linux-kernel - Re: [PATCH v2] usb: gadget: udc: core: Offload usb_udc_vbus

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Date:   Fri, 19 May 2023 08:44:57 -0700
From:   Badhri Jagan Sridharan <badhri@...gle.com>
To:     Alan Stern <stern@...land.harvard.edu>
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 v2] usb: gadget: udc: core: Offload usb_udc_vbus_handler processing

On Fri, May 19, 2023 at 8:07 AM Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Fri, May 19, 2023 at 10:49:49AM -0400, Alan Stern wrote:
> > On Fri, May 19, 2023 at 04:30:41AM +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.
> >
> > Look, this is way overkill.
> >
> > usb_udc_vbus_handler() has only two jobs to do: set udc->vbus and call
> > usb_udc_connect_control().  Furthermore, it gets called from only two
> > drivers: chipidea and max3420.
> >
> > Why not have the callers set udc->vbus themselves and then call
> > usb_gadget_{dis}connect() directly?  Then we could eliminate
> > usb_udc_vbus_handler() entirely.  And the unnecessary calls -- the ones
> > causing deadlocks -- from within udc_start() and udc_stop() handlers can
> > be removed with no further consequence.
> >
> > This approach simplifies and removes code.  Whereas your approach
> > complicates and adds code for no good reason.
>
> I changed my mind.
>
> After looking more closely, I found the comment in gadget.h about
> ->disconnect() callbacks happening in interrupt context.  This means we
> cannot use a mutex to protect the associated state, and therefore the
> connect_lock _must_ be a spinlock, not a mutex.

Quick observation so that I don't misunderstand.
I already see gadget->udc->driver->disconnect(gadget) being called with
udc_lock being held.

               mutex_lock(&udc_lock);
               if (gadget->udc->driver)
                       gadget->udc->driver->disconnect(gadget);
               mutex_unlock(&udc_lock);

The below patch seems to have introduced it:
1016fc0c096c USB: gadget: Fix obscure lockdep violation for udc_mutex

Are you referring to some other ->disconnect() callback ? If so, can you point
me to which one ?

>
> This also probably means that udc_start and udc_stop callbacks should
> not be invoked with the lock held.  In fact, you might want to avoid
> using the lock at all with gadget_bind_driver() and
> gadget_unbind_driver() -- use it only in the functions that these
> routines call.
>
> So it appears the whole connect_lock thing needs to be redesigned with
> these ideas in mind.  However, it's still true that the UDC drivers
> shouldn't try to set the connection state from within their udc_start
> and udc_stop callbacks, because the core takes care of this
> automatically.
>
> Alan Stern

Thanks for your inputs !
Badhri