linux-kernel - Re: [PATCH v6 1/2] usb: gadget: udc: core: Offload usb_udc_vbus

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Message-ID: <65faa454-c822-4163-be3d-940fb4a647c7@rowland.harvard.edu>
Date:   Wed, 7 Jun 2023 13:32:55 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Badhri Jagan Sridharan <badhri@...gle.com>
Cc:     gregkh@...uxfoundation.org, colin.i.king@...il.com,
        xuetao09@...wei.com, quic_eserrao@...cinc.com,
        water.zhangjiantao@...wei.com, francesco@...cini.it,
        alistair@...stair23.me, stephan@...hold.net, bagasdotme@...il.com,
        luca@...tu.xyz, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v6 1/2] usb: gadget: udc: core: Offload
 usb_udc_vbus_handler processing

On Thu, Jun 01, 2023 at 03:10:27AM +0000, Badhri Jagan Sridharan wrote:
> usb_udc_vbus_handler() can be invoked from interrupt context by irq
> handlers of the gadget drivers, however, usb_udc_connect_control() has
> to run in non-atomic context due to the following:
> a. Some of the gadget driver implementations expect the ->pullup
>    callback to be invoked in non-atomic context.
> b. usb_gadget_disconnect() acquires udc_lock which is a mutex.
> 
> Hence offload invocation of usb_udc_connect_control()
> to workqueue.
> 
> UDC should not be pulled up unless gadget driver is bound. The new flag
> "allow_connect" is now set by gadget_bind_driver() and cleared by
> gadget_unbind_driver(). This prevents work item to pull up the gadget
> even if queued when the gadget driver is already unbound.
> 
> Cc: stable@...r.kernel.org
> Fixes: 1016fc0c096c ("USB: gadget: Fix obscure lockdep violation for udc_mutex")
> Signed-off-by: Badhri Jagan Sridharan <badhri@...gle.com>
> ---
> Changes since v1:
> - Address Alan Stern's comment on usb_udc_vbus_handler invocation from
>   atomic context:
> * vbus_events_lock is now a spinlock and allocations in
> * usb_udc_vbus_handler are atomic now.
> 
> Changes since v2:
> - Addressing Alan Stern's comments:
> ** connect_lock is now held by callers of
> * usb_gadget_pullup_update_locked() and gadget_(un)bind_driver() does
> * notdirectly hold the lock.
> 
> ** Both usb_gadget_(dis)connect() and usb_udc_vbus_handler() would
> * set/clear udc->vbus and invoke usb_gadget_pullup_update_locked.
> 
> ** Add "unbinding" to prevent new connections after the gadget is being
> * unbound.
> 
> Changes since v3:
> ** Made a minor cleanup which I missed to do in v3 in
> * usb_udc_vbus_handler().
> 
> Changes since v4:
> - Addressing Alan Stern's comments:
> ** usb_udc_vbus_handler() now offloads invocation of usb_udc_connect_control()
> * from workqueue.
> 
> ** Dropped vbus_events list as this was redundant. Updating to the
> * latest value is suffice
> 
> Changes since v5:
> - Addressing Alan Stern's comments:
> ** Squashed allow_connect logic to this patch.
> ** Fixed comment length to wrap at 76
> ** Cancelling vbus_work in del_gadget()

Reviewed-by: Alan Stern <stern@...land.harvard.edu>