Re: [PATCH 2/2] USB: misc: Add onboard_usb_hub driver

[Matthias Kaehlcke <mka@chromium.org>]

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.

> 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.