battery: add "fancy" charge control

Inspired by this talk at FOSDEM:

https://fosdem.org/2024/schedule/event/fosdem-2024-2123-from-kernel-api-to-desktop-integration-how-do-we-integrate-battery-charge-limiting-in-the-desktop/

I wrote some "smart charge" logic for qtile. It probably needs better
detection of other kinds of docking stations, since all I have is a
thunderbolt one. But at least this is a start.

Signed-off-by: Tycho Andersen <[email protected]>

libqtile/widget/battery.py

@@ -42,6 +42,7 @@
 from typing import TYPE_CHECKING, NamedTuple
 
 from libqtile import bar, configurable, images
+from libqtile.command.base import expose_command
 from libqtile.images import Img
 from libqtile.log_utils import logger
 from libqtile.utils import send_notification
@@ -191,6 +192,12 @@ class _LinuxBattery(_Battery, configurable.Configurable):
             None,
             "Name of file with the current power draw in /sys/class/power_supply/battery_name",
         ),
+        (
+            "default_max_charge",
+            # hopefully a reasonable threshold to not kill peoples' batteries
+            90,
+            "The default max charge percentage (depends on battery support)",
+        ),
     ]
 
     filenames: dict = {}
@@ -214,6 +221,9 @@ def __init__(self, **config):
         self.add_defaults(_LinuxBattery.defaults)
         if isinstance(self.battery, int):
             self.battery = "BAT{}".format(self.battery)
+        self.charge_threshold_supported = True
+        self.override_charge_threshold = False
+        self.force_charge = False
 
     def _get_battery_name(self):
         if os.path.isdir(self.BAT_DIR):
@@ -267,7 +277,64 @@ def _get_param(self, name) -> tuple[str, str]:
 
         raise RuntimeError("Unable to read status for {}".format(name))
 
+    def connected_to_thunderbolt(self):
+        try:
+            sysfs = "/sys/bus/thunderbolt/devices"
+            entries = os.listdir(sysfs)
+            for e in entries:
+                try:
+                    name = Path(sysfs, e, "device_name").read_text()
+                except FileNotFoundError:
+                    continue
+                else:
+                    logger.debug("found dock %s", name)
+                    return True
+        except OSError:
+            logger.debug("failed to detect thunderbot %s", exc_info=True)
+        return False
+
+    def set_battery_charge_thresholds(self):
+        if not self.charge_threshold_supported:
+            return
+
+        start = 0
+        end = self.default_max_charge
+
+        # if we are thunderbolt docked, set the thresholds to 40/50, per
+        # https://support.lenovo.com/us/en/solutions/ht078208-how-can-i-increase-battery-life-thinkpad-and-lenovo-vbke-series-notebooks
+        if self.connected_to_thunderbolt():
+            start = 40
+            end = 50
+
+        # unless we have been explicitly asked to charge, then we should JFDI
+        if self.force_charge:
+            start = 0
+            end = 100
+
+        battery_dir = "/sys/class/power_supply"
+
+        path = os.path.join(battery_dir, self.battery, "charge_control_start_threshold")
+        try:
+            with open(path, "w+") as f:
+                f.write(str(start))
+        except FileNotFoundError:
+            self.charge_threshold_supported = False
+        except OSError:
+            self.charge_threshold_supported = False
+            logger.exception("Failed to write %s (did you set up udev rules?)", path)
+
+        path = os.path.join(battery_dir, self.battery, "charge_control_end_threshold")
+        try:
+            with open(path, "w+") as f:
+                f.write(str(end))
+        except FileNotFoundError:
+            self.charge_threshold_supported = False
+        except OSError:
+            self.charge_threshold_supported = False
+            logger.exception("Failed to write %s (did you set up udev rules?)", path)
+
     def update_status(self) -> BatteryStatus:
+        self.set_battery_charge_thresholds()
         stat = self._get_param("status_file")[0]
 
         if stat == "Full":
@@ -313,7 +380,67 @@ def update_status(self) -> BatteryStatus:
 
 
 class Battery(base.ThreadPoolText):
-    """A text-based battery monitoring widget currently supporting FreeBSD"""
+    """
+    A text-based battery monitoring widget supporting both Linux and FreeBSD.
+
+    The Linux version of this widget has functionality to charge "smartly"
+    (i.e. not to 100%) when the laptop is connected to a thunderbolt docking
+    station. To temporarily disable/re-enable this (e.g. if you know you're
+    going mobile and need to charge) use either:
+
+    .. code-block:: bash
+
+        qtile cmd-obj -o bar top widget battery -f charge_to_full
+        qtile cmd-obj -o bar top widget battery -f charge_dynamically
+
+    or bind a key to:
+
+    .. code-block:: python
+
+        Key([mod, "shift"], "c", lazy.widget['battery'].charge_to_full())
+        Key([mod, "shift"], "x", lazy.widget['battery'].charge_dynamically())
+
+    note that this functionality requires qtile to be able to write to certain
+    files in sysfs. The easiest way to persist this across reboots is via a
+    udev rule that sets g+w and ownership of the relevant files to the `sudo`
+    group, assuming the user qtile runs as is in that group.
+
+    This is slightly complicated, since the chage_control_{start,end}_threshold
+    files are not created by the device driver itself, but by the particular
+    ACPI module for your laptop. If we try to do the chown/chmod when the
+    device is added in udev, the files won't be present yet. So, we have to do
+    it when the ACPI module for the laptop is loaded.
+
+    For thinkpads, the udev rule looks like:
+
+    .. code-block:: bash
+
+        cat <<'EOF' | sudo tee /etc/udev/rules.d/99-qtile-battery.rules
+        ACTION=="add" KERNEL=="thinkpad_acpi" RUN+="/home/tycho/config/bin/qtile-battery"
+        EOF
+
+    and the qtile-battery script looks like:
+
+    .. code-block:: bash
+
+        #!/bin/bash -eu
+
+        GROUP=sudo
+        die() {
+            echo "$@"
+            exit 1
+        }
+
+        set_ownership() {
+            chgrp "$GROUP" $1 2>&1
+            chmod g+w $1
+        }
+
+        [ $# -eq 0 ] || die "Usage: $0"
+
+        set_ownership /sys/class/power_supply/BAT*/charge_control_end_threshold
+        set_ownership /sys/class/power_supply/BAT*/charge_control_start_threshold
+    """
 
     background: ColorsType | None
     low_background: ColorsType | None
@@ -359,6 +486,14 @@ def _configure(self, qtile, bar):
 
         base.ThreadPoolText._configure(self, qtile, bar)
 
+    @expose_command()
+    def charge_to_full(self):
+        self._battery.force_charge = True
+
+    @expose_command()
+    def charge_dynamically(self):
+        self._battery.force_charge = False
+
     @staticmethod
     def _load_battery(**config):
         """Function used to load the Battery object