linux-kernel - Re: [PATCH v2 1/1] usb: core: lpm: set lpm

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Date:	Sat, 13 Jun 2015 11:00:29 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Lu Baolu <baolu.lu@...ux.intel.com>
cc:	Mathias Nyman <mathias.nyman@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] usb: core: lpm: set lpm_capable for root hub
 device

On Sat, 13 Jun 2015, Lu Baolu wrote:

> Commit 25cd2882e2fc ("usb/xhci: Change how we indicate a host supports
> Link PM.") removed the code to set lpm_capable for USB 3.0 super-speed
> root hub. The intention of that change was to avoid touching usb core
> internal field, a.k.a. lpm_capable, and let usb core to set it by
> checking U1 and U2 exit latency values in the descriptor.
> 
> Usb core checks and sets lpm_capable in hub_port_init(). Unfortunately,
> root hub is a special usb device as it has no parent. Hub_port_init()
> will never be called for a root hub device. That means lpm_capable will
> by no means be set for the root hub. As the result, lpm isn't functional
> at all in Linux kernel.
> 
> This patch add the code to check and set lpm_capable when registering a
> root hub device. It could be back-ported to kernels as old as v3.15,
> that contains the Commit 25cd2882e2fc ("usb/xhci: Change how we indicate
> a host supports Link PM.").
> 
> Cc: stable@...r.kernel.org # 3.15
> Reported-by: Kevin Strasser <kevin.strasser@...ux.intel.com>
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---

> --- a/drivers/usb/core/usb.h
> +++ b/drivers/usb/core/usb.h
> @@ -82,6 +82,7 @@ extern int usb_runtime_suspend(struct device *dev);
>  extern int usb_runtime_resume(struct device *dev);
>  extern int usb_runtime_idle(struct device *dev);
>  extern int usb_set_usb2_hardware_lpm(struct usb_device *udev, int enable);
> +extern int usb_device_supports_lpm(struct usb_device *udev);
>  
>  #else
>  
> @@ -106,6 +107,11 @@ static inline int usb_set_usb2_hardware_lpm(struct usb_device *udev, int enable)
>  	return 0;
>  }
>  
> +static inline int usb_device_supports_lpm(struct usb_device *udev)
> +{
> +	return 0;
> +}
> +
>  #endif
>  
>  extern struct bus_type usb_bus_type;

In fact, usb_device_supports_lpm() is compiled even when CONFIG_PM
isn't set.  Maybe this should be changed.  But if you don't want to
change it now, you need to put the declaration outside the "#ifdef
CONFIG_PM" region.  As it is, your patch is still broken (did you try 
building it with CONFIG_PM unset?).

Alan Stern

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