linux-kernel - RE: [PATCH] usb: hub: try old enumeration scheme first for high speed devices

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Date:   Tue, 14 Aug 2018 10:39:53 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Zengtao (B)" <prime.zeng@...ilicon.com>
cc:     Roger Quadros <rogerq@...com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "mathias.nyman@...ux.intel.com" <mathias.nyman@...ux.intel.com>,
        "drinkcat@...omium.org" <drinkcat@...omium.org>,
        "felipe.balbi@...ux.intel.com" <felipe.balbi@...ux.intel.com>,
        "drake@...lessm.com" <drake@...lessm.com>,
        "mike.looijmans@...ic.nl" <mike.looijmans@...ic.nl>,
        "joe@...ches.com" <joe@...ches.com>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] usb: hub: try old enumeration scheme first for high
 speed devices

On Tue, 14 Aug 2018, Zengtao (B) wrote:

> Hi alan: 
> 
> >-----Original Message-----
> >From: linux-usb-owner@...r.kernel.org
> >[mailto:linux-usb-owner@...r.kernel.org] On Behalf Of Alan Stern
> >Sent: Friday, August 10, 2018 10:20 PM
> >To: Roger Quadros <rogerq@...com>
> >Cc: Zengtao (B) <prime.zeng@...ilicon.com>; gregkh@...uxfoundation.org;
> >mathias.nyman@...ux.intel.com; drinkcat@...omium.org;
> >felipe.balbi@...ux.intel.com; drake@...lessm.com; mike.looijmans@...ic.nl;
> >joe@...ches.com; linux-usb@...r.kernel.org; linux-kernel@...r.kernel.org
> >Subject: Re: [PATCH] usb: hub: try old enumeration scheme first for high speed
> >devices
> >
> >On Fri, 10 Aug 2018, Roger Quadros wrote:
> >
> >> Hi,
> >>
> >> On 10/08/18 18:01, Zeng Tao wrote:
> >> > The new scheme is required just to support legacy low and full-speed
> >> > devices. For high speed devices, it will slower the enumeration speed.
> >> > So in this patch we try the "old" enumeration scheme first for high
> >> > speed devices.
> >>
> >> How slow does it get? Is it significant?
> >> Do we risk breaking existing HS devices that work? I don't think we
> >> can be sure till we run this through testing.
> >
> >Indeed.  I am extremely skeptical about a patch like this, unless somebody can
> >show that Windows uses the "old" scheme for high-speed devices.
> 
> Yes, this is what the windows has done, you can refer to 
> https://blogs.msdn.microsoft.com/usbcoreblog/2013/04/11/usb-2-1-2-0-1-1-device-enumeration-changes-in-windows-8/

And that blog post is 5 years old!

Okay, I think we can go ahead and make this change.  However, you 
should update the patch description to mention what Microsoft did in 
Windows 8 and say that the new behavior matches theirs.

Also, as Roger mentioned, you should update the documentation to say 
that the old_scheme_first module parameter now applies only to low- and 
full-speed devices, since high- and SuperSpeed devices always use the 
old scheme first.

Alan Stern