Re: [PATCH] usb: hub: try old enumeration scheme first for high speed devices
From: Roger Quadros
Date: Thu Aug 16 2018 - 03:17:17 EST
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
>> 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:
>>>>> 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
>>>>>> 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
>>> Yes, this is what the windows has done, you can refer to
>> 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  that requires the old_scheme_first to be set in certain
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