idr: Add idr_alloc_u32 helper

All current users of idr_alloc_ext() actually want to allocate a u32
and idr_alloc_u32() fits their needs better.

Like idr_get_next(), it uses a 'nextid' argument which serves as both
a pointer to the start ID and the assigned ID (instead of a separate
minimum and pointer-to-assigned-ID argument).  It uses a 'max' argument
rather than 'end' because the semantics that idr_alloc has for 'end'
don't work well for unsigned types.

Since idr_alloc_u32() returns an errno instead of the allocated ID, mark
it as __must_check to help callers use it correctly.  Include copious
kernel-doc.  Chris Mi <chrism@mellanox.com> has promised to contribute
test-cases for idr_alloc_u32.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>

include/linux/idr.h

@@ -131,6 +131,8 @@ static inline int idr_alloc_ext(struct idr *idr, void *ptr,
 	return idr_alloc_cmn(idr, ptr, index, start, end, gfp, true);
 }
 
+int __must_check idr_alloc_u32(struct idr *, void *ptr, u32 *nextid,
+				unsigned long max, gfp_t);
 int idr_alloc_cyclic(struct idr *, void *entry, int start, int end, gfp_t);
 int idr_for_each(const struct idr *,
 		 int (*fn)(int id, void *p, void *data), void *data);

lib/idr.c

@@ -7,6 +7,37 @@
 DEFINE_PER_CPU(struct ida_bitmap *, ida_bitmap);
 static DEFINE_SPINLOCK(simple_ida_lock);
 
+/**
+ * idr_alloc_u32() - Allocate an ID.
+ * @idr: IDR handle.
+ * @ptr: Pointer to be associated with the new ID.
+ * @nextid: Pointer to an ID.
+ * @max: The maximum ID to allocate (inclusive).
+ * @gfp: Memory allocation flags.
+ *
+ * Allocates an unused ID in the range specified by @nextid and @max.
+ * Note that @max is inclusive whereas the @end parameter to idr_alloc()
+ * is exclusive.
+ *
+ * The caller should provide their own locking to ensure that two
+ * concurrent modifications to the IDR are not possible.  Read-only
+ * accesses to the IDR may be done under the RCU read lock or may
+ * exclude simultaneous writers.
+ *
+ * Return: 0 if an ID was allocated, -ENOMEM if memory allocation failed,
+ * or -ENOSPC if no free IDs could be found.  If an error occurred,
+ * @nextid is unchanged.
+ */
+int idr_alloc_u32(struct idr *idr, void *ptr, u32 *nextid,
+			unsigned long max, gfp_t gfp)
+{
+	unsigned long tmp = *nextid;
+	int ret = idr_alloc_ext(idr, ptr, &tmp, tmp, max + 1, gfp);
+	*nextid = tmp;
+	return ret;
+}
+EXPORT_SYMBOL_GPL(idr_alloc_u32);
+
 int idr_alloc_cmn(struct idr *idr, void *ptr, unsigned long *index,
 		  unsigned long start, unsigned long end, gfp_t gfp,
 		  bool ext)