power: Describe changing power source in power_supplies.md.
Update the Power Supplies document to describe how Chrome
instructs powerd to switch to a different power source.
BUG=b:65591971
TEST=none
Change-Id: I948033d54c1586583da459e70e6fb62571dd8587
Reviewed-on: https://chromium-review.googlesource.com/1187720
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Benson Leung <bleung@chromium.org>
diff --git a/power_manager/docs/power_supplies.md b/power_manager/docs/power_supplies.md
index f816c57..8262063 100644
--- a/power_manager/docs/power_supplies.md
+++ b/power_manager/docs/power_supplies.md
@@ -16,6 +16,9 @@
* Resume (after suspending)
* [udev] events concerning the `power_supply` subsystem
+The information is copied to [PowerSupplyProperties] protocol buffers that are
+included in `PowerSupplyPoll` D-Bus signals.
+
## Line Power
"Line power" refers to chargers or other external power sources; it's sometimes
@@ -53,9 +56,17 @@
If multiple line power directories are found, powerd will report all of them.
This typically happens in the case of Chrome OS devices with multiple USB Type-C
-ports. Only one power source may deliver power at a given time; Chrome reports
-the active source at `chrome://settings/power`. If multiple sources are
-available, the user may use this page to select which one to use.
+ports.
+
+Only one power source may deliver power at a given time; Chrome displays the
+active source at `chrome://settings/power`. If multiple sources are available,
+the user may use this page to select which one to use. Chrome makes a
+`SetPowerSource` D-Bus method call to powerd containing the ID of the desired
+source (taken from an earlier [PowerSupplyProperties] protobuf), and
+`PowerSupply::SetPowerSource` writes `0` to the device's
+`charge_control_limit_max` sysfs node. Chrome can also pass an empty ID to
+switch to battery power; powerd writes `-1` to the active power source's node in
+this case.
## Batteries
@@ -112,4 +123,5 @@
[sysfs]: https://en.wikipedia.org/wiki/Sysfs
[kernel documentation]: https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-class-power
[udev]: https://en.wikipedia.org/wiki/Udev
+[PowerSupplyProperties]: https://chromium.googlesource.com/chromiumos/platform/system_api/+/master/dbus/power_manager/power_supply_properties.proto
[Battery Notifications]: battery_notifications.md
