On 2/3/2025 6:40 AM, Heikki Krogerus wrote:
> On Mon, Jan 27, 2025 at 03:07:15PM -0800, Elson Roy Serrao wrote:
>> The role switch registration and set_role() can happen in parallel as they
>> are invoked independent of each other. There is a possibility that a driver
>> might spend significant amount of time in usb_role_switch_register() API
>> due to the presence of time intensive operations like component_add()
>> which operate under common mutex. This leads to a time window after
>> allocating the switch and before setting the registered flag where the set
>> role notifications are dropped. Below timeline summarizes this behavior
>>
>> Thread1 | Thread2
>> usb_role_switch_register() |
>> | |
>> ---> allocate switch |
>> | |
>> ---> component_add() | usb_role_switch_set_role()
>> | | |
>> | | --> Drop role notifications
>> | | since sw->registered
>> | | flag is not set.
>> | |
>> --->Set registered flag.|
>>
>> To avoid this, cache the last role received and set it once the switch
>> registration is complete. Since we are now caching the roles based on
>> registered flag, protect this flag with the switch mutex.
>
> Instead, why not just mark the switch registered from the get-go?
>
> diff --git a/drivers/usb/roles/class.c b/drivers/usb/roles/class.c
> index c58a12c147f4..cf38be82d397 100644
> --- a/drivers/usb/roles/class.c
> +++ b/drivers/usb/roles/class.c
> @@ -387,6 +387,8 @@ usb_role_switch_register(struct device *parent,
> dev_set_name(&sw->dev, "%s-role-switch",
> desc->name ? desc->name : dev_name(parent));
>
> + sw->registered = true;
> +
> ret = device_register(&sw->dev);
> if (ret) {
> put_device(&sw->dev);
> @@ -399,8 +401,6 @@ usb_role_switch_register(struct device *parent,
> dev_warn(&sw->dev, "failed to add component\n");
> }
>
> - sw->registered = true;
> -
> /* TODO: Symlinks for the host port and the device controller. */
>
> return sw;
>
Thank you for the feedback.
Yes that works as well. Wasn't entirely sure if we can set that flag from the get-go before calling device_register().
But guess we can reset the flag if device_register fails since that is the only failure path in role_switch_register().
I will upload v2 with this modification.
Best Regards,
Elson
