[PATCH v1] PM: runtime: Update last_busy before calling rpm_suspend() in rpm_idle()

["Rafael J. Wysocki" <rafael@kernel.org>]

From: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>

When rpm_idle() is about to call rpm_suspend(), there are no reasons
preventing the device from being suspended, but if the device uses an
autosuspend delay, it needs to be taken into account.

For this purpose, rpm_idle() adds RPM_AUTO to the flags passed to
rpm_suspend(), but doing that alone is insufficient because the point
in time from which the autosuspend delay is counted may be too early.

For example, if the device has children (or consumers) and the last
of them goes into runtime suspend, the parent may now become eligible
for suspending, but it may not be worthwhile for it to suspend until
a certain time later.

Quoting Alan Stern [1]:

 Let's say the last remaining active child goes into runtime suspend at
 time T.  Let's also say that the parent expects none of the children
 will need to resume until time T + D.  Is D sufficiently large to make
 it worthwhile for the parent to suspend and then resume?  If not, the
 parent should remain active.  If yes, the parent should autosuspend --
 and the length of time the parent should wait before autosuspending is
 on the order of the value of D (let's call it D*) that corresponds to
 this crossover point.  Thus the parent's autosuspend delay should be
 set to D*, or something close to it, in order to achieve both
 objectives.

Moreover, if the parent's autosuspend delay is set to D*, it should be
counted from time T at which the last child has suspended, but that does
not happen because the parent's last_busy timestamp is not updated at
time T.  That leads to undesirable parent suspends occurring between
child suspend and resume [2].

To address this issue, make rpm_idle() update the last_busy timestamp
of the device when it is about to call rpm_suspend() which most often
is right after the device's last child (or consumer) has gone into
runtime suspend.

Updating the last_busy timestamp when rpm_idle() returns an error
without calling rpm_suspend() is not useful because the device is then
going to remain active for an unknown amount of time and its autosuspend
delay needs to take effect later.

Link: https://lore.kernel.org/linux-pm/fc43eb13-816b-4cd0-9711-848fdab605ef@rowland.harvard.edu/ [1]
Link: https://lore.kernel.org/linux-pm/CAJZ5v0hKpjMsGQm8Cpa89x5wkNxqkggDNbp+eQ0S-E52DETWjQ@mail.gmail.com/ [2]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
 drivers/base/power/runtime.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/drivers/base/power/runtime.c
+++ b/drivers/base/power/runtime.c
@@ -560,7 +560,11 @@ static int rpm_idle(struct device *dev,
 
  out:
 	trace_rpm_return_int(dev, _THIS_IP_, retval);
-	return retval ? retval : rpm_suspend(dev, rpmflags | RPM_AUTO);
+	if (retval)
+		return retval;
+
+	pm_runtime_mark_last_busy(dev);
+	return rpm_suspend(dev, rpmflags | RPM_AUTO);
 }
 
 /**