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authorMatthew Wilcox <mawilcox@microsoft.com>2018-02-06 23:05:49 +0300
committerMatthew Wilcox <mawilcox@microsoft.com>2018-02-07 00:41:29 +0300
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tree7e74505b3e71b90f303d52b3bf66bc65d609e48c /Documentation/core-api/idr.rst
parent6ce711f2750031d12cec91384ac5cfa0a485b60a (diff)
downloadlinux-ac665d9423474e64e64b34b0e2cea43601b50d7d.tar.xz
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>
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+.. 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