[PATCH v2 11/19] cdev: have devfs_add_partition return existing identical partition, not NULL

[Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx>]

Starting with commit 7f9f45b9bfef ("devfs: Do not create overlapping
partitions"), any overlapping is disallowed. Overlapping can be useful
though to bridge the gap between partition described in DT and via
on-disk partition tables. Let's handle the case of identical partitions
specially and have it neither be an error or a duplicate partition, but
instead just return the existing partition and create a cdev link to it
if the name differs. This existing partition will be given a device tree
node and thus enabling schemes like:

  &{/state} {
  	backend = <&state_part>;
  };

  &mmc1 {
         partitions {
                 compatible = "fixed-partitions";
                 #address-cells = <2>;
                 #size-cells = <2>;

                 state_part: partition@5300000 {
                         label = "barebox-state";
			 /* will be folded with overlapping GPT partition if found */
                         reg = <0x0 0x5300000 0x0 0x100000>;
                 };
         };
  };

Signed-off-by: Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx>
---
v1 -> v2:
  - create link to existing cdev if name differs
  - add identical: goto label to make intent clearer
---
 fs/devfs-core.c | 57 +++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 46 insertions(+), 11 deletions(-)

diff --git a/fs/devfs-core.c b/fs/devfs-core.c
index 72a37918962b..c9f7fcfb076d 100644
--- a/fs/devfs-core.c
+++ b/fs/devfs-core.c
@@ -405,6 +405,12 @@ int devfs_remove(struct cdev *cdev)
 	return 0;
 }
 
+static bool region_identical(loff_t starta, loff_t lena,
+			     loff_t startb, loff_t lenb)
+{
+	return starta == startb && lena == lenb;
+}
+
 static bool region_overlap(loff_t starta, loff_t lena,
 			   loff_t startb, loff_t lenb)
 {
@@ -415,10 +421,22 @@ static bool region_overlap(loff_t starta, loff_t lena,
 	return 1;
 }
 
-static int check_overlap(struct cdev *cdev, const char *name, loff_t offset, loff_t size)
+/**
+ * check_overlap() - check overlap with existing partitions
+ * @cdev: parent cdev
+ * @name: partition name for informational purposes on conflict
+ * @offset: offset of new partition to be added
+ * @size: size of new partition to be added
+ *
+ * Return: NULL if no overlapping partition found or overlapping
+ *         partition if and only if it's identical in offset and size
+ *         to an existing partition. Otherwise, PTR_ERR(-EINVAL).
+ */
+static struct cdev *check_overlap(struct cdev *cdev, const char *name, loff_t offset, loff_t size)
 {
 	struct cdev *cpart;
 	loff_t cpart_offset;
+	int ret;
 
 	list_for_each_entry(cpart, &cdev->partitions, partition_entry) {
 		cpart_offset = cpart->offset;
@@ -431,20 +449,29 @@ static int check_overlap(struct cdev *cdev, const char *name, loff_t offset, lof
 		if (cpart->mtd)
 			cpart_offset = cpart->mtd->master_offset;
 
-		if (region_overlap(cpart_offset, cpart->size,
-				   offset, size))
+		if (region_identical(cpart_offset, cpart->size, offset, size)) {
+			ret = 0;
+			goto identical;
+		}
+
+		if (region_overlap(cpart_offset, cpart->size, offset, size)) {
+			ret = -EINVAL;
 			goto conflict;
+		}
 	}
 
-	return 0;
+	return NULL;
 
+identical:
 conflict:
-	pr_err("New partition %s (0x%08llx-0x%08llx) on %s "
-		"overlaps with partition %s (0x%08llx-0x%08llx), not creating it\n",
-		name, offset, offset + size - 1, cdev->name,
-		cpart->name, cpart_offset, cpart_offset + cpart->size - 1);
+	__pr_printk(ret ? MSG_WARNING : MSG_DEBUG,
+		    "New partition %s (0x%08llx-0x%08llx) on %s "
+		    "%s with partition %s (0x%08llx-0x%08llx), not creating it\n",
+		    name, offset, offset + size - 1, cdev->name,
+		    ret ? "conflicts" : "identical",
+		    cpart->name, cpart_offset, cpart_offset + cpart->size - 1);
 
-	return -EINVAL;
+	return ret ? ERR_PTR(ret) : cpart;
 }
 
 static struct cdev *__devfs_add_partition(struct cdev *cdev,
@@ -452,6 +479,7 @@ static struct cdev *__devfs_add_partition(struct cdev *cdev,
 {
 	loff_t offset, size;
 	static struct cdev *new;
+	struct cdev *overlap;
 
 	if (cdev_by_name(partinfo->name))
 		return ERR_PTR(-EEXIST);
@@ -482,8 +510,15 @@ static struct cdev *__devfs_add_partition(struct cdev *cdev,
 		return ERR_PTR(-EINVAL);
 	}
 
-	if (check_overlap(cdev, partinfo->name, offset, size))
-		return ERR_PTR(-EINVAL);
+	overlap = check_overlap(cdev, partinfo->name, offset, size);
+	if (overlap) {
+		if (!IS_ERR(overlap)) {
+			/* only fails with -EEXIST, which is fine */
+			(void)devfs_create_link(overlap, partinfo->name);
+		}
+
+		return overlap;
+	}
 
 	if (IS_ENABLED(CONFIG_MTD) && cdev->mtd) {
 		struct mtd_info *mtd;
-- 
2.39.2