Support 'discard sectors' operation.

We can benefit from knowing that the file system no longer cares about
the contents of certain sectors, by throwing them away immediately and
then never having to garbage collect them, and using the extra free
space to make our operations more efficient. Do so.
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

drivers/mtd/ftl.c

@@ -1005,6 +1005,29 @@ static int ftl_writesect(struct mtd_blktrans_dev *dev,
 	return ftl_write((void *)dev, buf, block, 1);
 }
 
+static int ftl_discardsect(struct mtd_blktrans_dev *dev,
+			   unsigned long sector, unsigned nr_sects)
+{
+	partition_t *part = (void *)dev;
+	uint32_t bsize = 1 << part->header.EraseUnitSize;
+
+	DEBUG(1, "FTL erase sector %ld for %d sectors\n",
+	      sector, nr_sects);
+
+	while (nr_sects) {
+		uint32_t old_addr = part->VirtualBlockMap[sector];
+		if (old_addr != 0xffffffff) {
+			part->VirtualBlockMap[sector] = 0xffffffff;
+			part->EUNInfo[old_addr/bsize].Deleted++;
+			if (set_bam_entry(part, old_addr, 0))
+				return -EIO;
+		}
+		nr_sects--;
+		sector++;
+	}
+
+	return 0;
+}
 /*====================================================================*/
 
 static void ftl_freepart(partition_t *part)
@@ -1069,6 +1092,7 @@ static struct mtd_blktrans_ops ftl_tr = {
 	.blksize 	= SECTOR_SIZE,
 	.readsect	= ftl_readsect,
 	.writesect	= ftl_writesect,
+	.discard	= ftl_discardsect,
 	.getgeo		= ftl_getgeo,
 	.add_mtd	= ftl_add_mtd,
 	.remove_dev	= ftl_remove_dev,