Re: [PATCH 0/12] PM / sleep: Driver flags for system suspend/resume

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Oct 16, 2017 at 03:12:35AM +0200, Rafael J. Wysocki wrote:
> Hi All,
> 
> Well, this took more time than expected, as I tried to cover everything I had
> in mind regarding PM flags for drivers.
> 
> This work was triggered by attempts to fix and optimize PM in the
> i2c-designware-platdev driver that ended up with adding a couple of
> flags to the driver's internal data structures for the tracking of
> device state (https://marc.info/?l=linux-acpi&m=150629646805636&w=2).
> That approach is sort of suboptimal, though, because other drivers will
> probably want to do similar things and if all of them need to use internal
> flags for that, quite a bit of code duplication may ensue at least.
> 
> That can be avoided in a couple of ways and one of them is to provide a means
> for drivers to tell the core what to do and to make the core take care of it
> if told to do so.  Hence, the idea to use driver flags for system-wide PM
> that was briefly discussed during the LPC in LA last month.
> 
> One of the flags considered at that time was to possibly cause the core
> to reuse the runtime PM callback path of a device for system suspend/resume.
> Admittedly, that idea didn't look too bad to me until I had started to try to
> implement it and I got to the PCI bus type's hibernation callbacks.  Then, I
> moved the patch I was working on to /dev/null right away.  I mean it.
> 
> No, this is not going to happen.  No way.
> 
> Moreover, that experience made me realize that the whole *idea* of using the
> runtime PM callback path for system-wide PM was actually totally bogus (sorry
> Ulf).
> 
> The whole point of having different callbacks pointers for different types of
> device transitions is because it may be necessary to do different things in
> those callbacks in general.  Now, if you consider runtime PM and system
> suspend/resume *only* and from a driver perspective, then yes, in some cases
> the same pair of callback routines may be used for all suspend-like and
> resume-like transitions of the device, but if you add hibernation to the mix,
> then it is not so clear any more unless the callbacks don't actually do any
> power management at all, but simply quiesce the device's activity and then
> activate it again.  Namely, changing power states of devices during the
> hibernation's "freeze" and "thaw" transitions rarely makes sense at all and
> the "restore" transition needs to be able to cope with uninitialized devices
> (in fact, it should be prepared to cope with devices in *any* state), so
> runtime PM is hardly suitable for them.  Still, if a *driver* choses to not
> do any real PM in its PM callbacks and leaves that to a middle layer (quite
> a few drivers do that), then it possibly can use one pair of callbacks in all
> cases and be happy, but middle layers pretty much have to use different
> callback routines for different transitions.
> 
> If you are a middle layer, your role is basically to do PM for a certain
> group of devices.  Thus you cannot really do the same in ->suspend or
> ->suspend_early and in ->runtime_suspend (because the former generally need to
> take device_may_wakeup() into account and the latter doesn't) and you shouldn't
> really do the same in ->suspend and ->freeze (becuase the latter shouldn't
> change the device's power state) and so on.  To put it bluntly, trying
> to use the ->runtime_suspend callback of a middle layer for anything other
> than runtime suspend is complete and utter nonsense.  At the same time, the
> ->runtime_resume callback of a middle layer may be reused to some extent,
> but even that doesn't cover the "thaw" transitions during hibernation.
> 
> What can work (and this is the only strategy that can work AFAICS) is to
> point different callback pointers *in* *a* *driver* to the same routine
> if the driver wants to reuse that code.  That actually will work for PCI
> and USB drivers today, at least most of the time, but unfortunately there
> are problems with it for, say, platform devices.
> 
> The first problem is the requirement to track the status of the device
> (suspended vs not suspended) in the callbacks, because the system-wide PM
> code in the PM core doesn't do that.  The runtime PM framework does it, so
> this means adding some extra code which isn't necessary for runtime PM to
> the callback routines and that is not particularly nice.
> 
> The second problem is that, if the driver wants to do anything in its
> ->suspend callback, it generally has to prevent runtime suspend of the
> device from taking place in parallel with that, which is quite cumbersome.
> Usually, that is taken care of by resuming the device from runtime suspend
> upfront, but generally doing that is wasteful (there may be no real need to
> resume the device except for the fact that the code is designed this way).
> 
> On top of the above, there are optimizations to be made, like leaving certain
> devices in suspend after system resume to avoid wasting time on waiting for
> them to resume before user space can run again and similar.
> 
> This patch series focuses on addressing those problems so as to make it
> easier to reuse callback routines by pointing different callback pointers
> to them in device drivers.  The flags introduced here are to instruct the
> PM core and middle layers (whatever they are) on how the driver wants the
> device to be handled and then the driver has to provide callbacks to match
> these instructions and the rest should be taken care of by the code above it.
> 
> The flags are introduced one by one to avoid making too many changes in
> one go and to allow things to be explained better (hopefully).  They mostly
> are mutually independent with some clearly documented exceptions.
> 
> The first three patches in the series are about an issue with the
> direct-complete optimization introduced some time ago in which some middle
> layers decide on whether or not to do the optimization without asking the
> drivers.  And, as it turns out, in some cases the drivers actually know
> better, so the new flags introduced by these patches are here for these
> drivers (and the DPM_FLAG_NEVER_SKIP one is really to avoid having to define
> ->prepare callbacks always returning zero).
> 
> The really interesting things start to happen in patches [4-9/12] which make it
> possible to avoid resuming devices from runtime suspend upfront during system
> suspend at least in some cases (and when direct-complete is not applied to the
> devices in question), but please refer to the changelogs for details.
> 
> The i2d-designware-platdev driver is used as the primary example in the series
> and the patches modifying it are based on some previous changes currently in
> linux-next AFAICS (the same applies to the intel-lpss driver), but these
> patches can wait until everything is properly merged.  They are included here
> mostly as illustration.
> 
> Overall, the series is based on the linux-next branch of the linux-pm.git tree
> with some extra patches on top of it and all of the names of new entities
> introduced in it are negotiable.

Thanks for the great explaination, I was wondering how your proposal
discussed at Plumbers was going to work out in the end :)

The patch series looks good to me (minor questions already sent on the
patches), but what does this mean for drivers?  Do they now have to do a
lot of work to take advantage of this, like you did for the
i2d-designware-platdev driver?  Or will things continue to work as-is
and it's only an opt-in type thing where the bus/driver wants to take
advantage of it?

thanks,

greg k-h