Prevent USB runtime PM (autosuspend) for AX88772* in bind.

usbnet enables runtime PM (autosuspend) by default, so disabling it via the usb_driver flag is ineffective. On AX88772B, autosuspend shows no measurable power saving with current driver (no link partner, admin up/down). The ~0.453 W -> ~0.248 W drop on v6.1 comes from phylib powering the PHY off on admin-down, not from USB autosuspend.

The real hazard is that with runtime PM enabled, ndo_open() (under RTNL) may synchronously trigger autoresume (usb_autopm_get_interface()) into asix_resume() while the USB PM lock is held. Resume paths then invoke phylink/phylib and MDIO, which also expect RTNL, leading to possible deadlocks or PM lock vs MDIO wake issues.

To avoid this, keep the device runtime-PM active by taking a usage reference in ax88772_bind() and dropping it in unbind(). A non-zero PM usage count blocks runtime suspend regardless of userspace policy (.../power/control - pm_runtime_allow/forbid), making this approach robust against sysfs overrides.

Holding a runtime-PM usage ref does not affect system-wide suspend; system sleep/resume callbacks continue to run as before.

Fixes: 4a2c7217cd5a ("net: usb: asix: ax88772: manage PHY PM from MAC") Reported-by: Hubert Wiśniewski hubert.wisniewski.25632@gmail.com Closes: https://lore.kernel.org/all/DCGHG5UJT9G3.2K1GHFZ3H87T0@gmail.com Tested-by: Hubert Wiśniewski hubert.wisniewski.25632@gmail.com Reported-by: Marek Szyprowski m.szyprowski@samsung.com Closes: https://lore.kernel.org/all/b5ea8296-f981-445d-a09a-2f389d7f6fdd@samsung.com Cc: stable@vger.kernel.org Signed-off-by: Oleksij Rempel o.rempel@pengutronix.de --- changes v3 - update comments and commit message changes v2: - Switch from pm_runtime_forbid()/allow() to pm_runtime_get_noresume()/put() as suggested by Alan Stern, to block autosuspend robustly. - Reword commit message to clarify the actual deadlock condition (autoresume under RTNL) as pointed out by Oliver Neukum. - Keep explanation in commit message, shorten in-code comment.

Link to the measurement results: https://lore.kernel.org/all/aMkPMa650kfKfmF4@pengutronix.de/ --- drivers/net/usb/asix_devices.c | 29 +++++++++++++++++++++++++++++ 1 file changed, 29 insertions(+)

diff --git a/drivers/net/usb/asix_devices.c b/drivers/net/usb/asix_devices.c index 792ddda1ad49..85bd5d845409 100644 --- a/drivers/net/usb/asix_devices.c +++ b/drivers/net/usb/asix_devices.c @@ -625,6 +625,21 @@ static void ax88772_suspend(struct usbnet *dev) asix_read_medium_status(dev, 1)); }

+/* Notes on PM callbacks and locking context: + * + * - asix_suspend()/asix_resume() are invoked for both runtime PM and + * system-wide suspend/resume. For struct usb_driver the ->resume() + * callback does not receive pm_message_t, so the resume type cannot + * be distinguished here. + * + * - The MAC driver must hold RTNL when calling phylink interfaces such as + * phylink_suspend()/resume(). Those calls will also perform MDIO I/O. + * + * - Taking RTNL and doing MDIO from a runtime-PM resume callback (while + * the USB PM lock is held) is fragile. Since autosuspend brings no + * measurable power saving here, we block it by holding a PM usage + * reference in ax88772_bind(). + */ static int asix_suspend(struct usb_interface *intf, pm_message_t message) { struct usbnet *dev = usb_get_intfdata(intf); @@ -919,6 +934,13 @@ static int ax88772_bind(struct usbnet *dev, struct usb_interface *intf) if (ret) goto initphy_err;

+ /* Keep this interface runtime-PM active by taking a usage ref. + * Prevents runtime suspend while bound and avoids resume paths + * that could deadlock (autoresume under RTNL while USB PM lock + * is held, phylink/MDIO wants RTNL). + */ + pm_runtime_get_noresume(&intf->dev); + return 0;

initphy_err: @@ -948,6 +970,8 @@ static void ax88772_unbind(struct usbnet *dev, struct usb_interface *intf) phylink_destroy(priv->phylink); ax88772_mdio_unregister(priv); asix_rx_fixup_common_free(dev->driver_priv); + /* Drop the PM usage ref taken in bind() */ + pm_runtime_put(&intf->dev); }

static void ax88178_unbind(struct usbnet *dev, struct usb_interface *intf) @@ -1600,6 +1624,11 @@ static struct usb_driver asix_driver = { .resume = asix_resume, .reset_resume = asix_resume, .disconnect = usbnet_disconnect, + /* usbnet enables autosuspend by default (supports_autosuspend=1). + * We keep runtime-PM active for AX88772* by taking a PM usage + * reference in ax88772_bind() (pm_runtime_get_noresume()) and + * dropping it in unbind(), which effectively blocks autosuspend. + */ .supports_autosuspend = 1, .disable_hub_initiated_lpm = 1, };