linux-kernel - Re: [PACTH,v6,1/2] usb: xhci: plat: Enable runtime PM

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Message-ID: <20160823032304.GA1781@google.com>
Date:   Mon, 22 Aug 2016 20:23:07 -0700
From:   Brian Norris <briannorris@...omium.org>
To:     robert.foss@...labora.com
Cc:     mathias.nyman@...el.com, gregkh@...uxfoundation.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Julius Werner <jwerner@...omium.org>,
        Andrew Bresticker <abrestic@...omium.org>,
        Felipe Balbi <felipe.balbi@...ux.intel.com>,
        Baolin Wang <baolin.wang@...aro.org>, zyx@...k-chips.com,
        wulf@...k-chips.com
Subject: Re: [PACTH,v6,1/2] usb: xhci: plat: Enable runtime PM

+ others

Hi Robert and Felipe,

I have a few questions for one or both of you. I'm not really an expert
on runtime PM, so please take my questions with a grain of salt.

On Wed, Aug 10, 2016 at 04:32:15PM -0400, robert.foss@...labora.com wrote:
> From: Robert Foss <robert.foss@...labora.com>
> 
> Enable runtime PM for the xhci-plat device so that the parent device
> may implement runtime PM.
> 
> Signed-off-by: Robert Foss <robert.foss@...labora.com>
> 
> Tested-by: Robert Foss <robert.foss@...labora.com>
> ---
>  drivers/usb/host/xhci-plat.c | 29 +++++++++++++++++++++++++++--
>  1 file changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/host/xhci-plat.c b/drivers/usb/host/xhci-plat.c
> index ed56bf9..ba4efe7 100644
> --- a/drivers/usb/host/xhci-plat.c
> +++ b/drivers/usb/host/xhci-plat.c
> @@ -246,6 +246,9 @@ static int xhci_plat_probe(struct platform_device *pdev)
>  	if (ret)
>  		goto dealloc_usb2_hcd;
>  
> +	pm_runtime_set_active(&pdev->dev);
> +	pm_runtime_enable(&pdev->dev);
> +

How does it help to enable PM runtime like this, if you don't have any
kind of runtime_{suspend,resume}() callbacks?

I suspect that this patch set was derived from the Chromium OS kernel
tree, where we were supporting a Tegra XHCI chipset:

https://chromium.googlesource.com/chromiumos/third_party/kernel/+/chromeos-3.10/drivers/usb/host/xhci-tegra.c#1920

It looks like the driver was refactored to not use xhci-plat.c before it
was upstreamed (and runtime PM support was dropped along the way).

So, I'm wondering how I might actually use this? Particularly, I'm
looking at trying out runtime suspend for a DWC3 controller in host
mode, and it looks like I'd have to do some layer-violating calls to
xhci_suspend()/xhci_resume() from the parent dwc3 device, or else
rewrite drivers/usb/dwc3/host.c to avoid using xhci-plat.c.

(I also see that Baolin, CC'd here, was interested in dwc3 [1].)

Or possibly an enlightening question for me: if you don't mind, how are
you utilizing runtime PM in conjunction with xhci-plat.c, Robert?
Presumably some other parent device/driver is doing some additional
management of the XHCI core?

Regards,
Brian

[1] [PATCH 4/4] usb: dwc3: core: Support the dwc3 host suspend/resume
    https://lkml.org/lkml/2016/7/15/181
    https://patchwork.kernel.org/patch/9231417/

>  	return 0;
>  
>  
> @@ -274,6 +277,8 @@ static int xhci_plat_remove(struct platform_device *dev)
>  	struct xhci_hcd	*xhci = hcd_to_xhci(hcd);
>  	struct clk *clk = xhci->clk;
>  
> +	pm_runtime_disable(&dev->dev);
> +
>  	usb_remove_hcd(xhci->shared_hcd);
>  	usb_phy_shutdown(hcd->usb_phy);
>  
> @@ -292,6 +297,13 @@ static int xhci_plat_suspend(struct device *dev)
>  {
>  	struct usb_hcd	*hcd = dev_get_drvdata(dev);
>  	struct xhci_hcd	*xhci = hcd_to_xhci(hcd);
> +	int ret;
> +
> +	ret = pm_runtime_get_sync(dev);
> +	if (ret < 0) {
> +		pm_runtime_put(dev);
> +		return ret;
> +	}
>  
>  	/*
>  	 * xhci_suspend() needs `do_wakeup` to know whether host is allowed
> @@ -301,15 +313,28 @@ static int xhci_plat_suspend(struct device *dev)
>  	 * reconsider this when xhci_plat_suspend enlarges its scope, e.g.,
>  	 * also applies to runtime suspend.
>  	 */
> -	return xhci_suspend(xhci, device_may_wakeup(dev));
> +	ret = xhci_suspend(xhci, device_may_wakeup(dev));
> +	pm_runtime_put(dev);
> +
> +	return ret;
>  }
>  
>  static int xhci_plat_resume(struct device *dev)
>  {
>  	struct usb_hcd	*hcd = dev_get_drvdata(dev);
>  	struct xhci_hcd	*xhci = hcd_to_xhci(hcd);
> +	int ret;
>  
> -	return xhci_resume(xhci, 0);
> +	ret = pm_runtime_get_sync(dev);
> +	if (ret < 0) {
> +		pm_runtime_put(dev);
> +		return ret;
> +	}
> +
> +	ret = xhci_resume(xhci, 0);
> +	pm_runtime_put(dev);
> +
> +	return ret;
>  }
>  
>  static const struct dev_pm_ops xhci_plat_pm_ops = {