Re: [linux-pm] [PATCH 02/11] PM: extend PM QoS with per-device wake-up constraints

[Mark Brown <broonie@opensource.wolfsonmicro.com>]

On Sat, Aug 20, 2011 at 11:35:58AM +0200, Rafael J. Wysocki wrote:

> I'm not sure what you mean by "exposing per-device wakeup latency constraints
> to user space".  I simply thought it might be useful to allow user space to
> add and remove those, in analogy to what it can do with the currently available
> PM QoS constraints.

I linked to the mails from Kevin...  To be honest I'm rather surprised
this stuff is getting exposed directly to userspace with write support.

> My point is that this won't prevent kernel subsystems from adding their
> own PM QoS constraints to devices and the constraints added by user space
> will only have effect if they make the devices stay active for longer
> times.  In consequence, they may only increase the energy usage of the system
> and shouldn't affect functionality.

Well, clearly that's not the only use - demand for increased energy
consumption by itself from users is generally quite low :)

> The reason why I think this may be useful is that I can readily imagine
> scenarios in which user space (think a distribution or system vendor) will
> want to do something that hasn't been anticipated by kernel developers
> and the ability to add per-device PM QoS constraints will allow it to do that

Coming at this from the embedded perspective modifying the kernel just
isn't an issue.  It's quite depressing in other cases too but some of
the circumstances you mentioned in previous messages are sensible do
make sense to me.  It does seem like this is a system specific problem
which we should be able to enable as part of the support for those
systems.

> without modifying the kernel.  Also, it will help to test and debug new drivers
> and subsystems.

If it were a debugfs facility I'd not be concerned.

> I don't want to prevent kernel subsystems from using their own interfaces to
> acquire user-level input that translates into PM QoS constraints.  I simply
> think it won't _hurt_ to provide user space with an additional level of
> control over per-device PM QoS constraints.

My experience has been that once you start providing an interface people
will find it, start using it and it takes a lot of work to convince them
that they ought to do something better.  This is totally reasonable from
when you look at things from their point of view, especially people who
aren't used to being able to improve anything except their own driver.

> > I'm additionally concerned that if we expose this stuff directly to userspace
> > that's an open invitation to driver authors to not even bother trying to make
> > the kernel figure this stuff out by itself and to instead tie the system
> > together with magic userspace.

> As I said, I don't think we can prevent that from happening anyway.
> Worst case people will add strange interfaces to drivers making them
> incompatible with anything except for their crazy user space.

That's not really a problem - if people are adding their own crazy
interfaces it's clear that they've done that.  It'll show up as a red
flag to anyone looking at their stuff and this will create pressure on
them to fix their code or at least do a better job for the next thing.

The goal isn't to tie people's hands to stop them doing silly things,
it's to make it clear that that is what they're doing.