On Wed, Nov 25, 2020 at 8:39 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote: > > On Thu, Nov 12, 2020 at 8:19 PM Furquan Shaikh <furquan@xxxxxxxxxx> wrote: > > > > On x86 Chromebooks, we have observed this issue for a long time now - > > when the system is powered off or rebooted, ACPI PM is not invoked and > > this results in PowerResource _OFF methods not being invoked for any > > of the devices. The _OFF methods are invoked correctly in case of > > suspend-to-idle (S0ix) and suspend-to-memory(S3). However, they do not > > get invoked when `poweroff` or `reboot` are triggered. > > > > One of the differences between suspend, hibernate and shutdown paths > > in Linux kernel is that the shutdown path does not use the typical > > device PM phases (prepare, freeze/suspend, poweroff) as used by > > suspend/hibernate. Instead the shutdown path makes use of > > .shutdown_pre() and .shutdown() callbacks. > > > > If I understand correctly, .shutdown() has been around for a long time > > and existed even before the PM callbacks were added. Thus, > > pm->poweroff() and .shutdown() are supposed to be analogous and > > consistent in the behavior. > > Well, not quite. > > ->shutdown() is expected to be a lightweight operation also suitable > for kexec() and similar situations where ->poweroff() may not work. > > > This is why runtime PM is disallowed by > > device_shutdown() before it calls .shutdown() (i.e. to keep behavior > > consistent for both paths). However, in practice, there are > > differences in behavior for the pm->poweroff() and .shutdown() paths > > since the shutdown path does not execute any PM domain operations. > > That's correct. > > > Because of this difference in behavior, shutdown path never invokes > > ACPI PM and thus the ACPI PowerResources are not turned off when the > > system is rebooted or powered off (sleep S5). On Chromebooks, it is > > critical to run the _OFF methods for poweroff/reboot in order to > > ensure that the device power off sequencing requirements are met. > > Currently, these requirements are violated which impact the > > reliability of devices over the lifetime of the platform. > > > > There are a few ways in which this can be addressed: > > > > 1. Similar to the case of hibernation, a new > > PMSG_POWEROFF/PM_EVENT_POWEROFF can be introduced to invoke device > > power management phases using `dpm_suspend_start(PMSG_POWEROFF)` and > > `dpm_suspend_end(PMSG_POWEROFF)`. However, as the shutdown path uses > > the class/bus/driver .shutdown() callbacks, adding dpm phases for > > poweroff complicates the order of operations. If the dpm phases are > > run before .shutdown() callbacks, then it will result in the callbacks > > accessing devices after they are powered off. If the .shutdown() > > callbacks are run before dpm phases, then the pm->poweroff() calls are > > made after the device shutdown is done. Since .shutdown() and > > pm->poweroff() are supposed to be analogous, having both calls in the > > shutdown path is not only redundant but also results in incorrect > > behavior. > > > > 2. Another option is to update device_shutdown() to make > > pm_domain.poweroff calls after the class/bus/driver .shutdown() is > > done. However, this suffers from the same problem as #1 above i.e. it > > is redundant and creates conflicting order of operations. > > > > 3. Third possible solution is to detach the device from the PM domain > > after it is shutdown. Currently, device drivers perform a detach > > operation only when the device is removed. However, in case of > > poweroff/reboot as the device is already shutdown, detaching PM domain > > will give it the opportunity to ensure that any power resources are > > correctly turned off before the system shuts down. > > 4. Make Chromebooks call something like hibernation_platform_enter() > on S5 entries (including reboot). Actually, Chromebooks do not support S4 and hence CONFIG_HIBERNATION. This is done for a number of reasons including security. Hence, I don't think using hibernation_platform_enter() would be an option. > > > Out of these, I think #3 makes the most sense as it does not introduce > > any conflicting operations. I verified that the following diff results > > in _OFF methods getting invoked in both poweroff and reboot cases: > > This won't work for PCI devices though, only for devices in the ACPI > PM domain, so it is not sufficient in general. That is true. The proposed solution only handles detaching of PM domains. I understand your point about this not working for any devices not part of the PM domain. The issues that we have observed in shutdown/reboot paths have been specific to ACPI power resources controlling the sequencing to external devices. > > > diff --git a/drivers/base/core.c b/drivers/base/core.c > > index 94df2ba1bbed..e55d65fbb4a9 100644 > > --- a/drivers/base/core.c > > +++ b/drivers/base/core.c > > @@ -23,6 +23,7 @@ > > #include <linux/of_device.h> > > #include <linux/genhd.h> > > #include <linux/mutex.h> > > +#include <linux/pm_domain.h> > > #include <linux/pm_runtime.h> > > #include <linux/netdevice.h> > > #include <linux/sched/signal.h> > > @@ -3230,6 +3231,8 @@ void device_shutdown(void) > > dev->driver->shutdown(dev); > > } > > > > + dev_pm_domain_detach(dev, true); > > + > > It generally makes sense to do this, because ->shutdown() is sort of > analogous to ->remove() from the driver model perspective, so if it is > sufficient for you, please feel free to send a formal patch with that > change. I will work on creating a formal patch and send it for review. > > > device_unlock(dev); > > if (parent) > > device_unlock(parent); > > > > This was discussed on the mailing list some time back[1] in the > > context of a different use case. However, the idea of detaching > > devices (on any bus) from the PM domain during shutdown is important > > to ensure correct power sequencing for the devices. > > > > One of the concerns that was raised on the above thread was slowing > > down the shutdown process when not needed. I think this can be handled > > by adding a sysfs attribute to allow platforms to decide if they need > > the ability to power off PM domains on shutdown/reboot path. > > If you need to do that on a per-platform basis, I would go for option > 4 above instead. For Chromebooks, I don't see a reason to do this on a per-platform basis. But, I wanted to make sure that the proposed change does not have any side-effect on any other user upstream. > > > Questions that I am looking to get feedback/comments are: > > > > 1. Is my assessment of the problem and understanding of the > > .shutdown() and pm.poweroff() correct? > > Not exactly. > > > 2. Does the solution #3 i.e. detaching PM domain after shutting down > > device on shutdown path makes sense? > > Yes, it does (to me), but no, it is not sufficient to address the > problem at hand. > > > 3. Are there other possible approaches to solve this problem that can > > be explored? > > Yes, there are. See option 4 above. > > > 4. Do we still have the performance concern about the shutdown path? I > > don’t think anything has changed since that thread, so this is > > probably still true. > > I think that it is the case. Whoever had had any performance concerns > regarding this before is still going to have them. > > > 5. Does the use of sysfs attribute make sense to let platform control > > if it wants to detach PM domains on shutdown path? > > That would be clunky IMV. I can start with the diff I pasted above as a formal patch to see if it makes sense. If more changes are required to it, I can work on them. > > > Sorry about the long thread and thank you so much for your time! > > No worries. Thanks again for your time. I really appreciate the insight and comments. - Furquan > > Thanks!
