Re: [PATCH v5 3/4] usb: host: xhci-plat: Create platform device for onboard hubs in probe()

From: Stephen Boyd <swboyd@chromium.org>
To: Krzysztof Kozlowski <krzk@kernel.org>,
	Matthias Kaehlcke <mka@chromium.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Rob Herring <robh+dt@kernel.org>,
	Frank Rowand <frowand.list@gmail.com>,
	devicetree@vger.kernel.org, Peter Chen <peter.chen@nxp.com>,
	Alan Stern <stern@rowland.harvard.edu>,
	Ravi Chandra Sadineni <ravisadineni@chromium.org>,
	Bastien Nocera <hadess@hadess.net>,
	linux-kernel@vger.kernel.org,
	Douglas Anderson <dianders@chromium.org>,
	linux-usb@vger.kernel.org,
	Mathias Nyman <mathias.nyman@intel.com>
Subject: Re: [PATCH v5 3/4] usb: host: xhci-plat: Create platform device for onboard hubs in probe()
Date: Thu, 11 Feb 2021 11:14:39 -0800	[thread overview]
Message-ID: <161307087919.1254594.11784819060723374369@swboyd.mtv.corp.google.com> (raw)
In-Reply-To: <YCRcIuCxB8nYi7/e@google.com>

Quoting Matthias Kaehlcke (2021-02-10 14:20:18)
> 
> On Wed, Feb 10, 2021 at 10:06:45PM +0100, Krzysztof Kozlowski wrote:
> > 
> > This looks hackish... what if later we have something else than hub?
> > Another if()?
> > 
> > What if hub could be connected to something else than XHCI controller?
> 
> In earlier versions this was standalone driver, which was more flexible and
> didn't require cooperation from the XHCI driver:
> 
> https://lore.kernel.org/patchwork/patch/1313001/
> 
> Rob Herring raised objections about the DT bindings, since the USB hub would be
> represented twice in the DT, once in the USB hierachry (with an explicit node or
> implicitly) plus a node for the platform device for the new driver:
> 
> https://lore.kernel.org/patchwork/patch/1305395/
> https://lore.kernel.org/patchwork/patch/1313000/
> 
> Alan Stern suggested to create the platform device in the XHCI platform driver:
> 
> https://lore.kernel.org/patchwork/patch/1313000/#1510227
> 
> I wasn't super happy about involving xhci-plat, but at least the code is minimal
> and all the device specific stuff is handled by the onboard_usb_hub driver.
> 
> If you have better suggestions that might satisfy all parties please let us
> know :)
> 

Is it possible to use the graph binding to connect the USB controller on
the SoC to the port on the hub? Then the hub would be a standalone node
at the root of DT connected to the USB controller (or phy) and xhci code
could probe the firmware to see if there's a graph connection downstream
that is a powered hub like this. I didn't see this idea mentioned in the
previous discussions, but maybe I missed it.