1.. SPDX-License-Identifier: GPL-2.0
2
3===============
4NVMEM Subsystem
5===============
6
7 Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
8
9This document explains the NVMEM Framework along with the APIs provided,
10and how to use it.
11
121. Introduction
13===============
14*NVMEM* is the abbreviation for Non Volatile Memory layer. It is used to
15retrieve configuration of SOC or Device specific data from non volatile
16memories like eeprom, efuses and so on.
17
18Before this framework existed, NVMEM drivers like eeprom were stored in
19drivers/misc, where they all had to duplicate pretty much the same code to
20register a sysfs file, allow in-kernel users to access the content of the
21devices they were driving, etc.
22
23This was also a problem as far as other in-kernel users were involved, since
24the solutions used were pretty much different from one driver to another, there
25was a rather big abstraction leak.
26
27This framework aims at solve these problems. It also introduces DT
28representation for consumer devices to go get the data they require (MAC
29Addresses, SoC/Revision ID, part numbers, and so on) from the NVMEMs. This
30framework is based on regmap, so that most of the abstraction available in
31regmap can be reused, across multiple types of buses.
32
33NVMEM Providers
34+++++++++++++++
35
36NVMEM provider refers to an entity that implements methods to initialize, read
37and write the non-volatile memory.
38
392. Registering/Unregistering the NVMEM provider
40===============================================
41
42A NVMEM provider can register with NVMEM core by supplying relevant
43nvmem configuration to nvmem_register(), on success core would return a valid
44nvmem_device pointer.
45
46nvmem_unregister(nvmem) is used to unregister a previously registered provider.
47
48For example, a simple qfprom case::
49
50  static struct nvmem_config econfig = {
51	.name = "qfprom",
52	.owner = THIS_MODULE,
53  };
54
55  static int qfprom_probe(struct platform_device *pdev)
56  {
57	...
58	econfig.dev = &pdev->dev;
59	nvmem = nvmem_register(&econfig);
60	...
61  }
62
63It is mandatory that the NVMEM provider has a regmap associated with its
64struct device. Failure to do would return error code from nvmem_register().
65
66Users of board files can define and register nvmem cells using the
67nvmem_cell_table struct::
68
69  static struct nvmem_cell_info foo_nvmem_cells[] = {
70	{
71		.name		= "macaddr",
72		.offset		= 0x7f00,
73		.bytes		= ETH_ALEN,
74	}
75  };
76
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),
81  };
82
83  nvmem_add_cell_table(&foo_nvmem_cell_table);
84
85Additionally it is possible to create nvmem cell lookup entries and register
86them with the nvmem framework from machine code as shown in the example below::
87
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",
93  };
94
95  nvmem_add_cell_lookups(&foo_nvmem_lookup, 1);
96
97NVMEM Consumers
98+++++++++++++++
99
100NVMEM consumers are the entities which make use of the NVMEM provider to
101read from and to NVMEM.
102
1033. NVMEM cell based consumer APIs
104=================================
105
106NVMEM cells are the data entries/fields in the NVMEM.
107The NVMEM framework provides 3 APIs to read/write NVMEM cells::
108
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);
111
112  void nvmem_cell_put(struct nvmem_cell *cell);
113  void devm_nvmem_cell_put(struct device *dev, struct nvmem_cell *cell);
114
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);
117
118`*nvmem_cell_get()` apis will get a reference to nvmem cell for a given id,
119and nvmem_cell_read/write() can then read or write to the cell.
120Once the usage of the cell is finished the consumer should call
121`*nvmem_cell_put()` to free all the allocation memory for the cell.
122
1234. Direct NVMEM device based consumer APIs
124==========================================
125
126In some instances it is necessary to directly read/write the NVMEM.
127To facilitate such consumers NVMEM framework provides below apis::
128
129  struct nvmem_device *nvmem_device_get(struct device *dev, const char *name);
130  struct nvmem_device *devm_nvmem_device_get(struct device *dev,
131					   const char *name);
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);
141
142Before the consumers can read/write NVMEM directly, it should get hold
143of nvmem_controller from one of the `*nvmem_device_get()` api.
144
145The difference between these apis and cell based apis is that these apis always
146take nvmem_device as parameter.
147
1485. Releasing a reference to the NVMEM
149=====================================
150
151When a consumer no longer needs the NVMEM, it has to release the reference
152to the NVMEM it has obtained using the APIs mentioned in the above section.
153The NVMEM framework provides 2 APIs to release a reference to the NVMEM::
154
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);
159
160Both these APIs are used to release a reference to the NVMEM and
161devm_nvmem_cell_put and devm_nvmem_device_put destroys the devres associated
162with this NVMEM.
163
164Userspace
165+++++++++
166
1676. Userspace binary interface
168==============================
169
170Userspace can read/write the raw NVMEM file located at::
171
172	/sys/bus/nvmem/devices/*/nvmem
173
174ex::
175
176  hexdump /sys/bus/nvmem/devices/qfprom0/nvmem
177
178  0000000 0000 0000 0000 0000 0000 0000 0000 0000
179  *
180  00000a0 db10 2240 0000 e000 0c00 0c00 0000 0c00
181  0000000 0000 0000 0000 0000 0000 0000 0000 0000
182  ...
183  *
184  0001000
185
1867. DeviceTree Binding
187=====================
188
189See Documentation/devicetree/bindings/nvmem/nvmem.txt
190