[PATCH 0/1] usb: dwc3: qcom: Keep wakeup option default off

[Peter Chen]

Current policy
--------------
When the controller node has the "wakeup-source" property, dwc3-qcom
calls device_init_wakeup(dev, true). USB is then a system wakeup
source by default. Users who do NOT want the system woken up by USB
have to disable it:

  echo disabled > /sys/devices/.../<dwc3 device>/power/wakeup

Proposed policy
---------------
"wakeup-source" only describes that the controller is capable of
waking up the system (see
Documentation/devicetree/bindings/power/wakeup-source.txt), so with
this patch the driver only marks the device as wakeup capable, and
power/wakeup starts out "disabled". Users who NEED USB as a wakeup
source have to enable it:

  echo enabled > /sys/devices/.../<dwc3 device>/power/wakeup

The motivation is:

- Users do not expect USB activity (connect/disconnect or USB line status
  changes) to wake up the system unless they asked for it.

- Power consumption. With wakeup enabled, host mode system suspend
  keeps PHYs powered (dwc3_core_exit() is skipped) and necessary logic
  for detecting wakeup must be on, so that the wakeup interrupts can
  fire. Today this extra power is spent by default, even on systems
  where USB is never used as a wakeup source and the controller could
  be fully powered off during suspend.

Comparison
----------
                       Current (default on)       Proposed (default off)
                       --------------------       ----------------------
Initial power/wakeup   enabled                    disabled
User action            write "disabled" if USB    write "enabled" if USB
                       wakeup is NOT wanted       wakeup IS wanted
Unexpected wakeups     possible (connect,         none unless the user
                       disconnect, noise)         opted in
Suspend power          higher, even if USB        lower, USB fully powered
                       wakeup is never used       off
Attached devices on    stay suspended, resume     reset-resumed or
resume                 quickly                    re-enumerated

Possible regression
-------------------
This is a user-visible change for existing platforms. Many Qualcomm
arm64 DTs set "wakeup-source" on the USB controller. With this patch:

- USB wakeup stops working until userspace (or a udev rule) writes
  "enabled" to the controller's power/wakeup.

- In host mode, because device_may_wakeup() is now false by default,
  system suspend takes the full power down path in
  dwc3_suspend_common() (dwc3_core_exit()). Platforms where this
  power down path has not been tested may fail to resume.

Peter Chen (1):
  usb: dwc3: qcom: Keep wakeup option default off

 drivers/usb/dwc3/dwc3-qcom.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

-- 
2.43.0