Re: Attempted summary of suspend-blockers LKML thread

From: david
Date: Wed Aug 04 2010 - 19:05:24 EST

On Wed, 4 Aug 2010, Rafael J. Wysocki wrote:

Subject: Re: Attempted summary of suspend-blockers LKML thread

On Wednesday, August 04, 2010, david@xxxxxxx wrote:
On Wed, 4 Aug 2010, Rafael J. Wysocki wrote:
In the suspend case, when you have frozen all applications, you can
sequentially disable all interrupts except for a few selected ("wakeup") ones
in a safe way. By disabling them, you ensure that the CPU will only be
"revived" by a limited set of events and that allows the system to stay
low-power for extended time intervals.

the benifit of this will depend on what wakeups you are able to avoid by
putting the hardware to sleep. Depending on the hardware, this may be not
matter that much.

That's correct, but evidently it does make a difference with the hardware
Android commonly runs on.

Ok, but is there a way to put some of this to sleep without involving a full suspend?

In addition, there are input devices like a touchscreen that could be
disabled even short of a full suspend, as long as there is some way to get
them reactivated.

That's correct again, but it doesn't matter too much as far as the difference
between "deep idle" and suspend is concerned. These devices are generally not
the interrupt sources that are difficult to shut down safely in the "idle" case.

To achieve the same result in the "idle" case, you'll need to have a mechanism
to disable interrupts (except for the "wakeup" ones) avoiding synchronization
problems (eg. situations in which task A, blocked on a "suspended" device
access, holds a mutex waited for by task B that needs to run so that we can
"suspend" another device). That, however, is a difficult problem.

I would say that the difficulty of the problem depends on the hardware and
how much userspace interaction is needed. If the kernel can disable the
hardware in the driver (so that it can wake it up again when accessed by
an application) it would seem that this isn't a problem.

Well, I guess it's useful to consider the sequence of events in the "idle"

We first detect that a CPU is idle, so it can be put into a C-state (or
equivalent). It's easy to put that CPU into such a state, but suppose we
want to do more and put all devices into low-power states at that point.
We have to ensure that there are no devices in the middle of DMA and that
the other CPUs are idle (that's kind of easy, but still). Next, we have to
figure out the right ordering in which to put devices into low-power states
(that is not trivial, because the tasks that use those devices may depend on
each other in various ways). If we decide to shut down clock source
interrupts, we must ensure that the timekeeping will be correct after that and
so on. Moreover, things get ugly if there are shared interrupts.

Suspend kind of works around these difficulties by taking the entire user
space out of the picture with one big sledgehammer called the freezer,
but these problems would have to be actually _solved_ in the "idle" case.

I'm not opposed to doing a suspend if the system is idle enough, I'm just questioning if suspend is really that different, or if it can be viewed as yet another low power mode (one that's significantly more expensive to transition in and out of)

David Lang
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