Re: [PATCH] usb: dwc3: avoid probe deferral when USB power supply is not available

[Thinh Nguyen <Thinh.Nguyen@synopsys.com>]

On Tue, Apr 07, 2026, Elson Serrao wrote:
> The dwc3 driver currently defers probe if the USB power supply is not yet
> registered. On some platforms, even though charging and power supply
> functionality is available during normal operation, there may exist
> minimal booting modes (such as recovery or diagnostic environments) where
> the relevant USB power supply device is not registered. In such cases,
> probe deferral prevents USB gadget operation entirely.
> 
> USB data functionality for basic operation does not inherently depend on
> the power supply framework, which is only required for enforcing VBUS
> current control. The configured VBUS current limit is typically enforced
> through the charger or PMIC power path. When charging functionality is
> unavailable, applying a current limit has no practical effect, reducing
> the benefit of strict probe-time enforcement in these environments.
> 
> Instead of deferring probe, register a power supply notifier when the
> USB power supply is not yet available. Cache the requested VBUS current
> limit and apply it once the matching power supply becomes available, as
> notified through the registered callback.
> 

This gets a bit cumbersome now that we need to consider some corner
cases around the notifier callback.

> Signed-off-by: Elson Serrao <elson.serrao@oss.qualcomm.com>
> ---
>  drivers/usb/dwc3/core.c   | 82 ++++++++++++++++++++++++++++++++-------
>  drivers/usb/dwc3/core.h   |  4 ++
>  drivers/usb/dwc3/gadget.c | 10 ++++-
>  3 files changed, 80 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
> index 161a4d58b2ce..20df0b287623 100644
> --- a/drivers/usb/dwc3/core.c
> +++ b/drivers/usb/dwc3/core.c
> @@ -2167,24 +2167,72 @@ static void dwc3_vbus_draw_work(struct work_struct *work)
>  	if (ret < 0)
>  		dev_dbg(dwc->dev, "Error (%d) setting vbus draw (%d mA)\n",
>  			ret, dwc->current_limit);
> +
> +	/* Unregister the psy notifier now that we have the power_supply reference */
> +	if (dwc->psy_nb.notifier_call) {

Is it possible for dwc3_core_remove() to happen here? If so, should we
do something about it?

> +		power_supply_unreg_notifier(&dwc->psy_nb);
> +		dwc->psy_nb.notifier_call = NULL;
> +	}
>  }
>  
> -static struct power_supply *dwc3_get_usb_power_supply(struct dwc3 *dwc)
> +static int dwc3_psy_notifier(struct notifier_block *nb,
> +			     unsigned long event, void *data)
> +{
> +	struct dwc3 *dwc = container_of(nb, struct dwc3, psy_nb);
> +	struct power_supply *psy = data;
> +	unsigned long flags;
> +
> +	if (strcmp(psy->desc->name, dwc->usb_psy_name) != 0)
> +		return NOTIFY_DONE;
> +
> +	/* Explicitly get the reference for this psy */
> +	psy = power_supply_get_by_name(dwc->usb_psy_name);
> +	if (!psy)
> +		return NOTIFY_DONE;
> +
> +	spin_lock_irqsave(&dwc->lock, flags);
> +	/*
> +	 * The USB power_supply may already be set. This can happen if notifier
> +	 * callbacks for the USB power_supply race, or if a previous notifier
> +	 * callback has already successfully fetched and associated the instance.
> +	 * In such cases, release the newly acquired reference and ignore
> +	 * subsequent notifications until the notifier is unregistered.
> +	 */
> +	if (dwc->usb_psy) {
> +		spin_unlock_irqrestore(&dwc->lock, flags);
> +		power_supply_put(psy);
> +		return NOTIFY_DONE;
> +	}
> +
> +	dwc->usb_psy = psy;
> +	if (dwc->current_limit != UINT_MAX)
> +		schedule_work(&dwc->vbus_draw_work);
> +	spin_unlock_irqrestore(&dwc->lock, flags);
> +
> +	return NOTIFY_OK;
> +}
> +
> +static void dwc3_get_usb_power_supply(struct dwc3 *dwc)
>  {
> -	struct power_supply *usb_psy;
> -	const char *usb_psy_name;
>  	int ret;
>  
> -	ret = device_property_read_string(dwc->dev, "usb-psy-name", &usb_psy_name);
> +	ret = device_property_read_string(dwc->dev, "usb-psy-name", &dwc->usb_psy_name);
>  	if (ret < 0)
> -		return NULL;
> -
> -	usb_psy = power_supply_get_by_name(usb_psy_name);
> -	if (!usb_psy)
> -		return ERR_PTR(-EPROBE_DEFER);
> +		return;
>  
>  	INIT_WORK(&dwc->vbus_draw_work, dwc3_vbus_draw_work);
> -	return usb_psy;
> +
> +	dwc->usb_psy = power_supply_get_by_name(dwc->usb_psy_name);
> +	if (!dwc->usb_psy) {

Is it possible for the power supply to register here?

> +		dwc->current_limit = UINT_MAX;
> +		dwc->psy_nb.notifier_call = dwc3_psy_notifier;
> +		ret = power_supply_reg_notifier(&dwc->psy_nb);
> +		if (ret) {
> +			dev_err(dwc->dev, "Failed to register power supply notifier: %d\n", ret);
> +			dwc->psy_nb.notifier_call = NULL;
> +			return;
> +		}
> +	}
>  }
>  
>  int dwc3_core_probe(const struct dwc3_probe_data *data)
> @@ -2232,9 +2280,9 @@ int dwc3_core_probe(const struct dwc3_probe_data *data)
>  
>  	dwc3_get_software_properties(dwc, &data->properties);
>  
> -	dwc->usb_psy = dwc3_get_usb_power_supply(dwc);
> -	if (IS_ERR(dwc->usb_psy))
> -		return dev_err_probe(dev, PTR_ERR(dwc->usb_psy), "couldn't get usb power supply\n");
> +	spin_lock_init(&dwc->lock);
> +
> +	dwc3_get_usb_power_supply(dwc);
>  
>  	if (!data->ignore_clocks_and_resets) {
>  		dwc->reset = devm_reset_control_array_get_optional_shared(dev);
> @@ -2286,7 +2334,6 @@ int dwc3_core_probe(const struct dwc3_probe_data *data)
>  		dwc->num_usb3_ports = 1;
>  	}
>  
> -	spin_lock_init(&dwc->lock);
>  	mutex_init(&dwc->mutex);
>  
>  	pm_runtime_get_noresume(dev);
> @@ -2354,6 +2401,8 @@ int dwc3_core_probe(const struct dwc3_probe_data *data)
>  err_assert_reset:
>  	reset_control_assert(dwc->reset);
>  err_put_psy:
> +	if (dwc->psy_nb.notifier_call)
> +		power_supply_unreg_notifier(&dwc->psy_nb);
>  	if (dwc->usb_psy)
>  		power_supply_put(dwc->usb_psy);
>  
> @@ -2410,6 +2459,11 @@ void dwc3_core_remove(struct dwc3 *dwc)
>  
>  	dwc3_free_event_buffers(dwc);
>  
> +	if (dwc->psy_nb.notifier_call) {
> +		power_supply_unreg_notifier(&dwc->psy_nb);
> +		dwc->psy_nb.notifier_call = NULL;
> +	}
> +
>  	if (dwc->usb_psy) {
>  		cancel_work_sync(&dwc->vbus_draw_work);
>  		power_supply_put(dwc->usb_psy);
> diff --git a/drivers/usb/dwc3/core.h b/drivers/usb/dwc3/core.h
> index a35b3db1f9f3..68171629c7bf 100644
> --- a/drivers/usb/dwc3/core.h
> +++ b/drivers/usb/dwc3/core.h
> @@ -1058,6 +1058,8 @@ struct dwc3_glue_ops {
>   * @role_switch_default_mode: default operation mode of controller while
>   *			usb role is USB_ROLE_NONE.
>   * @usb_psy: pointer to power supply interface.
> + * @usb_psy_name: name of the USB power supply
> + * @psy_nb: power supply notifier block
>   * @vbus_draw_work: Work to set the vbus drawing limit
>   * @current_limit: How much current to draw from vbus, in milliAmperes.
>   * @usb2_phy: pointer to USB2 PHY
> @@ -1246,6 +1248,8 @@ struct dwc3 {
>  	enum usb_dr_mode	role_switch_default_mode;
>  
>  	struct power_supply	*usb_psy;
> +	const char		*usb_psy_name;
> +	struct notifier_block	psy_nb;
>  	struct work_struct	vbus_draw_work;
>  	unsigned int		current_limit;

Would be nice to group these in a struct, but it's fine as is for now.

>  
> diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
> index 0a688904ce8c..4717c251596d 100644
> --- a/drivers/usb/dwc3/gadget.c
> +++ b/drivers/usb/dwc3/gadget.c
> @@ -3124,15 +3124,21 @@ static void dwc3_gadget_set_ssp_rate(struct usb_gadget *g,
>  static int dwc3_gadget_vbus_draw(struct usb_gadget *g, unsigned int mA)
>  {
>  	struct dwc3		*dwc = gadget_to_dwc(g);
> +	unsigned long		flags;
>  
>  	if (dwc->usb2_phy)
>  		return usb_phy_set_power(dwc->usb2_phy, mA);
>  
> -	if (!dwc->usb_psy)
> +	spin_lock_irqsave(&dwc->lock, flags);
> +	dwc->current_limit = mA;
> +	if (!dwc->usb_psy) {
> +		spin_unlock_irqrestore(&dwc->lock, flags);
> +		dev_dbg(dwc->dev, "Stored VBUS draw: %u mA (power supply not ready)\n", mA);
>  		return -EOPNOTSUPP;
> +	}
>  
> -	dwc->current_limit = mA;
>  	schedule_work(&dwc->vbus_draw_work);
> +	spin_unlock_irqrestore(&dwc->lock, flags);
>  
>  	return 0;
>  }
> -- 
> 2.34.1
> 

So, now if vbus_draw() is called without power supply registered, the
composite framework will just report 0mA because we don't have the power
supply yet? Beside a couple of comments above, it looks fine to me.

Thanks,
Thinh