linux-kernel - Re: [PATCH v2] usb: hub: Add delay for SuperSpeed hub resume to let links transit to U0

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Message-ID: <YbqmglqFbiTp4bkz@rowland.harvard.edu>
Date:   Wed, 15 Dec 2021 21:37:54 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Kai-Heng Feng <kai.heng.feng@...onical.com>
Cc:     gregkh@...uxfoundation.org, mathias.nyman@...ux.intel.com,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Chris Chiu <chris.chiu@...onical.com>,
        Bixuan Cui <cuibixuan@...wei.com>,
        Rajat Jain <rajatja@...gle.com>,
        Chunfeng Yun <chunfeng.yun@...iatek.com>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] usb: hub: Add delay for SuperSpeed hub resume to let
 links transit to U0

On Thu, Dec 16, 2021 at 08:23:29AM +0800, Kai-Heng Feng wrote:
> On Wed, Dec 15, 2021 at 11:47 PM Alan Stern <stern@...land.harvard.edu> wrote:
> >
> > On Wed, Dec 15, 2021 at 08:01:06PM +0800, Kai-Heng Feng wrote:

> > > --- a/drivers/usb/core/hub.c
> > > +++ b/drivers/usb/core/hub.c
> > > @@ -1110,7 +1110,10 @@ static void hub_activate(struct usb_hub *hub, enum hub_activation_type type)
> > >               } else {
> > >                       hub_power_on(hub, true);
> > >               }
> > > -     }
> > > +     /* Give some time on remote wakeup to let links to transit to U0 */
> > > +     } else if (hub_is_superspeed(hub->hdev))
> > > +             msleep(20);
> >
> > Where did the 20-ms number come from?  Is it mentioned in the spec as
> > the time required for a port to switch from U3 to U0?
> 
> The 20ms is the minimal time the device take to bring link back to U0 reliably.
> The spec mentioned device has to transit the link to U0 before
> signaling remote wakeup.
> So do you think this requires a new quirk?

No, I just wondered where you got the number from.  The patch looks fine; I 
can't think of any reasonable way to improve it.

Acked-by: Alan Stern <stern@...land.harvard.edu>

Alan Stern