Re: [RFC PATCH v2] USB: core: emit a device-level modalias

[Alan Stern <stern@rowland.harvard.edu>]

On Wed, Oct 07, 2026 at 09:11:30PM +0200, Giuseppe Piscitelli wrote:
> On the first insertion of an iPhone (05ac:12a8) after boot, an already
> running usbmuxd can select USB configuration 4 before the charging driver
> apple-mfi-fastcharge loads. usbmuxd is a userspace daemon that uses libusb
> to communicate with Apple devices; apple-mfi-fastcharge is a USB device
> driver that enables their higher charging current.
> 
> USB device uevents have no MODALIAS. The charging module instead loads
> from an interface uevent. Its registration reprobes the device, unbinds
> the generic driver and calls usb_set_configuration(udev, -1), tearing down
> the interfaces and interrupting usbmuxd's transfers.
> 
> Emit a modalias in the USB device uevent, using wildcards for interface
> fields because no particular interface is described by this event. This
> allows udev's kmod builtin to load matching device drivers while handling
> the device add event, before broadcasting that event to libudev listeners.
> libusb's udev hotplug backend listens for this broadcast, so usbmuxd sees
> the arrival after the charging module has finished loading and reprobed
> the device, rather than before a later interface event loads it.
> 
> A slow udev worker delays both module loading and the broadcast; it does
> not reverse this ordering. This addresses hotplug through the udev
> backend, not clients reading raw kernel events or scanning devices before
> udev has finished, and does not synchronize later manual module loading.
> 
> Signed-off-by: Giuseppe Piscitelli <ooonea@gmail.com>
> Assisted-by: OpenAI Codex
> ---
> v2:
> - Explain the iPhone, charging driver and usbmuxd interaction.
> - Describe synchronous module loading before the libudev broadcast,
>   including the ordering when udev runs slowly and its scope limits.
> - No code changes.

Much better, thank you.

Alan Stern