1The genalloc/genpool subsystem
  2==============================
  3
  4There are a number of memory-allocation subsystems in the kernel, each
  5aimed at a specific need.  Sometimes, however, a kernel developer needs to
  6implement a new allocator for a specific range of special-purpose memory;
  7often that memory is located on a device somewhere.  The author of the
  8driver for that device can certainly write a little allocator to get the
  9job done, but that is the way to fill the kernel with dozens of poorly
 10tested allocators.  Back in 2005, Jes Sorensen lifted one of those
 11allocators from the sym53c8xx_2 driver and posted_ it as a generic module
 12for the creation of ad hoc memory allocators.  This code was merged
 13for the 2.6.13 release; it has been modified considerably since then.
 14
 15.. _posted: https://lwn.net/Articles/125842/
 16
 17Code using this allocator should include <linux/genalloc.h>.  The action
 18begins with the creation of a pool using one of:
 19
 20.. kernel-doc:: lib/genalloc.c
 21   :functions: gen_pool_create		
 22
 23.. kernel-doc:: lib/genalloc.c
 24   :functions: devm_gen_pool_create
 25
 26A call to gen_pool_create() will create a pool.  The granularity of
 27allocations is set with min_alloc_order; it is a log-base-2 number like
 28those used by the page allocator, but it refers to bytes rather than pages.
 29So, if min_alloc_order is passed as 3, then all allocations will be a
 30multiple of eight bytes.  Increasing min_alloc_order decreases the memory
 31required to track the memory in the pool.  The nid parameter specifies
 32which NUMA node should be used for the allocation of the housekeeping
 33structures; it can be -1 if the caller doesn't care.
 34
 35The "managed" interface devm_gen_pool_create() ties the pool to a
 36specific device.  Among other things, it will automatically clean up the
 37pool when the given device is destroyed.
 38
 39A pool is shut down with:
 40
 41.. kernel-doc:: lib/genalloc.c
 42   :functions: gen_pool_destroy
 43
 44It's worth noting that, if there are still allocations outstanding from the
 45given pool, this function will take the rather extreme step of invoking
 46BUG(), crashing the entire system.  You have been warned.
 47
 48A freshly created pool has no memory to allocate.  It is fairly useless in
 49that state, so one of the first orders of business is usually to add memory
 50to the pool.  That can be done with one of:
 51
 52.. kernel-doc:: include/linux/genalloc.h
 53   :functions: gen_pool_add
 54
 55.. kernel-doc:: lib/genalloc.c
 56   :functions: gen_pool_add_owner
 57
 58A call to gen_pool_add() will place the size bytes of memory
 59starting at addr (in the kernel's virtual address space) into the given
 60pool, once again using nid as the node ID for ancillary memory allocations.
 61The gen_pool_add_virt() variant associates an explicit physical
 62address with the memory; this is only necessary if the pool will be used
 63for DMA allocations.
 64
 65The functions for allocating memory from the pool (and putting it back)
 66are:
 67
 68.. kernel-doc:: include/linux/genalloc.h
 69   :functions: gen_pool_alloc
 70
 71.. kernel-doc:: lib/genalloc.c
 72   :functions: gen_pool_dma_alloc
 73
 74.. kernel-doc:: lib/genalloc.c
 75   :functions: gen_pool_free_owner
 76
 77As one would expect, gen_pool_alloc() will allocate size< bytes
 78from the given pool.  The gen_pool_dma_alloc() variant allocates
 79memory for use with DMA operations, returning the associated physical
 80address in the space pointed to by dma.  This will only work if the memory
 81was added with gen_pool_add_virt().  Note that this function
 82departs from the usual genpool pattern of using unsigned long values to
 83represent kernel addresses; it returns a void * instead.
 84
 85That all seems relatively simple; indeed, some developers clearly found it
 86to be too simple.  After all, the interface above provides no control over
 87how the allocation functions choose which specific piece of memory to
 88return.  If that sort of control is needed, the following functions will be
 89of interest:
 90
 91.. kernel-doc:: lib/genalloc.c
 92   :functions: gen_pool_alloc_algo_owner
 93
 94.. kernel-doc:: lib/genalloc.c
 95   :functions: gen_pool_set_algo
 96
 97Allocations with gen_pool_alloc_algo() specify an algorithm to be
 98used to choose the memory to be allocated; the default algorithm can be set
 99with gen_pool_set_algo().  The data value is passed to the
100algorithm; most ignore it, but it is occasionally needed.  One can,
101naturally, write a special-purpose algorithm, but there is a fair set
102already available:
103
104- gen_pool_first_fit is a simple first-fit allocator; this is the default
105  algorithm if none other has been specified.
106
107- gen_pool_first_fit_align forces the allocation to have a specific
108  alignment (passed via data in a genpool_data_align structure).
109
110- gen_pool_first_fit_order_align aligns the allocation to the order of the
111  size.  A 60-byte allocation will thus be 64-byte aligned, for example.
112
113- gen_pool_best_fit, as one would expect, is a simple best-fit allocator.
114
115- gen_pool_fixed_alloc allocates at a specific offset (passed in a
116  genpool_data_fixed structure via the data parameter) within the pool.
117  If the indicated memory is not available the allocation fails.
118
119There is a handful of other functions, mostly for purposes like querying
120the space available in the pool or iterating through chunks of memory.
121Most users, however, should not need much beyond what has been described
122above.  With luck, wider awareness of this module will help to prevent the
123writing of special-purpose memory allocators in the future.
124
125.. kernel-doc:: lib/genalloc.c
126   :functions: gen_pool_virt_to_phys
127
128.. kernel-doc:: lib/genalloc.c
129   :functions: gen_pool_for_each_chunk
130
131.. kernel-doc:: lib/genalloc.c
132   :functions: gen_pool_has_addr
133
134.. kernel-doc:: lib/genalloc.c
135   :functions: gen_pool_avail
136
137.. kernel-doc:: lib/genalloc.c
138   :functions: gen_pool_size
139
140.. kernel-doc:: lib/genalloc.c
141   :functions: gen_pool_get
142
143.. kernel-doc:: lib/genalloc.c
144   :functions: of_gen_pool_get