linux-kernel - Re: [v15 2/6] usb: host: xhci-plat: Enable wakeup based on children wakeup status

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Date:   Mon, 9 May 2022 09:08:43 +0530
From:   Pavan Kondeti <quic_pkondeti@...cinc.com>
To:     Matthias Kaehlcke <mka@...omium.org>
CC:     Krishna Kurapati <quic_kriskura@...cinc.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        "Andy Gross" <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Felipe Balbi <balbi@...nel.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Doug Anderson <dianders@...omium.org>,
        Mathias Nyman <mathias.nyman@...el.com>,
        <devicetree@...r.kernel.org>, <linux-arm-msm@...r.kernel.org>,
        <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-pm@...r.kernel.org>, <quic_pkondeti@...cinc.com>,
        <quic_ppratap@...cinc.com>, <quic_vpulyala@...cinc.com>
Subject: Re: [v15 2/6] usb: host: xhci-plat: Enable wakeup based on children
 wakeup status

On Fri, May 06, 2022 at 08:36:31AM -0700, Matthias Kaehlcke wrote:
> On Thu, May 05, 2022 at 02:26:09PM +0530, Krishna Kurapati wrote:
> > device_wakeup_path() tells if any of the children devices needs
> > wakeup. Use this hint to enable/disable wakeup of our device. This
> > helps the parent device of xhci-plat (like sysdev) to retrieve
> > the wakeup setting via device_wakeup_path().
> > 
> > Signed-off-by: Krishna Kurapati <quic_kriskura@...cinc.com>
> > ---
> >  drivers/usb/host/xhci-plat.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/drivers/usb/host/xhci-plat.c b/drivers/usb/host/xhci-plat.c
> > index 649ffd8..ad585fa 100644
> > --- a/drivers/usb/host/xhci-plat.c
> > +++ b/drivers/usb/host/xhci-plat.c
> > @@ -415,6 +415,14 @@ static int __maybe_unused xhci_plat_suspend(struct device *dev)
> >  	if (pm_runtime_suspended(dev))
> >  		pm_runtime_resume(dev);
> >  
> > +	if (device_wakeup_path(dev)) {
> > +		if (!device_may_wakeup(dev))
> > +			device_wakeup_enable(dev);
> > +	} else {
> > +		if (device_may_wakeup(dev))
> > +			device_wakeup_disable(dev);
> > +	}
> 
> This code is not self-explantatory and deserves a comment.
> 
> Enabling/disabling wakeup for the purpose if signalling is a bit of a
> hack. It might be an acceptable hack as long as it has no side effects.
> However with the current implementation the wakeup state of the xHCI can
> be different after resuming than it was before going to suspend:
> 
> after boot
>   grep -h xhci /sys/class/wakeup/*/name
>     => xhci-hcd.14.auto
> 
> after suspend w/o wakeup capable device
>   grep -h xhci /sys/class/wakeup/*/name
>     => no results
> 
> after suspend with wakeup capable device
>   grep -h xhci /sys/class/wakeup/*/name
>     => xhci-hcd.14.auto
> 
> The hack shouldn't alter the wakeup state 'persistently', i.e. you'll have
> to restore it on resume, as in Pavan does in his reply to '[PATCH v14 2/7]
> PM / wakeup: Add device_children_wakeup_capable()' (it needs to be done
> conditionally though).

I am worried that we are not doing the right thing here. why should the
xhci-plat goes against the wishes of the user space policy here? Can we NOT
just do anything here? If some one wants xhci-plat to wakeup all the time,
dwc3 will be configured to wakeup the system provided that the support is
available. This way we don't break any existing users of xhci-plat i.e not
enabling wakeup from the kernel.

Thanks,
Pavan