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From: John Stultz <john.stultz@linaro.org>
Date: Tue, 13 Dec 2016 11:23:03 -0800
Message-ID: <CALAqxLW60PCgiRpXb9woTqeLqRkv=Y9m422ON5HC7nd=e7Rqcw@mail.gmail.com>
Subject: Re: [RFC][PATCH 5/5] usb: dwc2: Add a quirk to allow speed
 negotiation for Hisilicon Hi6220
To: Alan Stern <stern@rowland.harvard.edu>
Cc: lkml <linux-kernel@vger.kernel.org>, Chen Yu <chenyu56@huawei.com>,
        Wei Xu <xuwei5@hisilicon.com>, Guodong Xu <guodong.xu@linaro.org>,
        Amit Pundir <amit.pundir@linaro.org>, Rob Herring <robh+dt@kernel.org>,
        John Youn <johnyoun@synopsys.com>,
        Douglas Anderson <dianders@chromium.org>,
        Kishon Vijay Abraham I <kishon@ti.com>,
        Felipe Balbi <felipe.balbi@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux USB List <linux-usb@vger.kernel.org>
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On Tue, Dec 13, 2016 at 5:24 AM, Alan Stern <stern@rowland.harvard.edu> wrote:
> On Mon, 12 Dec 2016, John Stultz wrote:
>
>> From: Chen Yu <chenyu56@huawei.com>
>>
>> The Hi6220's usb controller is limited in that it does not
>> support "Split Transactions", so it does not support communicating
>> with low-speed and full-speed devices behind a high-speed hub.
>>
>> Thus it requires a quirk so that we can manually drop the usb
>> speed when low/full-speed are attached, and bump back to high
>> speed when they are removed.
>
> Just out of curiosity (I know nothing about this hardware), what

If your interested in details, page 12 of the pdf here has some details:
https://github.com/96boards/documentation/blob/master/ConsumerEdition/HiKey/AdditionalDocs/HiKey_Hardware_User_Manual_Rev0.2.pdf

There's also schematics for the board available (if you are interested
in that sort of stuff). You can find the USB bits on Page 4 here:
https://github.com/96boards/documentation/blob/master/ConsumerEdition/HiKey/HardwareDocs/HiKey_schematics_LeMaker_version_Rev_A1.pdf


> happens if there is a high-speed hub plugged into the host controller
> and both a high-speed and a full-speed device plugged into the hub?
>
> Do you end up forcing the high-speed device to run at full speed?

Yes. It drops back to full-speed.

thanks
-john