On Fri, Sep 27, 2024 at 09:26:51AM +0530, Krishna Chaitanya Chundru wrote: > when you get time can you look into this. > if there are no further concerns I will respin this patch. None from me. If Rafael thinks it's good, and your patch follows his suggestions, we should be good to go. > On 9/12/2024 9:00 PM, Rafael J. Wysocki wrote: > > On Thu, Sep 12, 2024 at 2:13 PM Krishna Chaitanya Chundru > > <quic_krichai@xxxxxxxxxxx> wrote: > > > > > > > > > > > > On 9/12/2024 5:27 PM, Rafael J. Wysocki wrote: > > > > On Thu, Sep 12, 2024 at 1:52 PM Krishna Chaitanya Chundru > > > > <quic_krichai@xxxxxxxxxxx> wrote: > > > > > > > > > > > > > > > > > > > > On 9/12/2024 5:12 PM, Rafael J. Wysocki wrote: > > > > > > 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? > > > > > > > > > > > PCIe controller can do certain actions like keeping low power state, > > > > > remove bandwidth votes etc as part of runtime suspend as when we know > > > > > the client drivers already runtime suspended. > > > > > > > > Surely they can, but enabling runtime PM for devices that have > > > > children with runtime PM disabled and where those children have > > > > children with runtime PM enabled is a bug. > > > > > > > we are trying to enable the runtime PM of host bridge here, so that we > > > can enable runtime PM of the controller. > > > > So this is a preliminary step. That was unclear to me. > > > > > If this change got accepted the child here(host bridge) runtime pm will > > > be enabled then i think there will no issue in enabling the runtime pm > > > of the controller then. > > > > > > > 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. > > > > > > > > > > > No we are having a different observations here. > > > > > Without this change the PCIe controller driver can go to runtime suspend > > > > > without considering the state of the client drivers i.e even when the > > > > > client drivers are active. > > > > > After adding this change we see the pcie controller is getting runtime > > > > > suspended only after the client drivers are runtime suspended which is > > > > > the expected behaviour. > > > > > > > > OK, but then when and how is it going to be resumed? > > > > > > sorry I am not expert of the pm framework here, what we observed is when > > > client drivers are trying to resume using runtime_get we see the > > > controller driver is also getting resume properly with this change. > > > let me dig in and see in code on how this is happening. > > > > > > Bjorn has this view on this change in previous v2 version[1] > > > "My expectation is that adding new functionality should only require > > > changes in drivers that want to take advantage of it. For example, if > > > we add runtime PM support in the controller driver, the result should > > > be functionally correct even if we don't update drivers for downstream > > > devices. > > > > > > If that's not the way it works, I suggest that would be a problem in > > > the PM framework. > > > > You can say so, but that's how it goes. > > > > If there are any devices with runtime PM disabled in a dependency > > chain, the runtime PM framework cannot follow that chain as a whole. > > If enabling runtime PM for a device leads to this situation, it is not > > correct. > > > > > The host bridge might be a special case because we don't have a > > > separate "host bridge" driver; that code is kind of integrated with > > > the controller drivers. So maybe it's OK to do controller + host > > > bridge runtime PM support at the same time, as long as any time we add > > > runtime PM to a controller, we sure it's also set up for the host > > > bridge" > > > > I think that you can enable runtime PM for host bridge devices in > > general, as long as they don't need to be resumed without resuming any > > of their children. > > > > If that's the case, resuming one of its children will also cause the > > host bridge to resume and all should be fine, although you also need > > to ensure that system-wide suspend handling is in agreement with this. > > > > I would suggest calling pm_runtime_no_callbacks() for the host bridge device. > > > > > [1] https://lore.kernel.org/all/20240307215505.GA632869@bhelgaas/ > > > > And this is the information to put into your patch changelog: > > > > 1. It is a property of the runtime PM framework that it can only > > follow continuous dependency chains. That is, if there is a device > > with runtime PM disabled in a dependency chain, runtime PM cannot be > > enabled for devices below it and above it in that chain both at the > > same time. > > > > 2. Because of the above, in order to enable runtime PM for a PCIe > > controller device, one needs to ensure that runtime PM is enabled for > > all devices in every dependency chain between it and any PCIe endpoint > > (as runtime PM is enabled for PCIe endpoints). > > > > 3. This means that runtime PM needs to be enabled for the host bridge > > device, which is present in all of these dependency chains. > > > > 4. After this change, the host bridge device will be runtime-suspended > > by the runtime PM framework automatically after suspending its last > > child and it will be runtime-resumed automatically before resuming its > > first child which will allow the runtime PM framework to track > > dependencies between the host bridge device and all of its > > descendants. > > > > Thanks!
