linux-kernel - Re: [RFC][PATCH 2/3] usb: roles: Add usb role switch notifier.

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Message-ID: <20191004080058.GD1048@kuha.fi.intel.com>
Date:   Fri, 4 Oct 2019 11:00:58 +0300
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     John Stultz <john.stultz@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Yu Chen <chenyu56@...wei.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Chunfeng Yun <chunfeng.yun@...iatek.com>,
        Felipe Balbi <balbi@...nel.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Jun Li <lijun.kernel@...il.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Linux USB List <linux-usb@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>
Subject: Re: [RFC][PATCH 2/3] usb: roles: Add usb role switch notifier.

On Thu, Oct 03, 2019 at 10:56:24PM +0200, Hans de Goede wrote:
> Hi,
> 
> On 03-10-2019 22:45, John Stultz wrote:
> > On Thu, Oct 3, 2019 at 4:26 AM Greg Kroah-Hartman
> > <gregkh@...uxfoundation.org> wrote:
> > > 
> > > On Wed, Oct 02, 2019 at 11:16:16PM +0000, John Stultz wrote:
> > > > From: Yu Chen <chenyu56@...wei.com>
> > > > 
> > > > This patch adds notifier for drivers want to be informed of the usb role
> > > > switch.
> > > 
> > > Ick, I hate notifiers, they always come back to cause problems.
> > > 
> > > What's just wrong with a "real" call to who ever needs to know this?
> > > And who does need to know this anyway?  Like Hans said, if we don't have
> > > a user for it, we should not add it.
> > 
> > So in this case, its used for interactions between the dwc3 driver and
> > the hikey960 integrated USB hub, which is controlled via gpio (which I
> > didn't submit here as I was trying to keep things short and
> > reviewable, but likely misjudged).
> > 
> > The HiKey960 has only one USB controller, but in order to support both
> > USB-C gadget/OTG and USB-A (host only) ports. When the USB-C
> > connection is attached, it powers down and disconnects the hub. When
> > the USB-C connection is detached, it powers the hub on and connects
> > the controller to the hub.
> 
> When you say one controller, do you mean 1 host and 1 gadget controller,
> or is this one of these lovely devices where a gadget controller gets
> abused as / confused with a proper host controller?
> 
> And since you are doing a usb-role-switch driver, I guess that the
> role-switch is integrated inside the SoC, so you only get one pair
> of USB datalines to the outside ?

Unless I'm mistaken, the dwc3 driver in this case is the
usb-role-switch. The DWC3 IP includes both USB dost and device blocks,
i.e. it's a dual role controller. Drivers like tcpm.c that negotiate
the actual role need to tell the outcome of the negotiation to the
dwc3 driver. So I think this part is OK.

The platform has also some kind of discrete switch for routing the
signals to either Standard-A (the hub) or Type-C connector, so it does
not represent the usb-role-switch. It should however affect the USB role,
as if that switch routes the data signals to the Standard-A port (to
the hub) instead of USB Type-C, the USB role needs to be fixed to host
mode.

I guess this series does not include the driver for that discrete
switch/mux. I don't remember/know how that switch was planned to be
handled.

> This does seem rather special, it might help if you can provide a diagram
> with both the relevant bits inside the SoC as well as what lives outside
> the Soc. even if it is in ASCII art...

thanks,

-- 
heikki