linux-kernel - Re: [linux-pm] [PATCH 1/8] PM: Add suspend block api.

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Message-ID: <1273268637.3542.130.camel@c-dwalke-linux.qualcomm.com>
Date:	Fri, 07 May 2010 14:43:57 -0700
From:	Daniel Walker <dwalker@...o99.com>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	Matthew Garrett <mjg@...hat.com>, Tony Lindgren <tony@...mide.com>,
	Brian Swetland <swetland@...gle.com>,
	Alan Stern <stern@...land.harvard.edu>,
	mark gross <mgross@...ux.intel.com>, markgross@...gnar.org,
	Len Brown <len.brown@...el.com>, linux-doc@...r.kernel.org,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [linux-pm] [PATCH 1/8] PM: Add suspend block api.

On Fri, 2010-05-07 at 14:35 -0700, Arve Hjønnevåg wrote:
> On Fri, May 7, 2010 at 2:30 PM, Daniel Walker <dwalker@...o99.com> wrote:
> > On Fri, 2010-05-07 at 22:03 +0100, Matthew Garrett wrote:
> >
> >> Here's a different example. A process is waiting for a keypress, but
> >> because it's badly written it's also drawing to the screen at 60 frames
> >> per second and preventing the system from every going to idle. How do
> >> you quiesce the system while still ensuring that the keypress will be
> >> delivered to the application?
> >
> > To me it's somewhat of a negative for suspend blockers. Since to solve
> > the problem you give above you would have to use a suspend blocker in an
> > asynchronous way (locked in an interrupt, released in a thread too)
> > assuming I understand your example. I've had my share of semaphore
> > nightmares, and I'm not too excited to see a protection scheme (i.e. a
> > lock) which allows asynchronous usage like suspend blockers.
> >
> 
> Why do you think this? The example in the documentation describe how
> we handle key events.

+- The Keypad driver gets an interrupt. It then calls suspend_block on the
+  keypad-scan suspend_blocker and starts scanning the keypad matrix.
+- The keypad-scan code detects a key change and reports it to the input-event
+  driver.
+- The input-event driver sees the key change, enqueues an event, and calls
+  suspend_block on the input-event-queue suspend_blocker.
+- The keypad-scan code detects that no keys are held and calls suspend_unblock
+  on the keypad-scan suspend_blocker.
+- The user-space input-event thread returns from select/poll, calls
+  suspend_block on the process-input-events suspend_blocker and then calls read
+  on the input-event device.
+- The input-event driver dequeues the key-event and, since the queue is now
+  empty, it calls suspend_unblock on the input-event-queue suspend_blocker.
+- The user-space input-event thread returns from read. If it determines that
+  the key should leave the screen off, it calls suspend_unblock on the
+  process_input_events suspend_blocker and then calls select or poll. The
+  system will automatically suspend again, since now no suspend blockers are
+  active.

This? Isn't this asynchronous on the input-event-queue since it's taken
in the interrupt , and release in the userspace thread?

Daniel

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