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

From: Johan Hovold
Date: Tue May 03 2016 - 03:51:46 EST

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@xxxxxxxxxxxxxxx> # 4.5
> > Signed-off-by: Johan Hovold <johan@xxxxxxxxxx>
> > ---
> >
> > 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.