Re: [RFC][PATCH 5/5] usb: dwc2: Add a quirk to allow speed negotiation for Hisilicon Hi6220

From: John Stultz
Date: Tue Dec 13 2016 - 14:32:42 EST

On Tue, Dec 13, 2016 at 5:24 AM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, 12 Dec 2016, John Stultz wrote:
>> From: Chen Yu <chenyu56@xxxxxxxxxx>
>> 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:

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:

> 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.