Comment 151 for bug 1730924

Hi everyone,

so Tsuchiya and I have been looking into this quite a lot for the last few weeks and we've identified several issues and tried to find the underlying reasons as far as possible.

We hope this information will be helpful for you in indentifying issues with the firmware and the driver so you don't have to take a shot in the dark. Because most issues and their reproducers are already extensively explained in this bugreport, we've listed what we suspect to be some of the root causes for those issues here:

1) The LTR (Latency Tolerance Reporting) messages sent by the firmware might be incorrect. With the newest firmware, the card always reports the maximum latency, which prevents the System from entering the lowest C-States (pc10 and slp_s0). I'm not sure whether that actually prevents ASPM L1.2 from working, because the command timeouts I see when ASPM L1.2 is enabled (other devices see them only with (Surface 3) L1 or (Surface Pro 6) L0s) are still happening.

2) Performing a PCI function level reset of the card with firmware redownloading only works when power-cycling the card by switching it to D3cold and then to D0 again, see [1], which implements this. As mentioned in that commit, this is a quirk that's also done by Windows and seems to be specific to only Surface devices.

3) For avoid a crash of the firmware when resuming from suspend, it's important to disable D3 for the PCI bridge, see [2]. This is another quirk that also seems to be applied by Windows.

4) There's also a bluetooth powersaving issue, which makes the bluetooth device never enter USB the suspend state as long as no LE device is connected: That's because (assuming the linux bluetooth stack works correctly) the firmware never stops sending interrupts with LE advertising reports, which can easily be confirmed by using the btmon utility and by disabling Bluetooth LE functionality in /etc/bluettoth/main.conf.

[1] https://github.com/kitakar5525/linux-surface-kernel/commit/6eba36073e85a64200f2c28e2a6ba0dbeb120bf2

[2] https://github.com/kitakar5525/linux-surface-kernel/commit/4606706679bf81ef41e701fc5e4d5b87adb983dd

Thank you!