power", 0)
+    end
+    self:_write_value(sysfs_directory .. "/brightness", value)
+end
+
+function KoboSysfsLight:_write_value(file, value)
+    local f = io.open(file, "w")
+    if not f then
+        logger.err("Could not open file: ", file)
+        return false
+    end
+    local ret, err_msg, err_code = f:write(value)
+    io.close(f)
+    if not ret then
+        logger.err("Write error: ", err_msg, err_code)
+        return false
+    end
+    return true
+end
+
+return KoboSysfsLight