Messages in this thread | | | Date | Sat, 15 Oct 2011 14:45:37 -0400 (EDT) | From | Alan Stern <> | Subject | Re: [: [RFC] wake up notifications and suspend blocking (aka more wakelock stuff)] |
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On Sat, 15 Oct 2011, NeilBrown wrote:
> On Thu, 13 Oct 2011 11:16:23 -0400 (EDT) Alan Stern > <stern@rowland.harvard.edu> wrote: > > > On Thu, 13 Oct 2011, NeilBrown wrote: > > > > > Nope, but I'm keen for you to convince me. Identify a wakeup event that > > > cannot be made visible to poll (or to user-space by some other > > > mechanism) before the wakeup_source needs to be deactivated. Or if I've > > > misunderstood what sort of notification is problematic, help me understand. > > > > Here's an example (just for kicks, not completely relevant to your > > discussion): A USB keyboard key release. Unlike key presses, key > > releases need not generate input events. If no processes are > > monitoring the raw keyboard event queue then the release is not visible > > to userspace at all, hence not visible before the wakeup_source needs > > to be deactivated. > > > > Alan Stern > > As you say, not completely relevant. > > If a tree falls in a forest with no one to here, does it make a sound? > > similarly if an event happens that no-one is looking for, is it visible? > It doesn't really matter.
That's a different question, but I'll answer it anyway: Yes, it does matter. If the kernel is unable to _know_ that nobody is looking for an event, it has to _assume_ that somebody is. Then what should happen if it turns out that nobody really is looking for it?
> So at most this is a case of "is not made visible" rather than "cannot be > made visible".
In this case it's the same thing. How can a key release be made visible?
> The key-release just needs to clear the "key is pressed" state so that > auto-repeat stops and if it was a modifier, the modification is discarded. > That is all trivially done in some kernel driver while the wakeup_source is > active.
In other words, if the event is discarded from within the kernel then the wakeup_source can be deactivated at that time. That's fine -- but it indicates that your original request above was phrased wrongly. You should have asked for an example of a wakeup_source which the kernel must not deactivate without a userspace handshake, but which cannot be made visible by poll or some other similar mechanism.
Alan Stern
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