Re: [PATCH v2 8/9] usb: dwc3: qcom: fix wakeup implementation

[Matthias Kaehlcke <mka@chromium.org>]

On Thu, Aug 04, 2022 at 05:10:00PM +0200, Johan Hovold wrote:
> It is the Qualcomm glue wakeup interrupts that may be able to wake the
> system from suspend and this can now be described in the devicetree.
> 
> Move the wakeup-source property handling over from the core driver and
> instead propagate the capability setting to the core device during
> probe.
> 
> This is needed as there is currently no way for the core driver to query
> the wakeup setting of the glue device, but it is the core driver that
> manages the PHY power state during suspend.
> 
> Also don't leave the PHYs enabled when system wakeup has been disabled
> through sysfs.
> 
> Fixes: 649f5c842ba3 ("usb: dwc3: core: Host wake up support from system suspend")
> Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
> ---
>  drivers/usb/dwc3/core.c      | 5 ++---
>  drivers/usb/dwc3/dwc3-qcom.c | 6 +++++-
>  2 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
> index 16d1f328775f..8c8e32651473 100644
> --- a/drivers/usb/dwc3/core.c
> +++ b/drivers/usb/dwc3/core.c
> @@ -1822,7 +1822,6 @@ static int dwc3_probe(struct platform_device *pdev)
>  
>  	platform_set_drvdata(pdev, dwc);
>  	dwc3_cache_hwparams(dwc);
> -	device_init_wakeup(&pdev->dev, of_property_read_bool(dev->of_node, "wakeup-source"));
>  
>  	spin_lock_init(&dwc->lock);
>  	mutex_init(&dwc->mutex);
> @@ -1984,7 +1983,7 @@ static int dwc3_suspend_common(struct dwc3 *dwc, pm_message_t msg)
>  		dwc3_core_exit(dwc);
>  		break;
>  	case DWC3_GCTL_PRTCAP_HOST:
> -		if (!PMSG_IS_AUTO(msg) && !device_can_wakeup(dwc->dev)) {
> +		if (!PMSG_IS_AUTO(msg) && !device_may_wakeup(dwc->dev)) {

Let me explain the rationale for why device_can_wakeup() was used here:

On QCOM SC7180 based Chromebooks we observe that the onboard USB hub consumes
~80 mW during system suspend when the PHYs are disabled, as opposed to ~17 mW
when the PHYs remain enabled. This is a significant delta when the device is
on a battery power.

The initial idea was to leave the PHYs always enabled (in a low power mode),
but then I dug up commit c4a5153e87fd ("usb: dwc3: core: Power-off core/PHYs
on system_suspend in host mode"), which provides a rationale for the PHYs
being powered off:

  Commit 689bf72c6e0d ("usb: dwc3: Don't reinitialize core during
  host bus-suspend/resume") updated suspend/resume routines to not
  power_off and reinit PHYs/core for host mode.
  It broke platforms that rely on DWC3 core to power_off PHYs to
  enter low power state on system suspend.

Unfortunately we don't know which platforms are impacted by this. The idea
behind using device_can_wakeup() was to use it as a proxy for platforms
that are *not* impacted. If a platform supports USB wakeup supposedly the
SoC can enter its low power mode during system suspend with the PHYs
enabled.

By now I'm not 100% sure if the above assumption is correct. I recently
saw allegations that the power consumption of a given QC SoC with USB
wakeup support drops significantly when wakeup is disabled (i.e. when
the PHYs are off), but haven't confirmed this yet.