1 The genalloc/genpool subsystem 2 ============================== 3 4 There are a number of memory-allocation subsystems in the kernel, each 5 aimed at a specific need. Sometimes, however, a kernel developer needs to 6 implement a new allocator for a specific range of special-purpose memory; 7 often that memory is located on a device somewhere. The author of the 8 driver for that device can certainly write a little allocator to get the 9 job done, but that is the way to fill the kernel with dozens of poorly 10 tested allocators. Back in 2005, Jes Sorensen lifted one of those 11 allocators from the sym53c8xx_2 driver and posted_ it as a generic module 12 for the creation of ad hoc memory allocators. This code was merged 13 for the 2.6.13 release; it has been modified considerably since then. 14 15 .. _posted: https://lwn.net/Articles/125842/ 16 17 Code using this allocator should include <linux/genalloc.h>. The action 18 begins 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 26 A call to gen_pool_create() will create a pool. The granularity of 27 allocations is set with min_alloc_order; it is a log-base-2 number like 28 those used by the page allocator, but it refers to bytes rather than pages. 29 So, if min_alloc_order is passed as 3, then all allocations will be a 30 multiple of eight bytes. Increasing min_alloc_order decreases the memory 31 required to track the memory in the pool. The nid parameter specifies 32 which NUMA node should be used for the allocation of the housekeeping 33 structures; it can be -1 if the caller doesn't care. 34 35 The "managed" interface devm_gen_pool_create() ties the pool to a 36 specific device. Among other things, it will automatically clean up the 37 pool when the given device is destroyed. 38 39 A pool is shut down with: 40 41 .. kernel-doc:: lib/genalloc.c 42 :functions: gen_pool_destroy 43 44 It's worth noting that, if there are still allocations outstanding from the 45 given pool, this function will take the rather extreme step of invoking 46 BUG(), crashing the entire system. You have been warned. 47 48 A freshly created pool has no memory to allocate. It is fairly useless in 49 that state, so one of the first orders of business is usually to add memory 50 to 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 58 A call to gen_pool_add() will place the size bytes of memory 59 starting at addr (in the kernel's virtual address space) into the given 60 pool, once again using nid as the node ID for ancillary memory allocations. 61 The gen_pool_add_virt() variant associates an explicit physical 62 address with the memory; this is only necessary if the pool will be used 63 for DMA allocations. 64 65 The functions for allocating memory from the pool (and putting it back) 66 are: 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 77 As one would expect, gen_pool_alloc() will allocate size< bytes 78 from the given pool. The gen_pool_dma_alloc() variant allocates 79 memory for use with DMA operations, returning the associated physical 80 address in the space pointed to by dma. This will only work if the memory 81 was added with gen_pool_add_virt(). Note that this function 82 departs from the usual genpool pattern of using unsigned long values to 83 represent kernel addresses; it returns a void * instead. 84 85 That all seems relatively simple; indeed, some developers clearly found it 86 to be too simple. After all, the interface above provides no control over 87 how the allocation functions choose which specific piece of memory to 88 return. If that sort of control is needed, the following functions will be 89 of 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 97 Allocations with gen_pool_alloc_algo() specify an algorithm to be 98 used to choose the memory to be allocated; the default algorithm can be set 99 with gen_pool_set_algo(). The data value is passed to the 100 algorithm; most ignore it, but it is occasionally needed. One can, 101 naturally, write a special-purpose algorithm, but there is a fair set 102 already 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. 118 119 There is a handful of other functions, mostly for purposes like querying 120 the space available in the pool or iterating through chunks of memory. 121 Most users, however, should not need much beyond what has been described 122 above. With luck, wider awareness of this module will help to prevent the 123 writing 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
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