linux-kernel - Re: [PATCH v1] usb: gadget: udc: core: Offload usb_udc_vbus

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Message-ID: <c7cb96ea-628d-4591-908c-d6ea572ef5a0@rowland.harvard.edu>
Date:   Wed, 17 May 2023 16:01:28 -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, peter.chen@...escale.com,
        balbi@...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,
        Francesco Dolcini <francesco.dolcini@...adex.com>
Subject: Re: [PATCH v1] usb: gadget: udc: core: Offload usb_udc_vbus_handler
 processing

On Wed, May 17, 2023 at 10:19:25AM -0700, Badhri Jagan Sridharan wrote:
> On Wed, May 17, 2023 at 7:44 AM Alan Stern <stern@...land.harvard.edu> wrote:
> >
> > On Wed, May 17, 2023 at 11:59:55AM +0000, Badhri Jagan Sridharan wrote:
> > > chipidea udc calls usb_udc_vbus_handler from udc_start gadget
> > > ops causing a deadlock. Avoid this by offloading usb_udc_vbus_handler
> > > processing.
> >
> > Surely that is the wrong approach.
> >
> > The real problem here is that usb_udc_vbus_handler() gets called from
> > within a udc_start routine.  But this is totally unnecessary, because
> > the UDC core will call usb_udc_connect_control_locked() itself, later on
> > during gadget_bind_driver().
> 
> Hi Alan,
> 
> usb_udc_vbus_handler sets the udc->vbus flag as well apart from
> calling usb_udc_connect_control_locked().  So, removing usb_udc_vbus_handler
> from chip specific start callback might prevent the controller from
> starting.
> 
> void usb_udc_vbus_handler(struct usb_gadget *gadget, bool status)
> {
> struct usb_udc *udc = gadget->udc;
> 
> mutex_lock(&udc->connect_lock);
> if (udc) {
> udc->vbus = status;
> usb_udc_connect_control_locked(udc);

Then add "udc->vbus = true;" at the appropriate spot in 
gadget_bind_driver().

Alan Stern

PS: I just noticed that in max3420_udc.c, the max_3420_vbus_handler() 
function calls usb_udc_vbus_handler() from within an interrupt handler.  
This won't work, since interrupt handlers aren't allowed to sleep and 
therefore can't lock mutexes.