Thunderbolt, direct-complete and long suspend/resume time of Suspend-to-idle

[Kai-Heng Feng]

Hi,

I am currently investigating long suspend and resume time of suspend-to-idle.
It's because Thunderbolt bridges need to wait for 1100ms [1] for runtime-resume on system suspend, and also for system resume.

I made a quick hack to the USB driver and xHCI driver to support direct-complete, but I failed to do so for the parent PCIe bridge as it always disables the direct-complete [2], since device_may_wakeup() returns true for the device:

	/* Avoid direct_complete to let wakeup_path propagate. */
		if (device_may_wakeup(dev) || dev->power.wakeup_path)
			dev->power.direct_complete = false;

Once the direct-complete is disabled, system suspend/resume is used hence the delay in [1] is making the resume really slow. 
So how do we make suspend-to-idle faster? I have some ideas but I am not sure if they are feasible:
- Make PM core know the runtime_suspend() already use the same wakeup as suspend(), so it doesn't need to use device_may_wakeup() check to determine direct-complete.
- Remove the DPM_FLAG_NEVER_SKIP flag in pcieport driver, and use pm_request_resume() in its complete() callback to prevent blocking the resume process.
- Reduce the 1100ms delay. Maybe someone knows the values used in macOS and Windows...

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/pci.c#n4621
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/base/power/main.c#n1748

Kai-Heng