dmaengine: edma: Document variables used for residue accounting

The granular residue accounting code uses certain variables specifically
for residue accounting. Document these in the structure declaration.
Also move around some elements and group them together.

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Joel Fernandes <joelf@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/dma/edma.c

@@ -70,12 +70,34 @@ struct edma_desc {
 	int				cyclic;
 	int				absync;
 	int				pset_nr;
+	struct edma_chan		*echan;
 	int				processed;
+
+	/*
+	 * The following 4 elements are used for residue accounting.
+	 *
+	 * - processed_stat: the number of SG elements we have traversed
+	 * so far to cover accounting. This is updated directly to processed
+	 * during edma_callback and is always <= processed, because processed
+	 * refers to the number of pending transfer (programmed to EDMA
+	 * controller), where as processed_stat tracks number of transfers
+	 * accounted for so far.
+	 *
+	 * - residue: The amount of bytes we have left to transfer for this desc
+	 *
+	 * - residue_stat: The residue in bytes of data we have covered
+	 * so far for accounting. This is updated directly to residue
+	 * during callbacks to keep it current.
+	 *
+	 * - sg_len: Tracks the length of the current intermediate transfer,
+	 * this is required to update the residue during intermediate transfer
+	 * completion callback.
+	 */
 	int				processed_stat;
-	u32				residue;
 	u32				sg_len;
+	u32				residue;
 	u32				residue_stat;
-	struct edma_chan		*echan;
+
 	struct edma_pset		pset[0];
 };