"Rafael J. Wysocki" <rjw@xxxxxxx> writes: > On Friday, February 11, 2011, Kevin Hilman wrote: >> "Rafael J. Wysocki" <rjw@xxxxxxx> writes: >> >> > On Monday, January 31, 2011, Rafael J. Wysocki wrote: >> >> On Monday, January 31, 2011, Alan Stern wrote: >> >> > On Mon, 31 Jan 2011, Kevin Hilman wrote: >> >> > >> >> > > I understand how this works, but frankly I'm still a bit fuzzy on why. >> >> > > >> >> > > I guess I'm still missing a good understanding of what "interfering with a >> >> > > system power transition" means, and why a runtime suspend qualifies as >> >> > > interfering but not a runtime resume. >> >> > >> >> > These are good questions. Rafael implemented this design originally; >> >> > my contribution was only to warn him of the potential for problems. >> >> > Therefore he should explain the rationale for the design. >> >> >> >> The reason why runtime resume is allowed during system power transitions is >> >> because in some cases during system suspend we simply have to resume devices >> >> that were previously runtime-suspended (for example, the PCI bus type does >> >> that). >> >> >> >> The reason why runtime suspend is not allowed during system power transitions >> >> if the following race: >> >> >> >> - A device has been suspended via a system suspend callback. >> >> - The runtime PM framework executes a (scheduled) suspend on that device, >> >> not knowing that it's already been suspended, which potentially results in >> >> accessing the device's registers in a low-power state. >> >> >> >> Now, it can be avoided if every driver does the right thing and checks whether >> >> the device is already suspended in its runtime suspend callback, but that would >> >> kind of defeat the purpose of the runtime PM framework, at least partially. >> > >> > In fact, I've just realized that the above race cannot really occur, because >> > pm_wq is freezable, so I'm proposing the following change. >> > >> > Of course, it still doesn't prevent user space from disabling the runtime PM >> > framework's helpers via /sys/devices/.../power/control. >> > >> > Thanks, >> > Rafael >> > >> > >> > --- >> > From: Rafael J. Wysocki <rjw@xxxxxxx> >> > Subject: PM: Allow pm_runtime_suspend() to succeed during system suspend >> > >> > The dpm_prepare() function increments the runtime PM reference >> > counters of all devices to prevent pm_runtime_suspend() from >> > executing subsystem-level callbacks. However, this was supposed to >> > guard against a specific race condition that cannot happen, because >> > the power management workqueue is freezable, so pm_runtime_suspend() >> > can only be called synchronously during system suspend and we can >> > rely on subsystems and device drivers to avoid doing that >> > unnecessarily. >> > >> > Make dpm_prepare() drop the runtime PM reference to each device >> > after making sure that runtime resume is not pending for it. >> > >> > Signed-off-by: Rafael J. Wysocki <rjw@xxxxxxx> >> > --- >> >> Yes! >> >> Acked-by: Kevin Hilman <khilman@xxxxxx> > > Well, I hope you realize that it doesn't help you a lot? > If you mean that because we still have to implement system PM methods because of /sys/devices/.../power/control, I agree. If something else, please explain. But to me it is still very helpful in terms of consistency and what driver writers would expect to happen if they used pm_runtime_suspend() in their system suspend method. Thanks, Kevin -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
