linux-kernel - Re: [PATCH] Revert "USB / PM: Allow USB devices to remain runtime-suspended when sleeping"

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Message-ID: <20160503075138.GB25025@localhost>
Date:	Tue, 3 May 2016 09:51:38 +0200
From:	Johan Hovold <johan@...nel.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Johan Hovold <johan@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	stable@...r.kernel.org
Subject: Re: [PATCH] Revert "USB / PM: Allow USB devices to remain
 runtime-suspended when sleeping"

On Mon, May 02, 2016 at 11:13:10AM -0400, Alan Stern wrote:
> On Mon, 2 May 2016, Johan Hovold wrote:
> 
> > This reverts commit e3345db85068ddb937fc0ba40dfc39c293dad977, which
> > broke system resume for a large class of devices.
> > 
> > Devices that after having been reset during resume need to be rebound
> > due to a missing reset_resume callback, are now left in a suspended
> > state. This specifically broke resume of common USB-serial devices,
> > which are now unusable after system suspend (until disconnected and
> > reconnected) when USB persist is enabled.
> > 
> > During resume, usb_resume_interface will set the needs_binding flag for
> > such interfaces, but unlike system resume, run-time resume does not
> > honour it.
> > 
> > Cc: stable <stable@...r.kernel.org>	# 4.5
> > Signed-off-by: Johan Hovold <johan@...nel.org>
> > ---
> > 
> > Greg, Alan,
> > 
> > This patch for v4.6-rc7 fixes a 4.5-regression that broke system suspend
> > for a large class of devices, including USB-serial devices, for example
> > when USB persist is enabled.
> > 
> > We may be able to find a way around this, but since it's a user-visible
> > regression and late in the rc-cycle, I believe reverting the offending
> > commit is the right thing to do.
> 
> The description of the problem doesn't sound right to me.  For 
> instance, would it help if usb_runtime_resume() did honor the 
> needs_binding flag?  I doubt it.  Things like the wakeup setting would 
> still be lost before the runtime resume occurred.
>
> I suspect the right answer is always to resume a USB device if it needs 
> a reset-resume, but otherwise allow it to remain in runtime suspend.

I was trying to describe how things broke but not necessarily to suggest
how this should be fixed. Could have put the last paragraph below the
cut-off line I guess.

> Reverting the patch for now is okay with me.  Tomeu may want to work on
> a better solution.  Part of the difficulty is that the PM core wants to
> know before suspending whether skipping resume will be okay, but the
> USB stack doesn't know until after the host controller has been 
> resumed.
> 
> In the end, we'll probably have the PM core call usb_resume all the 
> time, but usb_resume will leave the device in runtime suspend if it 
> can.  This isn't ideal but it may be the best we can do.

Thanks,
Johan