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

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Date:   Thu, 18 May 2023 21:29:45 -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 v1] usb: gadget: udc: core: Offload usb_udc_vbus_handler processing

On Wed, May 17, 2023 at 1:01 PM Alan Stern <stern@...land.harvard.edu> wrote:
>
> 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().


Not sure if I am misunderstanding something.
"udc->vbus = true" is set by usb_udc_vbus_handler based on invocation
from the chip level gadget driver and gadget_bind_driver() does not
seem to have the context for udc->vbus.
Do you still think it makes sense to add  "udc->vbus = true;" to
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.


Good point ! I didn't notice  that usb_udc_vbus_handler()  is invoked
from interrupt context as well.
I was looking at turning connect_lock into a spin lock. But looks like
udc_lock which is acquired
in usb_gadget_disconnect_locked is a mutex, So keeping connect_lock as
mutex and changing
vbus_events_lock into spin_lock is what that seems to be possible.
Sending out V2 of this patch
with these changes so that it's easier to see what I am referring to.
Eager to know your thoughts !

Thanks,
Badhri