Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 NVMEM SUBSYSTEM 2 Srinivas Kandagatla <srinivas.kandagatla@linaro.org> 3 4 This document explains the NVMEM Framework along with the APIs provided, 5 and how to use it. 6 7 1. Introduction 8 =============== 9 *NVMEM* is the abbreviation for Non Volatile Memory layer. It is used to 10 retrieve configuration of SOC or Device specific data from non volatile 11 memories like eeprom, efuses and so on. 12 13 Before this framework existed, NVMEM drivers like eeprom were stored in 14 drivers/misc, where they all had to duplicate pretty much the same code to 15 register a sysfs file, allow in-kernel users to access the content of the 16 devices they were driving, etc. 17 18 This was also a problem as far as other in-kernel users were involved, since 19 the solutions used were pretty much different from one driver to another, there 20 was a rather big abstraction leak. 21 22 This framework aims at solve these problems. It also introduces DT 23 representation for consumer devices to go get the data they require (MAC 24 Addresses, SoC/Revision ID, part numbers, and so on) from the NVMEMs. This 25 framework is based on regmap, so that most of the abstraction available in 26 regmap can be reused, across multiple types of buses. 27 28 NVMEM Providers 29 +++++++++++++++ 30 31 NVMEM provider refers to an entity that implements methods to initialize, read 32 and write the non-volatile memory. 33 34 2. Registering/Unregistering the NVMEM provider 35 =============================================== 36 37 A NVMEM provider can register with NVMEM core by supplying relevant 38 nvmem configuration to nvmem_register(), on success core would return a valid 39 nvmem_device pointer. 40 41 nvmem_unregister(nvmem) is used to unregister a previously registered provider. 42 43 For example, a simple qfprom case: 44 45 static struct nvmem_config econfig = { 46 .name = "qfprom", 47 .owner = THIS_MODULE, 48 }; 49 50 static int qfprom_probe(struct platform_device *pdev) 51 { 52 ... 53 econfig.dev = &pdev->dev; 54 nvmem = nvmem_register(&econfig); 55 ... 56 } 57 58 It is mandatory that the NVMEM provider has a regmap associated with its 59 struct device. Failure to do would return error code from nvmem_register(). 60 61 NVMEM Consumers 62 +++++++++++++++ 63 64 NVMEM consumers are the entities which make use of the NVMEM provider to 65 read from and to NVMEM. 66 67 3. NVMEM cell based consumer APIs 68 ================================= 69 70 NVMEM cells are the data entries/fields in the NVMEM. 71 The NVMEM framework provides 3 APIs to read/write NVMEM cells. 72 73 struct nvmem_cell *nvmem_cell_get(struct device *dev, const char *name); 74 struct nvmem_cell *devm_nvmem_cell_get(struct device *dev, const char *name); 75 76 void nvmem_cell_put(struct nvmem_cell *cell); 77 void devm_nvmem_cell_put(struct device *dev, struct nvmem_cell *cell); 78 79 void *nvmem_cell_read(struct nvmem_cell *cell, ssize_t *len); 80 int nvmem_cell_write(struct nvmem_cell *cell, void *buf, ssize_t len); 81 82 *nvmem_cell_get() apis will get a reference to nvmem cell for a given id, 83 and nvmem_cell_read/write() can then read or write to the cell. 84 Once the usage of the cell is finished the consumer should call *nvmem_cell_put() 85 to free all the allocation memory for the cell. 86 87 4. Direct NVMEM device based consumer APIs 88 ========================================== 89 90 In some instances it is necessary to directly read/write the NVMEM. 91 To facilitate such consumers NVMEM framework provides below apis. 92 93 struct nvmem_device *nvmem_device_get(struct device *dev, const char *name); 94 struct nvmem_device *devm_nvmem_device_get(struct device *dev, 95 const char *name); 96 void nvmem_device_put(struct nvmem_device *nvmem); 97 int nvmem_device_read(struct nvmem_device *nvmem, unsigned int offset, 98 size_t bytes, void *buf); 99 int nvmem_device_write(struct nvmem_device *nvmem, unsigned int offset, 100 size_t bytes, void *buf); 101 int nvmem_device_cell_read(struct nvmem_device *nvmem, 102 struct nvmem_cell_info *info, void *buf); 103 int nvmem_device_cell_write(struct nvmem_device *nvmem, 104 struct nvmem_cell_info *info, void *buf); 105 106 Before the consumers can read/write NVMEM directly, it should get hold 107 of nvmem_controller from one of the *nvmem_device_get() api. 108 109 The difference between these apis and cell based apis is that these apis always 110 take nvmem_device as parameter. 111 112 5. Releasing a reference to the NVMEM 113 ===================================== 114 115 When a consumer no longer needs the NVMEM, it has to release the reference 116 to the NVMEM it has obtained using the APIs mentioned in the above section. 117 The NVMEM framework provides 2 APIs to release a reference to the NVMEM. 118 119 void nvmem_cell_put(struct nvmem_cell *cell); 120 void devm_nvmem_cell_put(struct device *dev, struct nvmem_cell *cell); 121 void nvmem_device_put(struct nvmem_device *nvmem); 122 void devm_nvmem_device_put(struct device *dev, struct nvmem_device *nvmem); 123 124 Both these APIs are used to release a reference to the NVMEM and 125 devm_nvmem_cell_put and devm_nvmem_device_put destroys the devres associated 126 with this NVMEM. 127 128 Userspace 129 +++++++++ 130 131 6. Userspace binary interface 132 ============================== 133 134 Userspace can read/write the raw NVMEM file located at 135 /sys/bus/nvmem/devices/*/nvmem 136 137 ex: 138 139 hexdump /sys/bus/nvmem/devices/qfprom0/nvmem 140 141 0000000 0000 0000 0000 0000 0000 0000 0000 0000 142 * 143 00000a0 db10 2240 0000 e000 0c00 0c00 0000 0c00 144 0000000 0000 0000 0000 0000 0000 0000 0000 0000 145 ... 146 * 147 0001000 148 149 7. DeviceTree Binding 150 ===================== 151 152 See Documentation/devicetree/bindings/nvmem/nvmem.txt