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

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Date:   Wed, 10 Feb 2021 14:37:30 -0800
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Krzysztof Kozlowski <krzk@...nel.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

Hi Krzysztof,

On Wed, Feb 10, 2021 at 10:04:51PM +0100, Krzysztof Kozlowski wrote:
> 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

pwrseq was brought up in the discussion about this driver, but wasn't
deemed suitable for this use case which might require more complex
configurations:

https://lore.kernel.org/patchwork/patch/1313000/#1512725

> 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?

The driver could be extended to also cover other types of devices if desired.
Maybe it should be called usb-pwrseq then, even though it's not directly
related with the original pwrseq series.