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

[Oleksij Rempel <o.rempel@pengutronix.de>]

Hi Oliver,

On Wed, Sep 17, 2025 at 12:10:48PM +0200, Oliver Neukum wrote:
> Hi,
> 
> On 17.09.25 11:54, Oleksij Rempel wrote:
> 
> > 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 reasoning depends on netif_running() returning false during system resume.
> Is that guaranteed?

You’re right - there is no guarantee that netif_running() is false
during system resume. This change does not rely on that. If my wording
suggested otherwise, I’ll reword the commit message to make it explicit.

1) Runtime PM (autosuspend/autoresume)

Typical chain when user does ip link set dev <if> up while autosuspended:
rtnl_newlink (RTNL held)
  -> __dev_open -> usbnet_open
     -> usb_autopm_get_interface -> __pm_runtime_resume
        -> usb_resume_interface -> asix_resume

Here resume happens synchronously under RTNL (and with USB PM locking). If the
driver then calls phylink/phylib from resume (caller must hold RTNL; MDIO I/O),
we can deadlock or hit PM-lock vs MDIO wake issues.

Patch effect:
I forbid runtime PM per-interface in ax88772_bind(). This removes the
synchronous autoresume path.

2) System suspend/resume

Typical chain:
... dpm_run_callback (workqueue)
 -> usb_resume_interface -> asix_resume

This is not under RTNL, and no pm_runtime locking is involved. The patch does
not change this path and makes no assumption about netif_running() here.

If helpful, I can rework the commit message.

Best Regards,
Oleksij
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