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

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Date:   Thu, 3 Oct 2019 22:51:38 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     John Stultz <john.stultz@...aro.org>
Cc:     lkml <linux-kernel@...r.kernel.org>, Yu Chen <chenyu56@...wei.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        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>,
        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.

Hi,

On 03-10-2019 22:37, John Stultz wrote:
> On Thu, Oct 3, 2019 at 2:25 AM Hans de Goede <hdegoede@...hat.com> wrote:
>> On 03-10-2019 01:16, John Stultz wrote:
>>> From: Yu Chen <chenyu56@...wei.com>
>>>
>>> This patch adds notifier for drivers want to be informed of the usb role
>>> switch.
>>
>> I do not see any patches in this series actually using this new
>> notifier.
>>
>> Maybe it is best to drop this patch until we actually have in-kernel
>> users of this new API show up ?
> 
> Fair point. I'm sort of taking a larger patchset and trying to break
> it up into more easily reviewable chunks, but I guess here I mis-cut.
> 
> The user is the hikey960 gpio hub driver here:
>    https://git.linaro.org/people/john.stultz/android-dev.git/commit/?id=b06158a2d3eb00c914f12c76c93695e92d9af00f

Hmm, that seems to tie the TypeC data-role to the power-role, which
is not going to work with role swapping.

What is controlling the usb-role-switch, and thus ultimately
causing the notifier you are suggesting to get called ?

Things like TYPEC_VBUS_POWER_OFF and TYPEC_VBUS_POWER_ON
really beg to be modeled as a regulator and then the
Type-C controller (using e.g. the drivers/usb/typec/tcpm/tcpm.c
framework) can use that regulator to control things.
in case of the tcpm.c framework it can then use that
regulator to implement the set_vbus callback.

You really do not want to tie this do the usb_switch, both
because doing so ties the data and power-roles together
which is not supposed to happen and because role-swapping
requires careful timing of the VBUS on / off at different
moments then the moments when you actually set the mux/switch
for connecting the Dp/Dn lines to the host or gadget
controller.

The usb role switch abstraction is really only intended
for the data-lines switch and should not be tied together
with other stuff.

Regards,

Hans