Re: [PATCH net v1 1/1] net: usb: asix: forbid runtime PM to avoid PM/MDIO + RTNL deadlock

["Hubert Wiśniewski" <hubert.wisniewski.25632@gmail.com>]

On Wed Sep 17, 2025 at 3:54 PM CEST, Alan Stern wrote:
> 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)?

I have tested this. With this patch, it seems that writing "auto" to
power/control has no effect -- power/runtime_status remains "active" and
the device does not get suspended. But maybe there is a way to force the
suspension anyway?

> 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