> > If you want to suspend the whole system this is what you would likely
> > do anyway, as you potentially need to allocate a lot of ram or write
> > the system state to swap.
>
> Except on the most power-sensitive systems, which won't have swap
> space (or disks!) to worry about, just some flash and DRAM and a
> small battery that can, with care, handle several days' usage ...
True. It is not a device power management issue.
> > Plus, do we care? Isn't it a rare case? I smell premature optimisation.
>
> ... by routinely putting as many devices as possible into low
> power states (turning off or reducing clocks and power supplies),
> and leaving them in those states whenever possible.
>
> In those devices it's ludicrous to think about any system state
> other than "partially suspended" as the design center. And in
> the same vein, it's rather nonsensical to have a "PM core" which
> makes those systems even harder to build.
But we have this problem only if we have several low power state
which are able to change power state without waking up.
These are a minority. If you design for low power the prefered
state is off, which you cannot go lower from.
> > [..]
> >
> >>latter is true there's nothing much to worry about. Likewise, if the
> >>device has only one reduced-power mode (as with USB) there's no problem.
> >>In cases like these, waking up in order to go back to sleep seems silly.
> >
> >
> > Only if you think strictly in terms of USB. Consider the case of a bluetooth
> > host on USB.
>
> In which case, let that bluetooth driver cope with its PM quirks.
> Don't try to make the PM core code know about them all.
We want one for worst case, don't we? Designing with exception
planned up front is IMHO bad.
Regards
Oliver
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