linux-kernel - RE: [PATCH 2/2] USB: misc: Add onboard_usb

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Date:   Tue, 15 Sep 2020 07:05:38 +0000
From:   Peter Chen <peter.chen@....com>
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>,
        Alan Stern <stern@...land.harvard.edu>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Bastien Nocera <hadess@...ess.net>,
        Ravi Chandra Sadineni <ravisadineni@...omium.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        Stephen Boyd <swboyd@...omium.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Douglas Anderson <dianders@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Alexander A. Klimov" <grandmaster@...klimov.de>,
        Masahiro Yamada <masahiroy@...nel.org>
Subject: RE: [PATCH 2/2] USB: misc: Add onboard_usb_hub driver

  
> > > +	hub->cfg.power_off_in_suspend =
> of_property_read_bool(dev->of_node, "power-off-in-suspend");
> > > +	hub->cfg.wakeup_source = of_property_read_bool(dev->of_node,
> > > +"wakeup-source");
> >
> > Do you really need these two properties? If the device (and its
> > children if existed) has wakeup enabled, you keep power in suspend,
> > otherwise, you could close it, any exceptions?
> 
> That would work for my use case, but I'm not sure it's a universally good
> configuration.
> 
> I don't have a specific USB device in mind, but you could have a device that
> shouldn't lose it's context during suspend or keep operating autonomously (e.g.
> a sensor with a large buffer collecting samples). Not sure if something like this
> exists in the real though.
> 
> I'm not an expert, but it seems there are USB controllers with wakeup support
> which is always enabled. A board with such a controller then couldn't have a
> policy to power down the hub regardless of wakeup capable devices being
> connected.
> 

Whether or not it is a wakeup_source, it could get through its or its children's /sys/../power/wakeup
value, you have already used usb_wakeup_enabled_descendants to know it. If the onboard HUB
needs to reflect wakeup signal, it should not power off its regulator.

For another property power-off-in-suspend, I think it is also a user option, but not a hardware feature.

If (wakeup-source || ! power-off-in-suspend)
	power off;
else
	keep power;

Peter