mtd: handle partitioning on devices with 0 erasesize

erasesize is meaningful for flash devices but for SRAM there is no
concept of an erase block so erasesize is set to 0. When partitioning
these devices instead of ensuring partitions fall on erasesize
boundaries we ensure they fall on writesize boundaries.

Helped-by: Boris Brezillon <[e-mail]>
Signed-off-by: Chris Packham <[e-mail]>
Acked-by: Boris Brezillon <[e-mail]>
Signed-off-by: Brian Norris <[e-mail]>

drivers/mtd/mtdpart.c

···
 			const struct mtd_partition *part, int partno,
 			uint64_t cur_offset)
 {
+	int wr_alignment = (master->flags & MTD_NO_ERASE) ? master->writesize:
+							    master->erasesize;
 	struct mtd_part *slave;
+	u32 remainder;
 	char *name;
+	u64 tmp;
 
 	/* allocate the partition structure */
 	slave = kzalloc(sizeof(*slave), GFP_KERNEL);
···
 	if (slave->offset == MTDPART_OFS_APPEND)
 		slave->offset = cur_offset;
 	if (slave->offset == MTDPART_OFS_NXTBLK) {
+		tmp = cur_offset;
 		slave->offset = cur_offset;
-		if (mtd_mod_by_eb(cur_offset, master) != 0) {
-			/* Round up to next erasesize */
-			slave->offset = (mtd_div_by_eb(cur_offset, master) + 1) * master->erasesize;
+		remainder = do_div(tmp, wr_alignment);
+		if (remainder) {
+			slave->offset += wr_alignment - remainder;
 			printk(KERN_NOTICE "Moving partition %d: "
 			       "0x%012llx -> 0x%012llx\n", partno,
 			       (unsigned long long)cur_offset, (unsigned long long)slave->offset);
···
 		slave->mtd.erasesize = master->erasesize;
 	}
 
-	if ((slave->mtd.flags & MTD_WRITEABLE) &&
-	    mtd_mod_by_eb(slave->offset, &slave->mtd)) {
+	tmp = slave->offset;
+	remainder = do_div(tmp, wr_alignment);
+	if ((slave->mtd.flags & MTD_WRITEABLE) && remainder) {
 		/* Doesn't start on a boundary of major erase size */
 		/* FIXME: Let it be writable if it is on a boundary of
 		 * _minor_ erase size though */
 		slave->mtd.flags &= ~MTD_WRITEABLE;
-		printk(KERN_WARNING"mtd: partition \"%s\" doesn't start on an erase block boundary -- force read-only\n",
+		printk(KERN_WARNING"mtd: partition \"%s\" doesn't start on an erase/write block boundary -- force read-only\n",
 			part->name);
 	}
-	if ((slave->mtd.flags & MTD_WRITEABLE) &&
-	    mtd_mod_by_eb(slave->mtd.size, &slave->mtd)) {
+
+	tmp = slave->mtd.size;
+	remainder = do_div(tmp, wr_alignment);
+	if ((slave->mtd.flags & MTD_WRITEABLE) && remainder) {
 		slave->mtd.flags &= ~MTD_WRITEABLE;
-		printk(KERN_WARNING"mtd: partition \"%s\" doesn't end on an erase block -- force read-only\n",
+		printk(KERN_WARNING"mtd: partition \"%s\" doesn't end on an erase/write block -- force read-only\n",
 			part->name);
 	}