linux-kernel - Re: [PATCH v2 1/3] usb: core: lpm: fix usb3_hardware

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Date:	Fri, 13 Nov 2015 10:28:47 -0500 (EST)
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>,
	<stable@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] usb: core: lpm: fix usb3_hardware_lpm sysfs node

On Fri, 13 Nov 2015, Lu, Baolu wrote:

> On 11/13/2015 12:20 AM, Alan Stern wrote:
> > On Thu, 12 Nov 2015, Lu Baolu wrote:
> >
> >> Commit 655fe4effe0f ("usbcore: add sysfs support to xHCI usb3
> >> hardware LPM") introduced usb3_hardware_lpm sysfs node. This
> >> doesn't show the correct status of USB3 U1 and U2 LPM status.
> >>
> >> This patch fixes this by replacing usb3_hardware_lpm with two
> >> nodes, usb3_hardware_lpm_u1 (for U1) and usb3_hardware_lpm_u2
> >> (for U2), and recording the U1/U2 LPM status in right places.
> >>
> >> This patch should be back-ported to kernels as old as 4.3,
> >> that contains Commit 655fe4effe0f ("usbcore: add sysfs support
> >> to xHCI usb3 hardware LPM").
> >>
> >> Cc: stable@...r.kernel.org
> >> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> > ...
> >
> >> --- a/drivers/usb/core/hub.c
> >> +++ b/drivers/usb/core/hub.c
> >> @@ -3875,17 +3875,23 @@ static void usb_enable_link_state(struct usb_hcd *hcd, struct usb_device *udev,
> >>   		return;
> >>   	}
> >>   
> >> -	if (usb_set_lpm_timeout(udev, state, timeout))
> >> +	ret = usb_set_lpm_timeout(udev, state, timeout);
> >> +	if (ret)
> >>   		/* If we can't set the parent hub U1/U2 timeout,
> >>   		 * device-initiated LPM won't be allowed either, so let the xHCI
> >>   		 * host know that this link state won't be enabled.
> >>   		 */
> >>   		hcd->driver->disable_usb3_lpm_timeout(hcd, udev, state);
> >> -
> >>   	/* Only a configured device will accept the Set Feature U1/U2_ENABLE */
> >>   	else if (udev->actconfig)
> >>   		usb_set_device_initiated_lpm(udev, state, true);
> >>   
> >> +	if (!ret) {
> >> +		if (state == USB3_LPM_U1)
> >> +			udev->usb3_lpm_u1_enabled = 1;
> >> +		else if (state == USB3_LPM_U2)
> >> +			udev->usb3_lpm_u2_enabled = 1;
> >> +	}
> > This doesn't look right at all.  What happens if ret is 0 but the
> > device isn't configured?  You'll set the usb3_lpm_u*_enabled flag even
> > though LPM isn't really enabled.
> >
> > Don't you want to set these flags inside the
> > usb_set_device_initiated_lpm() function, where you know whether the
> > action succeeded?  And leave this routine unchanged?
> 
> My understand is that both hub and device can initiate LPM.
> As soon as usb_set_lpm_timeout(valid_timeout_value)
> returns 0, the hub-initiated LPM is enabled. Thus, LPM is
> enabled no matter the result of usb_set_device_initiated_lpm().
> The only difference is whether device is able to initiate LPM.
> 
> On disable side, as soon as usb_set_lpm_timeout(0) return 0,
> hub initiated LPM is disabled. Hub will disallows link to enter
> U1/U2 as well, even device is initiating LPM. Hence LPM
> is disabled as soon as hub LPM timeout set to 0, no matter
> device-initiated LPM is disabled or not.

Then maybe you can add a comment explaining this.

The patch still looks strange, though.  Your new code does this:

	ret = usb_set_lpm_timeout(...);
	if (ret)
		...
	else if (udev->actconfig)
		...
	if (!ret) {
		if (state == USB3_LPM_U1)
		...
	}

It would be better to do this:

	if (usb_set_lpm_timeout(...)) {
		...
	} else {
		if (udev->actconfig)
			...
		if (state == USB3_LPM_U1)
		...
	}

Alan Stern

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