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

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Date:   Wed, 16 Sep 2020 08:19:07 +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

On 20-09-15 16:03:45, Matthias Kaehlcke wrote:
> Hi Peter,
> 
> On Tue, Sep 15, 2020 at 07:05:38AM +0000, Peter Chen wrote:
> >   
> > > > > +	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.
> 
> I conceptually agree, but in practice there are some conflicting details:
> 
> wakeup for the hubs on my system is by default disabled, yet USB wakeup works
> regardless, so the flag doesn't really provide useful information. I guess we
> could still use it if there is no better way, but it doesn't seem ideal.
> 
> Similar for udev->bus->controller, according to sysfs it doesn't even have wakeup
> support. Please let me know if there is a reliable way to check if wakeup is
> enabled on the controller of a device.

Then, how could your code work, you use usb_wakeup_enabled_descendants
to get if HUB or the descendants under the HUB has wakeup enabled?

If you use dwc3, you need to enable xhci-plat.c's wakeup entry if your
system needs xHCI connect/disconnect wakeup event. I have one pending
patch to do it:

https://www.spinics.net/lists/linux-usb/msg199406.html

> 
> > 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.
> 
> Ok, I think you are suggesting a sysfs attribute instead of a DT property, that
> sounds good to me.

Yes.

-- 

Thanks,
Peter Chen