dmaengine: imx-sdma - correct the dma transfer residue calculation

The residue calculation was taking in consideration that dma
transaction status will be always retrieved in the dma callback
used to inform that dma transfer is complete. However this is not
the case for all subsystems that use dma. Some subsystems use a
timer to check the dma status periodically.

Therefore the calculation was updated and residue is calculated
accordingly by a) update the residue calculation taking in
consideration the last used buffer index by using *buf_ptail* variable
and b) chn_real_count (number of bytes transferred) is initialized to
zero, when dma channel is created, to avoid using an uninitialized
value in residue calculation when dma status is checked without
waiting dma complete event.

Signed-off-by: Nandor Han <[e-mail]>
Acked-by: Peter Senna Tschudin <[e-mail]>
Tested-by: Peter Senna Tschudin <[e-mail]>
Tested-by: Marek Vasut <[e-mail]>
Signed-off-by: Vinod Koul <[e-mail]>

drivers/dma/imx-sdma.c

···
  * @event_id1		for channels that use 2 events
  * @word_size		peripheral access size
  * @buf_tail		ID of the buffer that was processed
+ * @buf_ptail		ID of the previous buffer that was processed
  * @num_bd		max NUM_BD. number of descriptors currently handling
  */
 struct sdma_channel {
···
 	unsigned int			event_id1;
 	enum dma_slave_buswidth		word_size;
 	unsigned int			buf_tail;
+	unsigned int			buf_ptail;
 	unsigned int			num_bd;
 	unsigned int			period_len;
 	struct sdma_buffer_descriptor	*bd;
···
 		sdmac->chn_real_count = bd->mode.count;
 		bd->mode.status |= BD_DONE;
 		bd->mode.count = sdmac->period_len;
+		sdmac->buf_ptail = sdmac->buf_tail;
+		sdmac->buf_tail = (sdmac->buf_tail + 1) % sdmac->num_bd;
 
 		/*
 		 * The callback is called from the interrupt context in order
···
 		 */
 
 		dmaengine_desc_get_callback_invoke(&sdmac->desc, NULL);
-
-		sdmac->buf_tail++;
-		sdmac->buf_tail %= sdmac->num_bd;
 
 		if (error)
 			sdmac->status = old_status;
···
 	sdmac->flags = 0;
 
 	sdmac->buf_tail = 0;
+	sdmac->buf_ptail = 0;
+	sdmac->chn_real_count = 0;
 
 	dev_dbg(sdma->dev, "setting up %d entries for channel %d.\n",
 			sg_len, channel);
···
 	sdmac->status = DMA_IN_PROGRESS;
 
 	sdmac->buf_tail = 0;
+	sdmac->buf_ptail = 0;
+	sdmac->chn_real_count = 0;
 	sdmac->period_len = period_len;
 
 	sdmac->flags |= IMX_DMA_SG_LOOP;
···
 	u32 residue;
 
 	if (sdmac->flags & IMX_DMA_SG_LOOP)
-		residue = (sdmac->num_bd - sdmac->buf_tail) *
+		residue = (sdmac->num_bd - sdmac->buf_ptail) *
 			   sdmac->period_len - sdmac->chn_real_count;
 	else
 		residue = sdmac->chn_count - sdmac->chn_real_count;