idr: Add documentation

Move the idr kernel-doc to its own idr.rst file and add a few
paragraphs about how to use it.  Also add some more kernel-doc.
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>

Documentation/core-api/idr.rst

+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+=============
+ID Allocation
+=============
+
+:Author: Matthew Wilcox
+
+Overview
+========
+
+A common problem to solve is allocating identifiers (IDs); generally
+small numbers which identify a thing.  Examples include file descriptors,
+process IDs, packet identifiers in networking protocols, SCSI tags
+and device instance numbers.  The IDR and the IDA provide a reasonable
+solution to the problem to avoid everybody inventing their own.  The IDR
+provides the ability to map an ID to a pointer, while the IDA provides
+only ID allocation, and as a result is much more memory-efficient.
+
+IDR usage
+=========
+
+Start by initialising an IDR, either with :c:func:`DEFINE_IDR`
+for statically allocated IDRs or :c:func:`idr_init` for dynamically
+allocated IDRs.
+
+You can call :c:func:`idr_alloc` to allocate an unused ID.  Look up
+the pointer you associated with the ID by calling :c:func:`idr_find`
+and free the ID by calling :c:func:`idr_remove`.
+
+If you need to change the pointer associated with an ID, you can call
+:c:func:`idr_replace`.  One common reason to do this is to reserve an
+ID by passing a ``NULL`` pointer to the allocation function; initialise the
+object with the reserved ID and finally insert the initialised object
+into the IDR.
+
+Some users need to allocate IDs larger than ``INT_MAX``.  So far all of
+these users have been content with a ``UINT_MAX`` limit, and they use
+:c:func:`idr_alloc_u32`.  If you need IDs that will not fit in a u32,
+we will work with you to address your needs.
+
+If you need to allocate IDs sequentially, you can use
+:c:func:`idr_alloc_cyclic`.  The IDR becomes less efficient when dealing
+with larger IDs, so using this function comes at a slight cost.
+
+To perform an action on all pointers used by the IDR, you can
+either use the callback-based :c:func:`idr_for_each` or the
+iterator-style :c:func:`idr_for_each_entry`.  You may need to use
+:c:func:`idr_for_each_entry_continue` to continue an iteration.  You can
+also use :c:func:`idr_get_next` if the iterator doesn't fit your needs.
+
+When you have finished using an IDR, you can call :c:func:`idr_destroy`
+to release the memory used by the IDR.  This will not free the objects
+pointed to from the IDR; if you want to do that, use one of the iterators
+to do it.
+
+You can use :c:func:`idr_is_empty` to find out whether there are any
+IDs currently allocated.
+
+If you need to take a lock while allocating a new ID from the IDR,
+you may need to pass a restrictive set of GFP flags, which can lead
+to the IDR being unable to allocate memory.  To work around this,
+you can call :c:func:`idr_preload` before taking the lock, and then
+:c:func:`idr_preload_end` after the allocation.
+
+.. kernel-doc:: include/linux/idr.h
+   :doc: idr sync
+
+IDA usage
+=========
+
+.. kernel-doc:: lib/idr.c
+   :doc: IDA description
+
+Functions and structures
+========================
+
+.. kernel-doc:: include/linux/idr.h
+.. kernel-doc:: lib/idr.c

Documentation/core-api/index.rst

@@ -16,6 +16,7 @@ Core utilities
    atomic_ops
    refcount-vs-atomic
    cpu_hotplug
+   idr
    local_ops
    workqueue
    genericirq

Documentation/core-api/kernel-api.rst

@@ -103,18 +103,6 @@ CRC Functions
 .. kernel-doc:: lib/crc-itu-t.c
    :export:
 
-idr/ida Functions
------------------
-
-.. kernel-doc:: include/linux/idr.h
-   :doc: idr sync
-
-.. kernel-doc:: lib/idr.c
-   :doc: IDA description
-
-.. kernel-doc:: lib/idr.c
-   :export:
-
 Math Functions in Linux
 =======================
 

include/linux/idr.h

@@ -36,7 +36,6 @@ struct idr {
 	.idr_base = (base),						\
 	.idr_next = 0,							\
 }
-#define DEFINE_IDR(name)	struct idr name = IDR_INIT
 
 /**
  * IDR_INIT() - Initialise an IDR.
@@ -45,6 +44,15 @@ struct idr {
  */
 #define IDR_INIT	IDR_INIT_BASE(0)
 
+/**
+ * DEFINE_IDR() - Define a statically-allocated IDR
+ * @name: Name of IDR
+ *
+ * An IDR defined using this macro is ready for use with no additional
+ * initialisation required.  It contains no IDs.
+ */
+#define DEFINE_IDR(name)	struct idr name = IDR_INIT
+
 /**
  * idr_get_cursor - Return the current position of the cyclic allocator
  * @idr: idr handle
@@ -130,6 +138,12 @@ static inline void idr_init(struct idr *idr)
 	idr_init_base(idr, 0);
 }
 
+/**
+ * idr_is_empty() - Are there any IDs allocated?
+ * @idr: IDR handle.
+ *
+ * Return: %true if any IDs have been allocated from this IDR.
+ */
 static inline bool idr_is_empty(const struct idr *idr)
 {
 	return radix_tree_empty(&idr->idr_rt) &&