Re: [PATCH v5 5/5] usb: host: enable sideband transfer during system sleep

[Guan-Yu Lin <guanyulin@xxxxxxxxxx>]

On Mon, Oct 14, 2024 at 5:21 PM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Oct 14, 2024 at 08:50:29AM +0000, Guan-Yu Lin wrote:
> >
> > diff --git a/drivers/usb/core/driver.c b/drivers/usb/core/driver.c
> > index e713cf9b3dd2..eb85cbb1a2ff 100644
> > --- a/drivers/usb/core/driver.c
> > +++ b/drivers/usb/core/driver.c
> > @@ -1583,6 +1583,11 @@ int usb_suspend(struct device *dev, pm_message_t msg)
> >       struct usb_device       *udev = to_usb_device(dev);
> >       int r;
> >
> > +     if (msg.event == PM_EVENT_SUSPEND && usb_sideband_check(udev)) {
> > +             dev_dbg(dev, "device accessed via sideband\n");
> > +             return 0;
> > +     }
>
> What prevents the check from changing state right after you call this?
>

Maybe we should hold a lock before doing the check, and release the
lock after the entire system suspend period to ensure no sideband will
access the USB device after it has been suspended. However, I'm not
very confident about holding a lock over the entire suspend period.
Should we do so or use a boolean flag instead? I'm not sure if holding
a lock will prevent the system from entering low-power state or not.

> > +
> >       unbind_no_pm_drivers_interfaces(udev);
> >
> >       /* From now on we are sure all drivers support suspend/resume
> > @@ -1619,6 +1624,11 @@ int usb_resume(struct device *dev, pm_message_t msg)
> >       struct usb_device       *udev = to_usb_device(dev);
> >       int                     status;
> >
> > +     if (msg.event == PM_EVENT_RESUME && usb_sideband_check(udev)) {
> > +             dev_dbg(dev, "device accessed via sideband\n");
> > +             return 0;
> > +     }
>
> Same here, what's keeping the state from changing?
>
> > diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
> > index 2fdafbcbe44c..18c743ce5ac5 100644
> > --- a/drivers/usb/dwc3/core.c
> > +++ b/drivers/usb/dwc3/core.c
> > @@ -2550,8 +2550,15 @@ static int dwc3_runtime_idle(struct device *dev)
> >  static int dwc3_suspend(struct device *dev)
> >  {
> >       struct dwc3     *dwc = dev_get_drvdata(dev);
> > +     struct platform_device *xhci = dwc->xhci;
> >       int             ret;
> >
> > +     if (xhci && xhci_sideband_check(xhci->dev.driver_data)) {
>
> What could go wrong with poking into a random device structure's private
> data that you don't know the type of?  :(
>

We did observe that the driver_data was set to struct usb_hcd in the
"platform:xhci-hcd" module, which was the default linux xhci host
controller platform glue driver. Should we rewrite as "struct usb_hcd
*hcd = dev_get_drvdata(dev);" and then operate with hcd? Or even with
this kind of statement the idea is still unacceptable?



> > +             dev_dbg(dev, "device accessed via sideband\n");
> > +             return 0;
>
> I predict, that if this all does get implemented, we're going to have a
> lot of confusion of "why will my devices not go into suspend?"
> questions, right?
>

This feature should not influence anyone not using xhci sideband
framework. For those who use xhci sideband framework, the current
design seems to not support USB transfer during system suspend. I
think the goal for this patch is to enable system suspend while
holding necessary devices active to enable USB transfer via sideband,
and suspend all devices if we don't have USB transfers on it. The USB
devices are active only when there are USB transfers happening on it.
Based on this, I think users will have less concern on why the USB
devices are still active.

> How does userspace know if a device is controlled by a sideband path or
> not?  Is there some sysfs link somewhere, and does any tool show it
> anyway?
>
> thanks,
>
> greg k-h

I don't recall there's a mechanism to show sideband activity to
userspace. Is this need solely related to enabling USB transfer via
sideband during system suspend? Or is it related to generic sideband
functionality? If the latter is the case, maybe we should propose a
patch in series "Introduce QC USB SND audio offloading support" to
support this mechanism.

Regards,
Guan-Yu