I see this in powerd.LATEST after booting a ToT caroline device:

[0425/211435:INFO:udev.cc(392)] Adding device /sys/devices/pci0000:00/0000:00:15.0/i2c_designware.0/i2c-6/i2c-ATML0001:00/input/input7 with tags usable_when_laptop usable_when_tablet wakeup wakeup_disabled inhibit role_internal_touchscreen
[0425/211435:INFO:udev.cc(392)] Adding device /sys/devices/pci0000:00/0000:00:15.0/i2c_designware.0/i2c-6/i2c-ATML0001:00/input/input7/event7 with tags usable_when_laptop usable_when_tablet role_internal_touchscreen
...
[0425/211435:INFO:input_device_controller.cc(284)] Configuring devices for mode "laptop"
[0425/211435:INFO:input_device_controller.cc(215)] Un-inhibiting /sys/devices/pci0000:00/0000:00:15.0/i2c_designware.0/i2c-6/i2c-ATML0001:00/input/input7
...
[0425/211435:WARNING:input_device_controller.cc(199)] No power/wakeup sysattr available for /sys/devices/pci0000:00/0000:00:15.0/i2c_designware.0/i2c-6/i2c-ATML0001:00/input/input7

Is the warning message an issue? Here's the corresponding powerd code:
                                     
194 void InputDeviceController::SetWakeupFromS3(const system::TaggedDevice& device,
195                                             bool enabled) {
196   std::string parent_syspath;
197   if (!system::udev_util::FindWakeCapableParent(device.syspath(), udev_,
198                                                 &parent_syspath)) {
199     LOG(WARNING) << "No " << kPowerWakeup << " sysattr available for "
200                  << device.syspath();
201     return;
202   }
203   LOG(INFO) << (enabled ? "Enabling" : "Disabling") << " wakeup for "
204             << device.syspath() << " through " << parent_syspath;
205   udev_->SetSysattr(parent_syspath, kPowerWakeup,
206                     enabled ? kWakeupEnabled : kWakeupDisabled);
207 }

Despite this failing, the touchscreen doesn't wake from S3.

Is it safe for powerd to assume that if a device (and its ancestors) don't have a power/wakeup node, then the device won't wake from S3? If so, I can probably suppress this warning when |enabled| is false.