Re: [PATCH v2 1/3] driver core: add device_enumeration_failure_notify() helper

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Dec 24, 2025 at 11:58:06AM +0000, Akshay Gujar wrote:
> Hotpluggable buses can detect that a device is physically present, but
> enumeration may still fail early due to protocol-level errors. Today,
> such failures are only reported via kernel log messages, with no
> structured userspace notification.
> 
> Introduce device_enumeration_failure_notify(), a generic helper that
> emits a KOBJ_CHANGE uevent containing:
> 
>     DEVICE_ENUMERATION_FAILURE=<identifier>
> 
> The <identifier> string is provided by the bus layer and identifies the
> failing port or device instance in a bus-defined format.
> 
> This allows userspace to correlate repeated enumeration failures with
> specific ports or connectors without relying solely on kernel logs.
> 
> Signed-off-by: Akshay Gujar <Akshay.Gujar@harman.com>
> ---
>  drivers/base/core.c    | 30 ++++++++++++++++++++++++++++++
>  include/linux/device.h | 12 ++++++++++++
>  2 files changed, 42 insertions(+)
> 
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 40de2f51a1b1..4c70d9a6dc69 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -3747,6 +3747,36 @@ int device_add(struct device *dev)
>  }
>  EXPORT_SYMBOL_GPL(device_add);
>  
> +/**
> + * device_enumeration_failure_notify - send uevent for enumeration failure
> + * @parent: the device to send the uevent from
> + * @id_name: textual identifier for the failing device
> + *
> + * Emits a KOBJ_CHANGE uevent with:
> + *
> + *    DEVICE_ENUMERATION_FAILURE=<id_name>
> + *
> + * Buses such as USB/PCI may use this helper when hardware is detected
> + * but enumeration cannot proceed.
> + */
> +void device_enumeration_failure_notify(struct device *parent, const char *id_name)
> +{
> +	char *envp[2] = { NULL, NULL };
> +
> +	if (!parent || !id_name)
> +		return;
> +
> +	envp[0] = kasprintf(GFP_KERNEL,
> +			    "DEVICE_ENUMERATION_FAILURE=%s",
> +			    id_name);
> +	if (!envp[0])
> +		return;
> +
> +	kobject_uevent_env(&parent->kobj, KOBJ_CHANGE, envp);
> +	kfree(envp[0]);
> +}
> +EXPORT_SYMBOL_GPL(device_enumeration_failure_notify);
> +
>  /**
>   * device_register - register a device with the system.
>   * @dev: pointer to the device structure
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 0be95294b6e6..dedc5e9e0ade 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -1197,4 +1197,16 @@ static inline bool device_link_test(const struct device_link *link, u32 flags)
>  #define MODULE_ALIAS_CHARDEV_MAJOR(major) \
>  	MODULE_ALIAS("char-major-" __stringify(major) "-*")
>  
> +/**
> + * device_enumeration_failure_notify - notify userspace about enumeration failure
> + * @parent: device to emit the uevent from

Why is this called "parent"?  SHouldn't this just be the device that
caused the failure?

> + * @id_name: textual identifier for the failed endpoint/device instance

Any hints on what this is going to contain?  Should this be documented
somewhere?

thanks,

greg k-h