On Wed, Sep 17, 2025 at 11:54:57AM +0200, Oleksij Rempel wrote:
Forbid USB runtime PM (autosuspend) for AX88772* in bind.
usbnet enables runtime PM by default in probe, so disabling it via the usb_driver flag is ineffective. For AX88772B, autosuspend shows no measurable power saving in my tests (no link partner, admin up/down). The ~0.453 W -> ~0.248 W reduction on 6.1 comes from phylib powering the PHY off on admin-down, not from USB autosuspend.
With autosuspend active, resume paths may require calling phylink/phylib (caller must hold RTNL) and doing MDIO I/O. Taking RTNL from a USB PM resume can deadlock (RTNL may already be held), and MDIO can attempt a runtime-wake while the USB PM lock is held. Given the lack of benefit and poor test coverage (autosuspend is usually disabled by default in distros), forbid runtime PM here to avoid these hazards.
This affects only AX88772* devices (per-interface in bind). System sleep/resume is unchanged.
@@ -919,6 +935,16 @@ static int ax88772_bind(struct usbnet *dev, struct usb_interface *intf) if (ret) goto initphy_err;
- /* Disable USB runtime PM (autosuspend) for this interface.
* Rationale:* - No measurable power saving from autosuspend for this device.* - phylink/phylib calls require caller-held RTNL and do MDIO I/O,* which is unsafe from USB PM resume paths (possible RTNL already* held, USB PM lock held).* System suspend/resume is unaffected.*/- pm_runtime_forbid(&intf->dev);
Are you aware that the action of pm_runtime_forbid() can be reversed by the user (by writing "auto" to the .../power/control sysfs file)?
To prevent the user from re-enabling runtime PM, you should call pm_runtime_get_noresume() (and then of course pm_runtime_put() or equivalent while unbinding).
Alan Stern