Re: [PATCH v3 2/2] usb: core: add a pre_probe callback for device drivers that may defer

[James Clark]

On Tue, 03 Nov 2026 03:51:41 +0000, sashiko-bot@xxxxxxxxxx wrote:
> First, when udriver->pre_probe(udev) returns -ENODEV (for example from
> onboard_dev_usbdev_pre_probe() -> _find_onboard_dev()), usb_probe_device()
> still calls usb_autoresume_device(udev) and usb_generic_driver_probe(udev)
> to configure the device, bind all child interface drivers (triggering
> deferred probing), and emit USB_DEVICE_ADD, only to immediately tear them
> all down a few lines later in the error == -ENODEV block via
> usb_generic_driver_disconnect(udev) and usb_autosuspend_device(udev).

This is deliberate, and documented in the @pre_probe kerneldoc. An
error from pre_probe() other than -EPROBE_DEFER is handled as if
probe() had returned it, after the generic probe has run. This keeps
the -ENODEV fallback to the generic driver working as it does for
probe(). For onboard_usb_dev nothing changes: its -ENODEV used to come
from probe().

> Second, if udriver->pre_probe(udev) or udriver->probe(udev) fails with any
> other error (such as -EINVAL or -ENOMEM), usb_probe_device() jumps to
> err_pre_probe, which only calls udriver->disconnect(udev) and does not call
> usb_generic_driver_disconnect(udev) or usb_autosuspend_device(udev) to
> unwind the generic probe and runtime PM state. Because really_probe()
> clears udev->dev.driver to NULL when usb_probe_device() fails:
>
> - When udev is later unplugged, usb_disconnect() -> device_del() skips
>   usb_unbind_device() because udev->dev.driver is NULL, so
>   usb_notify_remove_device() (USB_DEVICE_REMOVE) is never sent. Subscribers
>   such as usbport_trig_notify() in drivers/usb/core/ledtrig-usbport.c never
>   remove the port entries added on USB_DEVICE_ADD, leaking struct
>   usbport_trig_port objects and sysfs attributes with a dangling port->hub
>   pointer to the freed struct usb_device, and usbdev_notify() in
>   drivers/usb/core/devio.c never runs usbdev_remove().

This was raised in Sashiko's review of v2. I missed that it can be fixed
when the device is unplugged. I will attempt to fix this in v4. I think
it can be fixed by recording in struct usb_device that USB_DEVICE_ADD
has been sent, and having usb_disconnect() send USB_DEVICE_REMOVE for a
device that has no driver.

> - For drivers with supports_autosuspend = 1 (such as
>   onboard_dev_usbdev_driver), usb_probe_device() does not take a runtime PM
>   reference, and hub_configure() enables autosuspend on hub devices; once
>   the device suspends, usb_resume_device() fails with -ENOTCONN because
>   udev->dev.driver is NULL.

This is a pre-existing bug that is new in this review.
usb_suspend_device() does a generic suspend for a device without a
driver, but usb_resume_device() refuses to resume one. I don't know
why resume is refused. If a generic resume is safe there, that would
fix it.

> Third, if udriver->pre_probe(udev) succeeds and udriver->probe(udev) then
> returns -ENODEV, the error == -ENODEV block runs
> usb_generic_driver_disconnect(udev) and usb_autosuspend_device(udev) before
> err_pre_probe calls udriver->disconnect(udev), reversing the teardown order
> used in usb_unbind_device():

I will fix this in v4.

James