x86: GEODE fix a race condition in the MFGPT timer tick

When we set the MFGPT timer tick, there is a chance that we'll
immediately assert an event. If for some reason the IRQ routing
for this clock has been setup for some other purpose, then we
could end up firing an interrupt into the SMM handler or worse.

This rearranges the timer tick init function to initalize the handler
before we set up the MFGPT clock to make sure that even if we get
an event, it will go to the handler.

Furthermore, in the handler we need to make sure that we clear the
event, even if the timer isn't running.

Signed-off-by: Jordan Crouse <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Acked-by: Ingo Molnar <[e-mail]>
Tested-by: Arnd Hannemann <[e-mail]>

arch/x86/kernel/mfgpt_32.c

···
 
 static irqreturn_t mfgpt_tick(int irq, void *dev_id)
 {
+	/* Turn off the clock (and clear the event) */
+	mfgpt_disable_timer(mfgpt_event_clock);
+
 	if (mfgpt_tick_mode == CLOCK_EVT_MODE_SHUTDOWN)
 		return IRQ_HANDLED;
-
-	/* Turn off the clock */
-	mfgpt_disable_timer(mfgpt_event_clock);
 
 	/* Clear the counter */
 	geode_mfgpt_write(mfgpt_event_clock, MFGPT_REG_COUNTER, 0);
···
 	}
 
 	mfgpt_event_clock = timer;
-	/* Set the clock scale and enable the event mode for CMP2 */
-	val = MFGPT_SCALE | (3 << 8);
-
-	geode_mfgpt_write(mfgpt_event_clock, MFGPT_REG_SETUP, val);
 
 	/* Set up the IRQ on the MFGPT side */
 	if (geode_mfgpt_setup_irq(mfgpt_event_clock, MFGPT_CMP2, irq)) {
···
 		       "mfgpt-timer:  Unable to set up the interrupt.\n");
 		goto err;
 	}
+
+	/* Set the clock scale and enable the event mode for CMP2 */
+	val = MFGPT_SCALE | (3 << 8);
+
+	geode_mfgpt_write(mfgpt_event_clock, MFGPT_REG_SETUP, val);
 
 	/* Set up the clock event */
 	mfgpt_clockevent.mult = div_sc(MFGPT_HZ, NSEC_PER_SEC, 32);