linux-kernel - Re: [PATCH v10 0/5] USB: misc: Add onboard_usb

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Date:   Wed, 12 May 2021 09:37:10 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Alexander Dahl <ada@...rsis.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Michal Simek <michal.simek@...inx.com>,
        devicetree@...r.kernel.org,
        Douglas Anderson <dianders@...omium.org>,
        linux-usb@...r.kernel.org, Peter Chen <peter.chen@...nel.org>,
        linux-kernel@...r.kernel.org, Stephen Boyd <swboyd@...omium.org>,
        Ravi Chandra Sadineni <ravisadineni@...omium.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Bastien Nocera <hadess@...ess.net>,
        Al Cooper <alcooperx@...il.com>,
        "Alexander A. Klimov" <grandmaster@...klimov.de>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Christian Lamparter <chunkeey@...glemail.com>,
        Colin Ian King <colin.king@...onical.com>,
        Dmitry Osipenko <digetx@...il.com>,
        Fabio Estevam <festevam@...il.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Mathias Nyman <mathias.nyman@...el.com>,
        Vinod Koul <vkoul@...nel.org>, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH v10 0/5] USB: misc: Add onboard_usb_hub driver

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