On 28 August 2017 at 03:30, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote: > On Friday, August 25, 2017 3:42:35 PM CEST Rafael J. Wysocki wrote: >> On Thursday, August 24, 2017 11:50:40 PM CEST Rafael J. Wysocki wrote: >> > On Thursday, August 24, 2017 6:35:49 PM CEST Rafael J. Wysocki wrote: >> > > On Thursday, August 24, 2017 11:15:26 AM CEST Ulf Hansson wrote: >> > >> > [cut] >> > >> > > [BTW, it is not entirely clear to me why it ever is necessary to runtime resume >> > > a device with direct_complete set after __device_suspend(), because it can only >> > > have direct_complete set at that point if all of the hierarchy below it has >> > > this flag set too and so runtime PM has to be disabled for all of those >> > > devices as well.] >> > >> > Which makes me realize that we should take a step back and look at what >> > problems there are. >> > >> > First, there are devices (I know about two examples so far and both are PCI) >> > that may need to be runtime resumed during system suspend for reasons other >> > than the ones checked by the ACPI PM domain (or the PCI bus type). There needs >> > to be a way to indicate that from the driver side. >> > >> > However, it still may be valuable to check the power-related conditions for >> > leaving the device in runtime suspend over system suspend/resume in case >> > it actually doesn't need to be runtime resumed during system suspend after >> > all. That's what the majority of my patch was about. >> > >> > The second problem is that the ACPI PM domain (and the PCI bus type) >> > runtime resumes all devices unconditionally in its ->suspend callback, >> > even though that may not be necessary for some devices. Therefore there >> > needs to be a way to indicate that too. That still would be good to >> > have *regardless* of the direct_complete mechanism, because the direct_complete >> > flag may not be set very often due to dependencies and then the >> > resume-during-suspend will take place unnecessarily. >> > >> > Accordingly, it looks like we need a "no need to resume me" flag in the first >> > place. That would indicate to interested pieces of code that, from the >> > driver perspective, the device doesn't need to be runtime resumed before >> > invoking its system suspend callbacks. This should be clear enough to everyone >> > IMO. >> > >> > [Note that if that flag is set for all devices, we may drop it along with >> > direct_complete, but before that happens both are needed.] >> >> I think we are in agreement that direct_complete will not be necessary any >> more when all drivers/bus types/PM domains and so on can do the "safe >> suspend", but we're not there yet. :-) >> >> > To address the first issue I would add something like the flag in the patches >> > I sent (but without the ACPI PM domain part which should be covered by the >> > "no need to resume me" flag above), because that allows the device's ->suspend >> > callback to run in principle and the driver may use that callback even to >> > runtime resume the device if that's what it wants to do. So something like >> > "run my ->suspend callback even though I might stay in runtime suspend". >> > >> > I would probably add driver_flags to dev_pm_info for that to set at the probe >> > time (and I would make the core clear that on driver removal). >> > >> > The complexity concern is there, but honestly I don't see a better way at >> > this point. >> >> So below is a prototype patch. It still is missing a documentation update, but >> other than that it should be complete unless I missed something. >> >> The way it works is that the SAFE_SUSPEND flag is not looked at by the core >> at all. The ACPI PM domain looks at it and the PCI bus type can be modified >> to take it into account in the future. That is what causes the "runtime resume >> during system suspend" to be skipped. >> >> In turn, the ALWAYS_SUSPEND flag is only looked at by the core and it causes >> the decision on whether or not to use direct_complete to be deferred to the >> __device_suspend_late() time. If you set it for a PCI device, the effect is >> equivalent to "no direct_complete". If you set it for a device in the ACPI >> PM domain, that depends on whether or not SAFE_SUSPEND is set. If it isn't >> set, the effect is equivalent to "no direct_complete" too, but if it is set, >> the core may still try to use direct_complete for the device, but it will >> make the decision on it in __device_suspend_late() and then it will not invoke >> the ->suspend_late callback for the device if it is still runtime suspended. >> [Note that you cannot runtime resume and runtime suspend again a device during >> system suspend, so if it is runtime suspended in __device_suspend_late(), it >> has been runtime suspend all the way since device_prepare().] >> >> So say you point the ->suspend_late and ->resume_early callbacks of >> the designware i2c driver to pm_runtime_force_suspend() and >> pm_runtime_force_resume(), respectively, and set both the SAFE_SUSPEND >> and ALWAYS_SUSPEND flags for the device. >> >> If system suspend is started and the device is not runtime suspended, >> direct_complete is not set for it and everything works as usual, so say >> the device is runtime suspended in device_prepare(). Then, the ACPI PM >> domain checks the other conditions for leaving it in runtime suspend and >> returns either 0 or a positive number from acpi_subsys_prepare(). >> >> If 0 is returned, direct_complete is not set by the core and >> acpi_subsys_suspend() is called. It checks the SAFE_SUSPEND flag and sees >> that the device need not be runtime resumed, so it invokes the driver's >> ->suspend callback (which is not present, so it doesn't do anything). >> Next, in __device_suspend_late(), acpi_subsys_suspend_late() is invoked >> and it calls pm_runtime_force_suspend(), which executes the driver's >> ->runtime_suspend() callback, and then (if successful) calls >> acpi_dev_suspend_late() to put the device into a low-power state. The >> resume path is a reverse of the above in this case. So far, so good. > > Well, not really, because if the device remains runtime suspended, > ->runtime_suspend() will not be called by pm_runtime_force_suspend() and > acpi_dev_suspend_late() should not be called then. > > So more changes in the ACPI PM domain are needed after all. Yes, that's what I thought as well. Anyway, let me cook a new version of the series - trying to address the first bits you have pointed out. Then we can continue with fine-tuning on top, addressing further optimizations of the ACPI PM domain. Kind regards Uffe -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
