dma/ep93xx_dma: prevent ep93xx_dma_tasklet() to reference an empty list

If dma_terminate_all() is called before the ep93xx_dma_tasklet() gets to run,
it tries to access an empty ->active list which results following OOPS:

  Internal error: Oops - undefined instruction: 0 [#1]
  CPU: 0    Not tainted  (3.2.0-rc1EP-1+ #1008)
  PC is at 0xc184c868
  LR is at ep93xx_dma_tasklet+0xec/0x164
  pc : [<c184c868>]    lr : [<c012b528>]    psr: 00000013
  sp : c02b7e70  ip : ffffffff  fp : c02b7ea4
  r10: 00000100  r9 : 80000013  r8 : c02b7e50
  r7 : c02b7e70  r6 : c02b7ea4  r5 : 000000a4  r4 : c02b7e70
  r3 : c02b751d  r2 : 8ae34598  r1 : c184c6e0  r0 : c02b7ea4
  Flags: nzcv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment kernel
  Control: c000717f  Table: c0004000  DAC: 00000017
  Process swapper (pid: 0, stack limit = 0xc02b6270)
  Stack: (0xc02b7e70 to 0xc02b8000)
  7e60:                                     c02b7ea4 c02b7e70 c0008b64 c02bd5c4
  7e80: c02d60e0 00000000 00000000 c02bd44c c02d60e0 00000100 c02b7ec4 c02b7ea8
  7ea0: c001c49c c012b44c 00000018 00000001 c02d60e0 c02b6000 c02b7f04 c02b7ec8
  7ec0: c001cbc0 c001c3e4 c02b7eec c02b7ed8 00000006 0000000a c02bf674 c02c458c
  7ee0: 00000011 00000000 c02b7f7c c0004000 41129200 c02b0c80 c02b7f14 c02b7f08
  7f00: c001cdd0 c001cb38 c02b7f34 c02b7f18 c000983c c001cd98 c0009a60 60000013
  7f20: fefb0001 c02b7f7c c02b7f44 c02b7f38 c0008190 c0009810 c02b7f9c c02b7f48
  7f40: c0008b64 c0008190 c02c2bf8 00000002 c02b7f90 60000013 c02b6000 c02d1504
  7f60: c02baa88 c02baa80 c0004000 41129200 c02b0c80 c02b7f9c c02b7fa0 c02b7f90
  7f80: c0009a54 c0009a60 60000013 ffffffff c02b7fbc c02b7fa0 c000a03c c0009a40
  7fa0: c02b80b0 c02b19dc c02b19d8 c02baa80 c02b7fcc c02b7fc0 c02384e4 c0009fd4
  7fc0: c02b7ff4 c02b7fd0 c029d924 c0238494 c029d49c 00000000 00000000 c02b19dc
  7fe0: c0007175 c02b803c 00000000 c02b7ff8 c000803c c029d700 00000000 00000000
  Backtrace:
  [<c012b43c>] (ep93xx_dma_tasklet+0x0/0x164) from [<c001c49c>] (tasklet_action+0xc8/0xdc)
  [<c001c3d4>] (tasklet_action+0x0/0xdc) from [<c001cbc0>] (__do_softirq+0x98/0x154)
   r7:c02b6000 r6:c02d60e0 r5:00000001 r4:00000018
  [<c001cb28>] (__do_softirq+0x0/0x154) from [<c001cdd0>] (irq_exit+0x48/0x50)
  [<c001cd88>] (irq_exit+0x0/0x50) from [<c000983c>] (handle_IRQ+0x3c/0x8c)
  [<c0009800>] (handle_IRQ+0x0/0x8c) from [<c0008190>] (asm_do_IRQ+0x10/0x14)
   r7:c02b7f7c r6:fefb0001 r5:60000013 r4:c0009a60
  [<c0008180>] (asm_do_IRQ+0x0/0x14) from [<c0008b64>] (__irq_svc+0x24/0xc0)
  Exception stack(0xc02b7f48 to 0xc02b7f90)
  7f40:                   c02c2bf8 00000002 c02b7f90 60000013 c02b6000 c02d1504
  7f60: c02baa88 c02baa80 c0004000 41129200 c02b0c80 c02b7f9c c02b7fa0 c02b7f90
  7f80: c0009a54 c0009a60 60000013 ffffffff
  [<c0009a30>] (default_idle+0x0/0x34) from [<c000a03c>] (cpu_idle+0x78/0xb0)
  [<c0009fc4>] (cpu_idle+0x0/0xb0) from [<c02384e4>] (rest_init+0x60/0x78)
   r7:c02baa80 r6:c02b19d8 r5:c02b19dc r4:c02b80b0
  [<c0238484>] (rest_init+0x0/0x78) from [<c029d924>] (start_kernel+0x234/0x278)
  [<c029d6f0>] (start_kernel+0x0/0x278) from [<c000803c>] (0xc000803c)
   r5:c02b803c r4:c0007175
  Code: 42555300 54535953 643d4d45 65766972 (53007372)

To make the code a bit more robust against things like these, we modify
ep93xx_dma_get_active() to return NULL in case of empty ->active list and make
sure that callers handle this correctly.

Reported-by: Rafal Prylowski <prylowski@metasoft.pl>
Signed-off-by: Mika Westerberg <mika.westerberg@iki.fi>
Acked-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>

drivers/dma/ep93xx_dma.c

@@ -246,6 +246,9 @@ static void ep93xx_dma_set_active(struct ep93xx_dma_chan *edmac,
 static struct ep93xx_dma_desc *
 ep93xx_dma_get_active(struct ep93xx_dma_chan *edmac)
 {
+	if (list_empty(&edmac->active))
+		return NULL;
+
 	return list_first_entry(&edmac->active, struct ep93xx_dma_desc, node);
 }
 
@@ -263,16 +266,22 @@ ep93xx_dma_get_active(struct ep93xx_dma_chan *edmac)
  */
 static bool ep93xx_dma_advance_active(struct ep93xx_dma_chan *edmac)
 {
+	struct ep93xx_dma_desc *desc;
+
 	list_rotate_left(&edmac->active);
 
 	if (test_bit(EP93XX_DMA_IS_CYCLIC, &edmac->flags))
 		return true;
 
+	desc = ep93xx_dma_get_active(edmac);
+	if (!desc)
+		return false;
+
 	/*
 	 * If txd.cookie is set it means that we are back in the first
 	 * descriptor in the chain and hence done with it.
 	 */
-	return !ep93xx_dma_get_active(edmac)->txd.cookie;
+	return !desc->txd.cookie;
 }
 
 /*
@@ -327,9 +336,15 @@ static void m2p_hw_shutdown(struct ep93xx_dma_chan *edmac)
 
 static void m2p_fill_desc(struct ep93xx_dma_chan *edmac)
 {
-	struct ep93xx_dma_desc *desc = ep93xx_dma_get_active(edmac);
+	struct ep93xx_dma_desc *desc;
 	u32 bus_addr;
 
+	desc = ep93xx_dma_get_active(edmac);
+	if (!desc) {
+		dev_warn(chan2dev(edmac), "M2P: empty descriptor list\n");
+		return;
+	}
+
 	if (ep93xx_dma_chan_direction(&edmac->chan) == DMA_MEM_TO_DEV)
 		bus_addr = desc->src_addr;
 	else
@@ -491,7 +506,13 @@ static void m2m_hw_shutdown(struct ep93xx_dma_chan *edmac)
 
 static void m2m_fill_desc(struct ep93xx_dma_chan *edmac)
 {
-	struct ep93xx_dma_desc *desc = ep93xx_dma_get_active(edmac);
+	struct ep93xx_dma_desc *desc;
+
+	desc = ep93xx_dma_get_active(edmac);
+	if (!desc) {
+		dev_warn(chan2dev(edmac), "M2M: empty descriptor list\n");
+		return;
+	}
 
 	if (edmac->buffer == 0) {
 		writel(desc->src_addr, edmac->regs + M2M_SAR_BASE0);
@@ -669,24 +690,30 @@ static void ep93xx_dma_tasklet(unsigned long data)
 {
 	struct ep93xx_dma_chan *edmac = (struct ep93xx_dma_chan *)data;
 	struct ep93xx_dma_desc *desc, *d;
-	dma_async_tx_callback callback;
-	void *callback_param;
+	dma_async_tx_callback callback = NULL;
+	void *callback_param = NULL;
 	LIST_HEAD(list);
 
 	spin_lock_irq(&edmac->lock);
+	/*
+	 * If dma_terminate_all() was called before we get to run, the active
+	 * list has become empty. If that happens we aren't supposed to do
+	 * anything more than call ep93xx_dma_advance_work().
+	 */
 	desc = ep93xx_dma_get_active(edmac);
-	if (desc->complete) {
-		edmac->last_completed = desc->txd.cookie;
-		list_splice_init(&edmac->active, &list);
+	if (desc) {
+		if (desc->complete) {
+			edmac->last_completed = desc->txd.cookie;
+			list_splice_init(&edmac->active, &list);
+		}
+		callback = desc->txd.callback;
+		callback_param = desc->txd.callback_param;
 	}
 	spin_unlock_irq(&edmac->lock);
 
 	/* Pick up the next descriptor from the queue */
 	ep93xx_dma_advance_work(edmac);
 
-	callback = desc->txd.callback;
-	callback_param = desc->txd.callback_param;
-
 	/* Now we can release all the chained descriptors */
 	list_for_each_entry_safe(desc, d, &list, node) {
 		/*
@@ -706,13 +733,22 @@ static void ep93xx_dma_tasklet(unsigned long data)
 static irqreturn_t ep93xx_dma_interrupt(int irq, void *dev_id)
 {
 	struct ep93xx_dma_chan *edmac = dev_id;
+	struct ep93xx_dma_desc *desc;
 	irqreturn_t ret = IRQ_HANDLED;
 
 	spin_lock(&edmac->lock);
 
+	desc = ep93xx_dma_get_active(edmac);
+	if (!desc) {
+		dev_warn(chan2dev(edmac),
+			 "got interrupt while active list is empty\n");
+		spin_unlock(&edmac->lock);
+		return IRQ_NONE;
+	}
+
 	switch (edmac->edma->hw_interrupt(edmac)) {
 	case INTERRUPT_DONE:
-		ep93xx_dma_get_active(edmac)->complete = true;
+		desc->complete = true;
 		tasklet_schedule(&edmac->tasklet);
 		break;