Re: [PATCH v4] PCI: Enable runtime pm of the host bridge

[Rafael J. Wysocki]

On Fri, Aug 16, 2024 at 10:45 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>
> [+cc Rafael, Mayank, Markus (when people have commented on previous
> versions, please cc them on new versions).  I'm still hoping Rafael
> will have a chance to chime in]
>
> On Mon, Jul 08, 2024 at 10:19:40AM +0530, Krishna chaitanya chundru wrote:
> > The Controller driver is the parent device of the PCIe host bridge,
> > PCI-PCI bridge and PCIe endpoint as shown below.
> >
> >         PCIe controller(Top level parent & parent of host bridge)
> >                         |
> >                         v
> >         PCIe Host bridge(Parent of PCI-PCI bridge)
> >                         |
> >                         v
> >         PCI-PCI bridge(Parent of endpoint driver)
> >                         |
> >                         v
> >                 PCIe endpoint driver
> >
> > Now, when the controller device goes to runtime suspend, PM framework
> > will check the runtime PM state of the child device (host bridge) and
> > will find it to be disabled.
>
> I guess "will find it to be disabled"  means the child (host bridge)
> has runtime PM disabled, not that the child device is disabled, right?
>
> > So it will allow the parent (controller
> > device) to go to runtime suspend. Only if the child device's state was
> > 'active' it will prevent the parent to get suspended.
>
> Can we include a hint like the name of the function where the PM
> framework decides this?  Maybe this is rpm_check_suspend_allowed()?
>
> rpm_check_suspend_allowed()  checks ".ignore_children", which sounds
> like it could be related, and AFAICS .ignore_children == false here,
> so .child_count should be relevant.
>
> But I'm still confused about why we can runtime suspend a bridge that
> leads to devices that are not suspended.

That should only be possible if runtime PM is disabled for those devices.

> > Since runtime PM is disabled for host bridge, the state of the child
> > devices under the host bridge is not taken into account by PM framework
> > for the top level parent, PCIe controller. So PM framework, allows
> > the controller driver to enter runtime PM irrespective of the state
> > of the devices under the host bridge. And this causes the topology
> > breakage and also possible PM issues like controller driver goes to
> > runtime suspend while endpoint driver is doing some transfers.

Why is it a good idea to enable runtime PM for a PCIe controller?

> What does "topology breakage" mean?  Do you mean something other than
> the fact that an endpoint DMA might fail if the controller is
> suspended?
>
> > So enable runtime PM for the host bridge, so that controller driver
> > goes to suspend only when all child devices goes to runtime suspend.

This by itself makes sense to me.

> IIUC, the one-sentence description here is that previously, the PCI
> host controller could be runtime suspended even while an endpoint was
> active, which caused DMA failures.  And this patch changes that so the
> host controller is only runtime suspended after the entire hierarchy
> below it is runtime suspended?  Is that right?
>
> > Signed-off-by: Krishna chaitanya chundru <quic_krichai@xxxxxxxxxxx>
> > ---
> > Changes in v4:
>
> (Note: v4 applies cleanly to v6.10-rc1 and to v6.11-rc1 with a small
> offset).
>
> > - Changed pm_runtime_enable() to devm_pm_runtime_enable() (suggested by mayank)
> > - Link to v3: https://lore.kernel.org/lkml/20240609-runtime_pm-v3-1-3d0460b49d60@xxxxxxxxxxx/
> > Changes in v3:
> > - Moved the runtime API call's from the dwc driver to PCI framework
> >   as it is applicable for all (suggested by mani)
> > - Updated the commit message.
> > - Link to v2: https://lore.kernel.org/all/20240305-runtime_pm_enable-v2-1-a849b74091d1@xxxxxxxxxxx
> > Changes in v2:
> > - Updated commit message as suggested by mani.
> > - Link to v1: https://lore.kernel.org/r/20240219-runtime_pm_enable-v1-1-d39660310504@xxxxxxxxxxx
> > ---
> >
> > ---
> >  drivers/pci/probe.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > index 8e696e547565..fd49563a44d9 100644
> > --- a/drivers/pci/probe.c
> > +++ b/drivers/pci/probe.c
> > @@ -3096,6 +3096,10 @@ int pci_host_probe(struct pci_host_bridge *bridge)
> >       }
> >
> >       pci_bus_add_devices(bus);
> > +
> > +     pm_runtime_set_active(&bridge->dev);
> > +     devm_pm_runtime_enable(&bridge->dev);
> > +
> >       return 0;
> >  }
> >  EXPORT_SYMBOL_GPL(pci_host_probe);

This will effectively prevent the host bridge from being
runtime-suspended at all IIUC, so the PCIe controller will never
suspend too after this change.

If this is the intended behavior, I would suggest saying that
explicitly in the changelog.