ARM: 7136/1: pl330: Fix a race condition

If two requests have been submitted and one of them is running, if you
call pl330_chan_ctrl(ch_id, PL330_OP_START), there's a window of time
between the spin_lock_irqsave() and the _state() check in which the
running transaction may finish.  In that case, we don't receive the
interrupt (because they are disabled), but _start() sees that the DMA
is stopped, so it starts it.  The problem is that it sends the
transaction that has just finished again, because pl330_update()
hasn't mark it as done yet.

This patch fixes this race condition by not calling _start() if the
DMA is already executing transactions.  When interrupts are reenabled,
pl330_update() will call _start().

Reference: <1317892206-3600-1-git-send-email-javi.merino@arm.com>
Signed-off-by: NJavi Merino <javi.merino@arm.com>
Acked-by: NJassi Brar <jassi.brar@samsung.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/common/pl330.c

@@ -1546,7 +1546,7 @@ int pl330_chan_ctrl(void *ch_id, enum pl330_chan_op op)
 
 		/* Start the next */
 	case PL330_OP_START:
-		if (!_start(thrd))
+		if (!_thrd_active(thrd) && !_start(thrd))
 			ret = -EIO;
 		break;