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References: <20220826075839.292615-1-raychi@google.com> <Ywje7UCqXridmRpw@rowland.harvard.edu>
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From:   Ray Chi <raychi@google.com>
Date:   Sat, 27 Aug 2022 01:53:45 +0800
Message-ID: <CAPBYUsBBGTdLcBoRRup14hZDQYNbpRqu8Az7ZCVxP7WmctySBg@mail.gmail.com>
Subject: Re: [PATCH] usb: core: stop USB enumeration if too many retries
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        mathias.nyman@linux.intel.com, linux-usb@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Albert Wang <albertccwang@google.com>,
        Badhri Jagan Sridharan <badhri@google.com>,
        Puma Hsu <pumahsu@google.com>
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On Fri, Aug 26, 2022 at 10:55 PM Alan Stern <stern@rowland.harvard.edu> wrote:
>
> On Fri, Aug 26, 2022 at 03:58:39PM +0800, Ray Chi wrote:
> > If a broken accessory connected to a USB host, usbcore might
> > keep doing enumeration retries and it will take a long time to
> > cause system unstable.
> >
> > This patch provides a quirk to specific USB ports of the hub to
> > stop USB enumeration if needed.
>
> Why only to specific ports?

The specific port means it is connected to a broken accessory.

>
> > Signed-off-by: Ray Chi <raychi@google.com>
> > ---
> >  drivers/usb/core/hub.c | 33 +++++++++++++++++++++++++++++++++
> >  include/linux/usb.h    |  3 +++
> >  2 files changed, 36 insertions(+)
> >
> > diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
> > index 2633acde7ac1..0f4097440ffb 100644
> > --- a/drivers/usb/core/hub.c
> > +++ b/drivers/usb/core/hub.c
> > @@ -3081,6 +3081,28 @@ static int hub_port_reset(struct usb_hub *hub, int port1,
> >       return status;
> >  }
> >
> > +/* Stop enumerate if the port met errors and quirk is set */
> > +static bool hub_port_stop_enumerate(struct usb_hub *hub, int port1, int retries)
> > +{
> > +     struct usb_port *port_dev = hub->ports[port1 - 1];
> > +     struct usb_device *hdev = hub->hdev;
> > +
> > +     if (retries < (PORT_INIT_TRIES - 1) / 2)
> > +             return false;
> > +
> > +     /*
> > +      * Some USB hosts can't take a long time to keep doing enumeration
> > +      * retry. After doing half of the retries, we would turn off the port
> > +      * power to stop enumeration if the quirk is set.
> > +      */
> > +     if (port_dev->quirks & USB_PORT_QUIRK_STOP_ENUM) {
> > +             usb_hub_set_port_power(hdev, hub, port1, false);
>
> Why turn the port power off?  Aren't there better ways to stop the
> enumeration attempts?  When will the power ever get turned back on?
>

Because the broken accessory is still connected to the port. Even if we stop
the enumeration attempts, the port change event will issue a new port
initialization.

The implementation is used for dual-role devices, the port power could
be turned on
when the host mode restarts again.

> Why not use the initial_descriptor_timeout module parameter for this
> purpose?  That's the sort of thing it was meant for.
>

As I mentioned above, the usbcore driver will keep doing enumeration attempts if
the broken accessory is still connected. It never stops. This is why I
want to turn
off the port to stop enumeration.

> Alan Stern

Thanks,
Ray