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

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Date:   Thu, 3 Oct 2019 13:45:59 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     lkml <linux-kernel@...r.kernel.org>, Yu Chen <chenyu56@...wei.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Chunfeng Yun <chunfeng.yun@...iatek.com>,
        Felipe Balbi <balbi@...nel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        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 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.

This is pretty HiKey960 specific, so I think the notifier is useful to
let the gpio hub logic tie into the role switching events.

Suggestions for alternative approaches?

thanks
-john