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

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Date:   Thu, 16 Aug 2018 10:16:48 +0300
From:   Roger Quadros <rogerq@...com>
To:     "Zengtao (B)" <prime.zeng@...ilicon.com>,
        Alan Stern <stern@...land.harvard.edu>
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

On 16/08/18 09:21, Zengtao (B) wrote:
> Hi alan:
> 
>> -----Original Message-----
>> From: Alan Stern [mailto:stern@...land.harvard.edu]
>> Sent: Tuesday, August 14, 2018 10:40 PM
>> To: Zengtao (B) <prime.zeng@...ilicon.com>
>> Cc: Roger Quadros <rogerq@...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 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.
>>
> Okay, I will update it the change log in v2.
> 
>> 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.
>>
> 
> Since we should have dedicated enumeration flow for SS, HIGH, low and full speed devices,
> So I think the old_scheme_first and use_both_schemes parameters should be removed.
> What do you think about it?

I think we should retain them as some host controllers can have issues
and these parameters give some control to system integrators to workaround if required.

I'm aware of one errata [1] that requires the old_scheme_first to be set in certain
circumstances.

[1] http://www.ti.com/lit/er/sprz429l/sprz429l.pdf
Section i897

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