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

[Roger Quadros]

On 16/08/18 09:21, Zengtao (B) wrote:
> Hi alan:
> 
>> -----Original Message-----
>> From: Alan Stern [mailto:stern@xxxxxxxxxxxxxxxxxxx]
>> Sent: Tuesday, August 14, 2018 10:40 PM
>> To: Zengtao (B) <prime.zeng@xxxxxxxxxxxxx>
>> Cc: Roger Quadros <rogerq@xxxxxx>; gregkh@xxxxxxxxxxxxxxxxxxx;
>> mathias.nyman@xxxxxxxxxxxxxxx; drinkcat@xxxxxxxxxxxx;
>> felipe.balbi@xxxxxxxxxxxxxxx; drake@xxxxxxxxxxxx; mike.looijmans@xxxxxxxx;
>> joe@xxxxxxxxxxx; linux-usb@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
>> 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@xxxxxxxxxxxxxxx
>>>> [mailto:linux-usb-owner@xxxxxxxxxxxxxxx] On Behalf Of Alan Stern
>>>> Sent: Friday, August 10, 2018 10:20 PM
>>>> To: Roger Quadros <rogerq@xxxxxx>
>>>> Cc: Zengtao (B) <prime.zeng@xxxxxxxxxxxxx>;
>>>> gregkh@xxxxxxxxxxxxxxxxxxx; mathias.nyman@xxxxxxxxxxxxxxx;
>>>> drinkcat@xxxxxxxxxxxx; felipe.balbi@xxxxxxxxxxxxxxx;
>>>> drake@xxxxxxxxxxxx; mike.looijmans@xxxxxxxx; joe@xxxxxxxxxxx;
>>>> linux-usb@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
>>>> 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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