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

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Date:   Tue, 13 Dec 2016 08:24:08 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     John Stultz <john.stultz@...aro.org>
cc:     lkml <linux-kernel@...r.kernel.org>, Chen Yu <chenyu56@...wei.com>,
        Wei Xu <xuwei5@...ilicon.com>,
        Guodong Xu <guodong.xu@...aro.org>,
        Amit Pundir <amit.pundir@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        John Youn <johnyoun@...opsys.com>,
        Douglas Anderson <dianders@...omium.org>,
        Kishon Vijay Abraham I <kishon@...com>,
        Felipe Balbi <felipe.balbi@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        <linux-usb@...r.kernel.org>
Subject: Re: [RFC][PATCH 5/5] usb: dwc2: Add a quirk to allow speed negotiation
 for Hisilicon Hi6220

On Mon, 12 Dec 2016, John Stultz wrote:

> From: Chen Yu <chenyu56@...wei.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 
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?

Alan Stern