1 .. SPDX-License-Identifier: GPL-2.0
7 Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
9 This document explains the NVMEM Framework along with the APIs provided,
14 *NVMEM* is the abbreviation for Non Volatile Memory layer. It is used to
15 retrieve configuration of SOC or Device specific data from non volatile
16 memories like eeprom, efuses and so on.
18 Before this framework existed, NVMEM drivers like eeprom were stored in
19 drivers/misc, where they all had to duplicate pretty much the same code to
20 register a sysfs file, allow in-kernel users to access the content of the
21 devices they were driving, etc.
23 This was also a problem as far as other in-kernel users were involved, since
24 the solutions used were pretty much different from one driver to another, there
25 was a rather big abstraction leak.
27 This framework aims at solve these problems. It also introduces DT
28 representation for consumer devices to go get the data they require (MAC
29 Addresses, SoC/Revision ID, part numbers, and so on) from the NVMEMs. This
30 framework is based on regmap, so that most of the abstraction available in
31 regmap can be reused, across multiple types of buses.
36 NVMEM provider refers to an entity that implements methods to initialize, read
37 and write the non-volatile memory.
39 2. Registering/Unregistering the NVMEM provider
40 ===============================================
42 A NVMEM provider can register with NVMEM core by supplying relevant
43 nvmem configuration to nvmem_register(), on success core would return a valid
46 nvmem_unregister(nvmem) is used to unregister a previously registered provider.
48 For example, a simple qfprom case::
50 static struct nvmem_config econfig = {
55 static int qfprom_probe(struct platform_device *pdev)
58 econfig.dev = &pdev->dev;
59 nvmem = nvmem_register(&econfig);
63 It is mandatory that the NVMEM provider has a regmap associated with its
64 struct device. Failure to do would return error code from nvmem_register().
66 Users of board files can define and register nvmem cells using the
67 nvmem_cell_table struct::
69 static struct nvmem_cell_info foo_nvmem_cells[] = {
77 static struct nvmem_cell_table foo_nvmem_cell_table = {
78 .nvmem_name = "i2c-eeprom",
79 .cells = foo_nvmem_cells,
80 .ncells = ARRAY_SIZE(foo_nvmem_cells),
83 nvmem_add_cell_table(&foo_nvmem_cell_table);
85 Additionally it is possible to create nvmem cell lookup entries and register
86 them with the nvmem framework from machine code as shown in the example below::
88 static struct nvmem_cell_lookup foo_nvmem_lookup = {
89 .nvmem_name = "i2c-eeprom",
90 .cell_name = "macaddr",
91 .dev_id = "foo_mac.0",
92 .con_id = "mac-address",
95 nvmem_add_cell_lookups(&foo_nvmem_lookup, 1);
100 NVMEM consumers are the entities which make use of the NVMEM provider to
101 read from and to NVMEM.
103 3. NVMEM cell based consumer APIs
104 =================================
106 NVMEM cells are the data entries/fields in the NVMEM.
107 The NVMEM framework provides 3 APIs to read/write NVMEM cells::
109 struct nvmem_cell *nvmem_cell_get(struct device *dev, const char *name);
110 struct nvmem_cell *devm_nvmem_cell_get(struct device *dev, const char *name);
112 void nvmem_cell_put(struct nvmem_cell *cell);
113 void devm_nvmem_cell_put(struct device *dev, struct nvmem_cell *cell);
115 void *nvmem_cell_read(struct nvmem_cell *cell, ssize_t *len);
116 int nvmem_cell_write(struct nvmem_cell *cell, void *buf, ssize_t len);
118 `*nvmem_cell_get()` apis will get a reference to nvmem cell for a given id,
119 and nvmem_cell_read/write() can then read or write to the cell.
120 Once the usage of the cell is finished the consumer should call
121 `*nvmem_cell_put()` to free all the allocation memory for the cell.
123 4. Direct NVMEM device based consumer APIs
124 ==========================================
126 In some instances it is necessary to directly read/write the NVMEM.
127 To facilitate such consumers NVMEM framework provides below apis::
129 struct nvmem_device *nvmem_device_get(struct device *dev, const char *name);
130 struct nvmem_device *devm_nvmem_device_get(struct device *dev,
132 void nvmem_device_put(struct nvmem_device *nvmem);
133 int nvmem_device_read(struct nvmem_device *nvmem, unsigned int offset,
134 size_t bytes, void *buf);
135 int nvmem_device_write(struct nvmem_device *nvmem, unsigned int offset,
136 size_t bytes, void *buf);
137 int nvmem_device_cell_read(struct nvmem_device *nvmem,
138 struct nvmem_cell_info *info, void *buf);
139 int nvmem_device_cell_write(struct nvmem_device *nvmem,
140 struct nvmem_cell_info *info, void *buf);
142 Before the consumers can read/write NVMEM directly, it should get hold
143 of nvmem_controller from one of the `*nvmem_device_get()` api.
145 The difference between these apis and cell based apis is that these apis always
146 take nvmem_device as parameter.
148 5. Releasing a reference to the NVMEM
149 =====================================
151 When a consumer no longer needs the NVMEM, it has to release the reference
152 to the NVMEM it has obtained using the APIs mentioned in the above section.
153 The NVMEM framework provides 2 APIs to release a reference to the NVMEM::
155 void nvmem_cell_put(struct nvmem_cell *cell);
156 void devm_nvmem_cell_put(struct device *dev, struct nvmem_cell *cell);
157 void nvmem_device_put(struct nvmem_device *nvmem);
158 void devm_nvmem_device_put(struct device *dev, struct nvmem_device *nvmem);
160 Both these APIs are used to release a reference to the NVMEM and
161 devm_nvmem_cell_put and devm_nvmem_device_put destroys the devres associated
167 6. Userspace binary interface
168 ==============================
170 Userspace can read/write the raw NVMEM file located at::
172 /sys/bus/nvmem/devices/*/nvmem
176 hexdump /sys/bus/nvmem/devices/qfprom0/nvmem
178 0000000 0000 0000 0000 0000 0000 0000 0000 0000
180 00000a0 db10 2240 0000 e000 0c00 0c00 0000 0c00
181 0000000 0000 0000 0000 0000 0000 0000 0000 0000
186 7. DeviceTree Binding
187 =====================
189 See Documentation/devicetree/bindings/nvmem/nvmem.txt