linux-kernel - Re: [PATCH v5 0/4] USB: misc: Add onboard_usb

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Date:   Wed, 10 Feb 2021 22:04:51 +0100
From:   Krzysztof Kozlowski <krzk@...nel.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org, Peter Chen <peter.chen@....com>,
        Stephen Boyd <swboyd@...omium.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Ravi Chandra Sadineni <ravisadineni@...omium.org>,
        Bastien Nocera <hadess@...ess.net>,
        linux-kernel@...r.kernel.org,
        Douglas Anderson <dianders@...omium.org>,
        linux-usb@...r.kernel.org, Al Cooper <alcooperx@...il.com>,
        "Alexander A. Klimov" <grandmaster@...klimov.de>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Mathias Nyman <mathias.nyman@...el.com>,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH v5 0/4] USB: misc: Add onboard_usb_hub driver

On Wed, Feb 10, 2021 at 09:10:35AM -0800, Matthias Kaehlcke wrote:
> This series adds the onboard_usb_hub_driver, the corresponding
> device tree bindings and creation of onboard_usb_hub platform in
> the xhci-plat driver during probe().
> 
> 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.

It seems you are re-developing the power sequence drivers which perform
exactly this. Peter Chen some time ago was bringing power sequence to
USB devices, but I lost track where this ended up.

Some of his (and my) very old work (2017...) can be found here:
https://github.com/krzk/linux/tree/wip/odroid-u3-usb3503-pwrseq

Instead of adding custom driver hiding some USB hub implementation,
power sequence seems a generic solution. What if you need to power cycle
other embedded USB device? Not a hub?

I was not aware of previous discussions so maybe I am repeating
someone.

Best regards,
Krzysztof