Re: [PATCH v10 0/5] USB: misc: Add onboard_usb_hub driver

From: Matthias Kaehlcke <mka@chromium.org>
To: Alexander Dahl <ada@thorsis.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Alan Stern <stern@rowland.harvard.edu>,
	Rob Herring <robh+dt@kernel.org>,
	Frank Rowand <frowand.list@gmail.com>,
	Michal Simek <michal.simek@xilinx.com>,
	devicetree@vger.kernel.org,
	Douglas Anderson <dianders@chromium.org>,
	linux-usb@vger.kernel.org, Peter Chen <peter.chen@kernel.org>,
	linux-kernel@vger.kernel.org, Stephen Boyd <swboyd@chromium.org>,
	Ravi Chandra Sadineni <ravisadineni@chromium.org>,
	Krzysztof Kozlowski <krzk@kernel.org>,
	Bastien Nocera <hadess@hadess.net>,
	Al Cooper <alcooperx@gmail.com>,
	"Alexander A. Klimov" <grandmaster@al2klimov.de>,
	Andy Gross <agross@kernel.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	Christian Lamparter <chunkeey@googlemail.com>,
	Colin Ian King <colin.king@canonical.com>,
	Dmitry Osipenko <digetx@gmail.com>,
	Fabio Estevam <festevam@gmail.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Mathias Nyman <mathias.nyman@intel.com>,
	Vinod Koul <vkoul@kernel.org>,
	linux-arm-msm@vger.kernel.org
Subject: Re: [PATCH v10 0/5] USB: misc: Add onboard_usb_hub driver
Date: Wed, 12 May 2021 09:37:10 -0700	[thread overview]
Message-ID: <YJwENo/DmcyMR8bw@google.com> (raw)
In-Reply-To: <YJuBmlPSaJlyVuzW@ada-deb-carambola.ifak-system.com>

Hi Alexander,

On Wed, May 12, 2021 at 09:19:54AM +0200, Alexander Dahl wrote:
> Hello Matthias,
> 
> just a curious informal question, see below.
> 
> Am Tue, May 11, 2021 at 03:52:18PM -0700 schrieb Matthias Kaehlcke:
> > This series adds:
> > - the onboard_usb_hub_driver
> > - glue in the xhci-plat driver to create the onboard_usb_hub
> >   platform device if needed
> > - a device tree binding for the Realtek RTS5411 USB hub controller
> > - device tree changes that add RTS5411 entries for the QCA SC7180
> >   based boards trogdor and lazor
> > - a couple of stubs for platform device functions to avoid
> >   unresolved symbols with certain kernel configs
> > 
> > The main issue the driver addresses is that a USB hub needs to be
> > powered before it can be discovered. For discrete onboard hubs (an
> > example for such a hub is the Realtek RTS5411) this is often solved
> > by supplying the hub with an 'always-on' regulator, which is kind
> > of a hack. Some onboard hubs may require further initialization
> > steps, like changing the state of a GPIO or enabling a clock, which
> > requires even more hacks. This driver creates a platform device
> > representing the hub which performs the necessary initialization.
> > Currently it only supports switching on a single regulator, support
> > for multiple regulators or other actions can be added as needed.
> > Different initialization sequences can be supported based on the
> > compatible string.
> 
> This sounds like it would be useful for other hub controllers as well?
> For example, would the Microchip USB3503 (former SMSC,
> drivers/usb/misc/usb3503.c, [1]) fall into this category? That chip is
> used on the "Cubietech Cubietruck Plus" for example.

usb3503.c provides two 'separate' USB3503 drivers (which share some
code), a i2c client driver and a platform driver. IIUC on a system with
an USB3503 only one of these drivers is used. Theoretically it should be
feasible to extend the onboard_usb_hub driver to cover the functionality
of the platform driver in usb3503.c (essentially to control GPIOs and
clocks at initialization time and suspend/resume). Another question is
whether that would be desirable, since the i2c and the platform driver
share code, which then would be duplicated in the i2c and onboard_usb_hub
driver, unless a way is found to keep sharing that code.

The i2c driver can't be completely replaced by the onboard_usb_hub
driver, due to the i2c communications. It might be possible to have the
i2c driver and the onboard_usb_hub collaborate, however I expect it
would take a certain effort to design and implement a solid solution.

Thanks

Matthias