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

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Date:   Tue, 14 Aug 2018 07:35:00 +0000
From:   "Zengtao (B)" <prime.zeng@...ilicon.com>
To:     Alan Stern <stern@...land.harvard.edu>,
        Roger Quadros <rogerq@...com>
CC:     "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

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/

>
>Alan Stern