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

From: Heikki Krogerus <heikki.krogerus@linux.intel.com>
To: Hans de Goede <hdegoede@redhat.com>
Cc: John Stultz <john.stultz@linaro.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Yu Chen <chenyu56@huawei.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Rob Herring <robh+dt@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Chunfeng Yun <chunfeng.yun@mediatek.com>,
	Felipe Balbi <balbi@kernel.org>,
	Andy Shevchenko <andy.shevchenko@gmail.com>,
	Jun Li <lijun.kernel@gmail.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Linux USB List <linux-usb@vger.kernel.org>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
	<devicetree@vger.kernel.org>
Subject: Re: [RFC][PATCH 2/3] usb: roles: Add usb role switch notifier.
Date: Fri, 4 Oct 2019 11:12:30 +0300	[thread overview]
Message-ID: <20191004081230.GE1048@kuha.fi.intel.com> (raw)
In-Reply-To: <b8695418-9d3a-96a6-9587-c9a790f49740@redhat.com>

On Thu, Oct 03, 2019 at 10:51:38PM +0200, Hans de Goede wrote:
> Hi,
> 
> On 03-10-2019 22:37, John Stultz wrote:
> > On Thu, Oct 3, 2019 at 2:25 AM Hans de Goede <hdegoede@redhat.com> wrote:
> > > On 03-10-2019 01:16, John Stultz wrote:
> > > > From: Yu Chen <chenyu56@huawei.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.

Hear, hear.

-- 
heikki