Re: [PATCH] usb: xhci: Disable runtime PM suspend for quirky controllers.

[Shawn Nematbakhsh <shawnn@chromium.org>]

Hi Sarah,

I will resubmit the patch with these changes shortly.

On Fri, Aug 9, 2013 at 10:22 AM, Sarah Sharp
<sarah.a.sharp@linux.intel.com> wrote:
> Hi Shawn,
>
> I noticed that the ChromeOS kernel tree is still using this particular
> patch, and thought it was probably time to revisit it.
>
> On Sat, May 25, 2013 at 09:57:57AM -0700, Shawn Nematbakhsh wrote:
>> Hi Sarah and Alan,
>>
>> Thanks for the comments. I will make the following revisions:
>>
>> 1. Call pm_runtime_get_noresume only when the first device is connected,
>> and call pm_runtime_put when the last device is disconnected.
>> 2. Wrap the calls in an ifdef CONFIG_USB_DEFAULT_PERSIST.
>> 3. Make sure that all pm_runtime_get_noresume calls have a corresponding
>> pm_runtime_put somewhere (originally the intent was to disable runtime
>> suspend "forever", but no longer).
>>
>> In principle, would the patch be acceptable with these revisions?
>
> The thread petered off with all other options turning out to be
> dead-ends, so yes, if you made those changes, you could get that patch
> upstream.  I would like the ChromeOS kernel to be as close to upstream
> as possible, so please resubmit this patch with those changes.
>
> Thanks,
> Sarah Sharp
>
>>
>> On Sat, May 25, 2013 at 7:11 AM, Alan Stern <stern@rowland.harvard.edu>wrote:
>>
>> > On Fri, 24 May 2013, Sarah Sharp wrote:
>> >
>> > > On Fri, May 24, 2013 at 11:12:57AM -0700, Shawn Nematbakhsh wrote:
>> > > > If a USB controller with XHCI_RESET_ON_RESUME goes to runtime suspend,
>> > > > a reset will be performed upon runtime resume. Any previously suspended
>> > > > devices attached to the controller will be re-enumerated at this time.
>> > > > This will cause problems, for example, if an open system call on the
>> > > > device triggered the resume (the open call will fail).
>> > >
>> > > That's ugly, but disabling runtime PM is going to have a big impact on
>> > > the power consumption of those systems.
>> > >
>> > > Alan, do you think this is really the right thing to be doing here?  It
>> > > feels like userspace should just be able to deal with devices
>> > > disconnecting on resume.  After all, there are lots of USB devices that
>> > > can't handle USB device suspend at all.
>> >
>> > This is a complicated issue.  It depends on the runtime PM settings for
>> > both the device and the host controller.
>> >
>> > As just one aspect, consider the fact that if it wants to, userspace
>> > can already prevent the controller from going into runtime suspend.
>> > Always preventing this at the kernel level, even when no devices are
>> > plugged in, does seem too heavy-handed.
>> >
>> > > Shouldn't userspace just disable runtime PM for the USB device classes
>> > > that don't have a reset resume callback?
>> >
>> > That's not so easy, because the kernel changes over time.  Userspace
>> > has no general way to tell which drivers have reset-resume support.
>> >
>> > > > Note that this change is only relevant when persist_enabled is not set
>> > > > for USB devices.
>> > >
>> > > Could we at least wrap the call in an ifdef CONFIG_USB_DEFAULT_PERSIST?
>> > > That way if people have USB persist turned off in their configuration,
>> > > their host will still be able to suspend.
>> >
>> > Not just that; the patch is incorrect on the face of it...
>> >
>> > > > @@ -4687,6 +4687,12 @@ int xhci_gen_setup(struct usb_hcd *hcd,
>> > xhci_get_quirks_t get_quirks)
>> > > >
>> > > >     get_quirks(dev, xhci);
>> > > >
>> > > > +   /* If we are resetting upon resume, we must disable runtime PM.
>> > > > +    * Otherwise, an open() syscall to a device on our runtime
>> > suspended
>> > > > +    * controller will trigger controller reset and device
>> > re-enumeration */
>> > > > +   if (xhci->quirks & XHCI_RESET_ON_RESUME)
>> > > > +           pm_runtime_get_noresume(dev);
>> > > > +
>> >
>> > It adds a pm_runtime_get call with no corresponding pm_runtime_put.
>> >
>> > Alan Stern
>> >
>> >