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1The genalloc/genpool subsystem2==============================3 4There are a number of memory-allocation subsystems in the kernel, each5aimed at a specific need. Sometimes, however, a kernel developer needs to6implement a new allocator for a specific range of special-purpose memory;7often that memory is located on a device somewhere. The author of the8driver for that device can certainly write a little allocator to get the9job done, but that is the way to fill the kernel with dozens of poorly10tested allocators. Back in 2005, Jes Sorensen lifted one of those11allocators from the sym53c8xx_2 driver and posted_ it as a generic module12for the creation of ad hoc memory allocators. This code was merged13for 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 action18begins with the creation of a pool using one of:19 20.. kernel-doc:: lib/genalloc.c21 :functions: gen_pool_create 22 23.. kernel-doc:: lib/genalloc.c24 :functions: devm_gen_pool_create25 26A call to gen_pool_create() will create a pool. The granularity of27allocations is set with min_alloc_order; it is a log-base-2 number like28those 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 a30multiple of eight bytes. Increasing min_alloc_order decreases the memory31required to track the memory in the pool. The nid parameter specifies32which NUMA node should be used for the allocation of the housekeeping33structures; it can be -1 if the caller doesn't care.34 35The "managed" interface devm_gen_pool_create() ties the pool to a36specific device. Among other things, it will automatically clean up the37pool when the given device is destroyed.38 39A pool is shut down with:40 41.. kernel-doc:: lib/genalloc.c42 :functions: gen_pool_destroy43 44It's worth noting that, if there are still allocations outstanding from the45given pool, this function will take the rather extreme step of invoking46BUG(), crashing the entire system. You have been warned.47 48A freshly created pool has no memory to allocate. It is fairly useless in49that state, so one of the first orders of business is usually to add memory50to the pool. That can be done with one of:51 52.. kernel-doc:: include/linux/genalloc.h53 :functions: gen_pool_add54 55.. kernel-doc:: lib/genalloc.c56 :functions: gen_pool_add_owner57 58A call to gen_pool_add() will place the size bytes of memory59starting at addr (in the kernel's virtual address space) into the given60pool, once again using nid as the node ID for ancillary memory allocations.61The gen_pool_add_virt() variant associates an explicit physical62address with the memory; this is only necessary if the pool will be used63for DMA allocations.64 65The functions for allocating memory from the pool (and putting it back)66are:67 68.. kernel-doc:: include/linux/genalloc.h69 :functions: gen_pool_alloc70 71.. kernel-doc:: lib/genalloc.c72 :functions: gen_pool_dma_alloc73 74.. kernel-doc:: lib/genalloc.c75 :functions: gen_pool_free_owner76 77As one would expect, gen_pool_alloc() will allocate size< bytes78from the given pool. The gen_pool_dma_alloc() variant allocates79memory for use with DMA operations, returning the associated physical80address in the space pointed to by dma. This will only work if the memory81was added with gen_pool_add_virt(). Note that this function82departs from the usual genpool pattern of using unsigned long values to83represent kernel addresses; it returns a void * instead.84 85That all seems relatively simple; indeed, some developers clearly found it86to be too simple. After all, the interface above provides no control over87how the allocation functions choose which specific piece of memory to88return. If that sort of control is needed, the following functions will be89of interest:90 91.. kernel-doc:: lib/genalloc.c92 :functions: gen_pool_alloc_algo_owner93 94.. kernel-doc:: lib/genalloc.c95 :functions: gen_pool_set_algo96 97Allocations with gen_pool_alloc_algo() specify an algorithm to be98used to choose the memory to be allocated; the default algorithm can be set99with gen_pool_set_algo(). The data value is passed to the100algorithm; most ignore it, but it is occasionally needed. One can,101naturally, write a special-purpose algorithm, but there is a fair set102already available:103 104- gen_pool_first_fit is a simple first-fit allocator; this is the default105 algorithm if none other has been specified.106 107- gen_pool_first_fit_align forces the allocation to have a specific108 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 the111 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 a116 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 querying120the space available in the pool or iterating through chunks of memory.121Most users, however, should not need much beyond what has been described122above. With luck, wider awareness of this module will help to prevent the123writing of special-purpose memory allocators in the future.124 125.. kernel-doc:: lib/genalloc.c126 :functions: gen_pool_virt_to_phys127 128.. kernel-doc:: lib/genalloc.c129 :functions: gen_pool_for_each_chunk130 131.. kernel-doc:: lib/genalloc.c132 :functions: gen_pool_has_addr133 134.. kernel-doc:: lib/genalloc.c135 :functions: gen_pool_avail136 137.. kernel-doc:: lib/genalloc.c138 :functions: gen_pool_size139 140.. kernel-doc:: lib/genalloc.c141 :functions: gen_pool_get142 143.. kernel-doc:: lib/genalloc.c144 :functions: of_gen_pool_get145