dmaengine: stm32-dma: fix potential race between pause and resume

When disabling dma channel, a TCF flag is set and as TCIE is enabled, an
interrupt is raised.
On a busy system, the interrupt may have latency and the user can ask for
dmaengine_resume while stm32-dma driver has not yet managed the complete
pause (backup of registers to restore state in resume).
To avoid such a case, instead of waiting the interrupt to backup the
registers, do it just after disabling the channel and discard Transfer
Complete interrupt in case the channel is paused.

Change-Id: I8d07cd3a2cbb9ff3056b104fea6cab10d90c32fd
Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/linux-stm32/+/252593
Reviewed-by: CITOOLS <MDG-smet-aci-reviews@list.st.com>
Reviewed-by: Fabien DESSENNE <fabien.dessenne@foss.st.com>

Author: ADESTM

drivers/dma/stm32-dma.c

@@ -1065,6 +1065,8 @@ static void stm32_dma_handle_chan_paused(struct stm32_dma_chan *chan)
 
 	chan->chan_reg.dma_sndtr = stm32_dma_read(dmadev, STM32_DMA_SNDTR(chan->id));
 
+	chan->status = DMA_PAUSED;
+
 	dev_dbg(chan2dev(chan), "vchan %pK: paused\n", &chan->vchan);
 }
 
@@ -1182,9 +1184,7 @@ static irqreturn_t stm32_dma_chan_irq(int irq, void *devid)
 	if (status & STM32_DMA_TCI) {
 		stm32_dma_irq_clear(chan, STM32_DMA_TCI);
 		if (scr & STM32_DMA_SCR_TCIE) {
-			if (chan->status == DMA_PAUSED && !(scr & STM32_DMA_SCR_EN))
-				stm32_dma_handle_chan_paused(chan);
-			else
+			if (chan->status != DMA_PAUSED)
 				stm32_dma_handle_chan_done(chan, scr);
 		}
 		status &= ~STM32_DMA_TCI;
@@ -1237,13 +1237,11 @@ static int stm32_dma_pause(struct dma_chan *c)
 		return -EPERM;
 
 	spin_lock_irqsave(&chan->vchan.lock, flags);
+
 	ret = stm32_dma_disable_chan(chan);
-	/*
-	 * A transfer complete flag is set to indicate the end of transfer due to the stream
-	 * interruption, so wait for interrupt
-	 */
 	if (!ret)
-		chan->status = DMA_PAUSED;
+		stm32_dma_handle_chan_paused(chan);
+
 	spin_unlock_irqrestore(&chan->vchan.lock, flags);
 
 	return ret;