Date:   Thu, 28 Dec 2017 08:51:34 -0800
From:   Tony Lindgren <tony@...mide.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        JeffyChen <jeffy.chen@...k-chips.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Shawn Lin <shawn.lin@...k-chips.com>,
        Brian Norris <briannorris@...omium.org>,
        Doug Anderson <dianders@...omium.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>
Subject: Re: [RFC PATCH v11 4/5] PCI / PM: Add support for the PCIe WAKE#
 signal for OF

* Rafael J. Wysocki <rafael@...nel.org> [171228 12:21]:
> On Thu, Dec 28, 2017 at 5:22 AM, Tony Lindgren <tony@...mide.com> wrote:
> > * Rafael J. Wysocki <rafael@...nel.org> [171228 00:51]:
> >> On Wed, Dec 27, 2017 at 4:08 PM, Tony Lindgren <tony@...mide.com> wrote:
> >> > * Rafael J. Wysocki <rjw@...ysocki.net> [171227 01:00]:
> >> >> On Tuesday, December 26, 2017 2:06:47 AM CET JeffyChen wrote:
> >> >> > Hi Rafael,
> >> >> >
> >> >> > Thanks for your reply :)
> >> >> >
> >> >> > On 12/26/2017 08:11 AM, Rafael J. Wysocki wrote:
> >> >> > >> >+
> >> >> > >> >+       dn = pci_device_to_OF_node(ppdev);
> >> >> > >> >+       if (!dn)
> >> >> > >> >+               return 0;
> >> >> > >> >+
> >> >> > >> >+       irq = of_irq_get_byname(dn, "wakeup");
> >> >> > > Why is this a property of the bridge and not of the device itself?
> >> >> >
> >> >> > That is suggested by Brian, because in that way, the wakeup pin would
> >> >> > not "tied to what exact device is installed (or no device, if it's a slot)."
> >> >>
> >> >> But I don't think it works when there are two devices using different WAKE#
> >> >> interrupt lines under the same bridge.  Or how does it work then?
> >> >
> >> > It won't work currently for multiple devices but adding more than
> >> > one wakeriq per device is doable. And I think we will have other
> >> > cases where multiple wakeirqs are connected to a single device, so
> >> > that issue should be sorted out sooner or later.
> >> >
> >> > And if requesting wakeirq for the PCI WAKE# lines at the PCI
> >> > controller does the job, then maybe that's all we need to start with.
> >>
> >> These are expected to be out-of-band, so not having anything to do
> >> with the Root Complex.
> >>
> >> In-band PME Messages go through the PCIe hierarchy, but that is a
> >> standard mechanism and it is supported already.
> >>
> >> WAKE# are platform-specific, pretty much by definition and I guess
> >> that on most ARM boards they are just going to be some kind of GPIO
> >> pins.
> >
> > OK. So probably supporting the following two configurations
> > should be enough then:
> >
> > 1. One or more WAKE# lines configured as a wakeirq for the PCI
> >    controller
> >
> >    When the wakeirq calls pm_wakeup_event() for the PCI controller
> >    device driver, the PCI controller wakes up and can deal with
> >    it's child devices
> 
> But this shouldn't be necessary at all.  Or if it is, I wonder why
> that's the case.

Well Brian had a concern where we would have to implement PM runtime
for all device drivers for PCI devices.

> I'm assuming that we're talking about PCI Express here, which has two
> wakeup mechanisms defined, one of which is based on using PME Messages
> (Beacon) and the second one is WAKE#:
> 
> "The WAKE# mechanism uses sideband signaling to implement wakeup
> functionality. WAKE# is
> an “open drain” signal asserted by components requesting wakeup and
> observed by the associated
> power controller."
> 
> (from PCIe Base Spec 3.0).  [And there's a diagram showing the routing
> of WAKE# in two cases in Figure 5-4: Conceptual Diagrams Showing Two
> Example Cases of WAKE# Routing.]

Thanks for the pointer, I had not seen that :) So the use cases
I was trying to describe above are similar to the wiring in the
PCIe Base Spec 3.0 "Figure 5-4" , but numbered the other way around.

> Note that WAKE# is defined to be "observed by the associated power
> controller", so I'm not sure what the PCI controller's role in the
> handing of it is at all.

To me it seems the "switch" part stays at least partially powered
and then in-band PME messages are used after host is woken up to
figure out which WAKE# triggered?

> > 2. Optionally a WAKE# line from a PCI device configured as wakeirq
> >    for the PCI device driver
> >
> >    In this case calling the PM runtime resume in the child
> >    PCI device will also wake up the parent PCI controller,
> >    and then the PCI controller can deal with it's children
> >
> > Seems like this series is pretty close to 1 above except
> > we need to have a list of wakeirqs per device instead of
> > just one. And option 2 should already work as long as the
> > PCI device driver parses and configures the wakeirq.
> 
> Well, this is confusing, because as I said above, option 1 doesn't
> look relevant even.

So isn't my option 1 above similar to the PCIe spec "Figure 5-4"
case 2? Anyways, let's standardize on the "Figure 5-4" naming
from now to avoid confusion :)

> >> > Then in addition to that, we could do the following to allow
> >> > PCI devices to request the wakeirq from the PCI controller:
> >> >
> >> > 1. PCI controller or framework implements a chained irq for
> >> >    the WAKE# lines assuming it can mask/unmask the WAKE# lines
> >> >
> >> > 2. PCI devices then can just request the wakeirq from the PCI
> >> >    controller
> >> >
> >> > And that's about it. Optionally we could leave out the dependency
> >> > to having PCI devices implement PM runtime and just resume the
> >> > parent (PCI controller) if PCI devices has not implemented
> >> > PM runtime.
> >>
> >> So if my understanding is correct, DT should give you the WAKE# IRQ
> >> for the given endpoint PCI device and you only are expected to request
> >> it.   The rest should just follow from the other pieces of information
> >> in the DT.
> >
> > Yeah and it seems that we should allow configuring both cases
> > 1 and 2 above.
> >
> >> With the quite obvious caveat that the same IRQ may be used as WAKE#
> >> for multiple endpoint devices (which BTW need not be under the same
> >> bridge even).
> >
> > And with the shared interrupts we can't do the masking/unmasking
> > automatically..
> 
> Or we need to use reference counting (so actually the wakeup IRQs are
> not dedicated).

Yeah. FYI, for the dedicated wakeirq cases I have, we need to keep
them masked during runtime to avoid tons of interrupts as they
are often wired to the RX pins.

> >> >> > >> >+       if (irq == -EPROBE_DEFER)
> >> >> > > Braces here, please.
> >> >> > ok, will fix in the next version.
> >> >> >
> >> >> > >
> >> >> > >> >+               return irq;
> >> >> > >> >+       /* Ignore other errors, since a missing wakeup is non-fatal. */
> >> >> > >> >+       else if (irq < 0) {
> >> >> > >> >+               dev_info(&pdev->dev, "cannot get wakeup interrupt: %d\n", irq);
> >> >> > >> >+               return 0;
> >> >> > >> >+       }
> >> >> > >> >+
> >> >> > >> >+       device_init_wakeup(&pdev->dev, true);
> >> >> > > Why do you call this before dev_pm_set_dedicated_wake_irq()?
> >> >> >
> >> >> > hmmm, i thought so too, but it turns out the dedicated wake irq
> >> >> > framework requires device_init_wakeup(dev, true) before attach the wake irq:
> >> >> >
> >> >> > int device_wakeup_attach_irq(struct device *dev,
> >> >> >                               struct wake_irq *wakeirq)
> >> >> > {
> >> >> >          struct wakeup_source *ws;
> >> >> >
> >> >> >          ws = dev->power.wakeup;
> >> >> >          if (!ws) {
> >> >> >                  dev_err(dev, "forgot to call device_init_wakeup?\n");
> >> >> >                  return -EINVAL;
> >> >> >
> >> >>
> >> >> Well, that's a framework issue, fair enough.
> >> >>
> >> >> That said, what if user space removes the wakeup source from under you
> >> >> concurrently via sysfs?  Tony?
> >> >
> >> > Hmm sounds racy, need to take a look.
> >>
> >> Not only racy, as I don't see anything to prevent user space from
> >> making the dev->power.wakeup wakeup source go away via sysfs at any
> >> time *after* the IRQ has been requested.
> >
> > Currently nothing happens with wakeirqs if there's no struct
> > wakeup_source. On device_wakeup_enable() we call device_wakeup_attach()
> > that just copies dev->power.wakeirq to ws->wakeirq. And when struct
> > wake_source is freed the device should be active and wakeirq
> > disabled. Or are you seeing other issues here?
> 
> I'm suspicious about one thing, but I need to look deeper into the code. :-)

OK. My response time will be laggy this week in case you find
something that needs urgent fixing :)

Regards,

Tony

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