linux-kernel - Re: [PATCH] usb: dwc3: gadget: lower informal user notifaction dequeue operation

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Message-ID: <20230324172413.lpd3cces7byoe7tk@synopsys.com>
Date:   Fri, 24 Mar 2023 17:24:13 +0000
From:   Thinh Nguyen <Thinh.Nguyen@...opsys.com>
To:     Marco Felsch <m.felsch@...gutronix.de>
CC:     Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "balbi@...nel.org" <balbi@...nel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>
Subject: Re: [PATCH] usb: dwc3: gadget: lower informal user notifaction
 dequeue operation

On Fri, Mar 24, 2023, Marco Felsch wrote:
> Hi,
> 
> On 23-03-23, Thinh Nguyen wrote:
> > Hi,
> > 
> > On Thu, Mar 23, 2023, Marco Felsch wrote:
> > > Printing an error message during usb_ep_dequeue() is more confusing than
> > > helpful since the usb_ep_dequeue() could be call during unbind() just
> > > in case that everything is canceld before unbinding the driver. Lower
> > > the dev_err() message to dev_dbg() to keep the message for developers.
> > > 
> > > Fixes: fcd2def66392 ("usb: dwc3: gadget: Refactor dwc3_gadget_ep_dequeue")
> > > Signed-off-by: Marco Felsch <m.felsch@...gutronix.de>
> > > ---
> > >  drivers/usb/dwc3/gadget.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
> > > index 89dcfac01235f..6699db26cc7b5 100644
> > > --- a/drivers/usb/dwc3/gadget.c
> > > +++ b/drivers/usb/dwc3/gadget.c
> > > @@ -2106,7 +2106,7 @@ static int dwc3_gadget_ep_dequeue(struct usb_ep *ep,
> > >  		}
> > >  	}
> > >  
> > > -	dev_err(dwc->dev, "request %pK was not queued to %s\n",
> > > +	dev_dbg(dwc->dev, "request %pK was not queued to %s\n",
> > >  		request, ep->name);
> > >  	ret = -EINVAL;
> > >  out:
> > > -- 
> > > 2.30.2
> > > 
> > 
> > How were you able to reproduce this error message?
> 
> We use the driver within barebox where we do have support for fastboot.
> During the driver unbind usb_ep_dequeue() is called which throw this
> error.

I mean which gadget/function driver did you use.

> 
> > During unbind(), the function driver would typically call to
> > usb_ep_disable(). Before the call usb_ep_disable() completes, all queued
> > and incompleted requests are expected to be returned with -ESHUTDOWN.
> 
> So the unbind() function driver should use usb_ep_disable() instead of
> usb_ep_dequeue()?

No, it can do whatever it wants. I'm just pointing out the typical
behavior when this case happens during unbind().

> 
> > For you to see this error, this means that the function driver issued
> > usb_ep_dequeue() to an already disabled endpoint, and the request was
> > probably already given back.
> 
> The unbind() just calls usb_ep_dequeue() which isn't forbidden according
> the API doc. We just want to ensure that the request is cancled if any.

It's not forbidden, and it's not unexpected for this message to be
generated if usb_ep_dequeue() is called after usb_ep_disable(). However,
knowing the behavior of usb_ep_disable(), does it make sense to call
usb_ep_dequeue() after usb_ep_disable() completes? (I'm assuming this is
what happened in your case from the commit description).

> 
> > Even though this error message is not critical and shouldn't affect the
> > driver's behavior, it's better to fix the function driver to handle this
> > race.
> 
> As you have pointed out: 'it is not criticial' and therefore we shouldn't
> use dev_err() for non crictical information since this can cause
> user-space confusion.

I noted this particular case that it's not critical because we know
where/when it happened because you pointed out that it occurs during
unbind(). However, in any case, we want to notify that the
usb_ep_dequeue() was used on a wrong request, allowing the user to
review and fix this if needed.

Thanks,
Thinh