Re: [RFC PATCH v2] usb: core: add sysfs entry for usb device state

[Alan Stern]

On Wed, May 31, 2023 at 01:01:34AM +0000, Roy Luo wrote:
> Expose usb device state to userland as the information is useful in
> detecting non-compliant setups and diagnosing enumeration failures.
> For example:
> - End-to-end signal integrity issues: the device would fail port reset
>   repeatedly and thus be stuck in POWERED state.
> - Charge-only cables (missing D+/D- lines): the device would never enter
>   POWERED state as the HC would not see any pullup.
> 
> What's the status quo?
> We do have error logs such as "Cannot enable. Maybe the USB cable is bad?"
> to flag potential setup issues, but there's no good way to expose them to
> userspace.
> 
> Why add a sysfs entry in struct usb_port instead of struct usb_device?
> The struct usb_device is not device_add() to the system until it's in
> ADDRESS state hence we would miss the first two states. The struct
> usb_port is a better place to keep the information because its life
> cycle is longer than the struct usb_device that is attached to the port.
> 
> Signed-off-by: Roy Luo <royluo@xxxxxxxxxx>
> ---
> Changes since v1:
> * Address Greg Kroah-Hartman's comment: replace sysfs_notify with
>   sysfs_notify_dirent and remove the workqueue.
> * Address Alan Stern's comment: replicate port_dev->child->state
>   in port_dev->state instead of using a pointer to link them.
> ---

Reviewed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>