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

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Date:   Wed, 11 Mar 2020 13:39:51 +0800
From:   Kai-Heng Feng <kai.heng.feng@...onical.com>
To:     Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:     "Shih-Yuan Lee (FourDollars)" <sylee@...onical.com>,
        Tiffany <tiffany.wang@...onical.com>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Thunderbolt, direct-complete and long suspend/resume time of
 Suspend-to-idle

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