xref: /linux/Documentation/driver-api/nvmem.rst (revision 84b9b44b) 
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.
30
31NVMEM Providers
32+++++++++++++++
33
34NVMEM provider refers to an entity that implements methods to initialize, read
35and write the non-volatile memory.
36
372. Registering/Unregistering the NVMEM provider
38===============================================
39
40A NVMEM provider can register with NVMEM core by supplying relevant
41nvmem configuration to nvmem_register(), on success core would return a valid
42nvmem_device pointer.
43
44nvmem_unregister(nvmem) is used to unregister a previously registered provider.
45
46For example, a simple nvram case::
47
48  static int brcm_nvram_probe(struct platform_device *pdev)
49  {
50	struct nvmem_config config = {
51		.name = "brcm-nvram",
52		.reg_read = brcm_nvram_read,
53	};
54	...
55	config.dev = &pdev->dev;
56	config.priv = priv;
57	config.size = resource_size(res);
58
59	devm_nvmem_register(&config);
60  }
61
62Users of board files can define and register nvmem cells using the
63nvmem_cell_table struct::
64
65  static struct nvmem_cell_info foo_nvmem_cells[] = {
66	{
67		.name		= "macaddr",
68		.offset		= 0x7f00,
69		.bytes		= ETH_ALEN,
70	}
71  };
72
73  static struct nvmem_cell_table foo_nvmem_cell_table = {
74	.nvmem_name		= "i2c-eeprom",
75	.cells			= foo_nvmem_cells,
76	.ncells			= ARRAY_SIZE(foo_nvmem_cells),
77  };
78
79  nvmem_add_cell_table(&foo_nvmem_cell_table);
80
81Additionally it is possible to create nvmem cell lookup entries and register
82them with the nvmem framework from machine code as shown in the example below::
83
84  static struct nvmem_cell_lookup foo_nvmem_lookup = {
85	.nvmem_name		= "i2c-eeprom",
86	.cell_name		= "macaddr",
87	.dev_id			= "foo_mac.0",
88	.con_id			= "mac-address",
89  };
90
91  nvmem_add_cell_lookups(&foo_nvmem_lookup, 1);
92
93NVMEM Consumers
94+++++++++++++++
95
96NVMEM consumers are the entities which make use of the NVMEM provider to
97read from and to NVMEM.
98
993. NVMEM cell based consumer APIs
100=================================
101
102NVMEM cells are the data entries/fields in the NVMEM.
103The NVMEM framework provides 3 APIs to read/write NVMEM cells::
104
105  struct nvmem_cell *nvmem_cell_get(struct device *dev, const char *name);
106  struct nvmem_cell *devm_nvmem_cell_get(struct device *dev, const char *name);
107
108  void nvmem_cell_put(struct nvmem_cell *cell);
109  void devm_nvmem_cell_put(struct device *dev, struct nvmem_cell *cell);
110
111  void *nvmem_cell_read(struct nvmem_cell *cell, ssize_t *len);
112  int nvmem_cell_write(struct nvmem_cell *cell, void *buf, ssize_t len);
113
114`*nvmem_cell_get()` apis will get a reference to nvmem cell for a given id,
115and nvmem_cell_read/write() can then read or write to the cell.
116Once the usage of the cell is finished the consumer should call
117`*nvmem_cell_put()` to free all the allocation memory for the cell.
118
1194. Direct NVMEM device based consumer APIs
120==========================================
121
122In some instances it is necessary to directly read/write the NVMEM.
123To facilitate such consumers NVMEM framework provides below apis::
124
125  struct nvmem_device *nvmem_device_get(struct device *dev, const char *name);
126  struct nvmem_device *devm_nvmem_device_get(struct device *dev,
127					   const char *name);
128  struct nvmem_device *nvmem_device_find(void *data,
129			int (*match)(struct device *dev, const void *data));
130  void nvmem_device_put(struct nvmem_device *nvmem);
131  int nvmem_device_read(struct nvmem_device *nvmem, unsigned int offset,
132		      size_t bytes, void *buf);
133  int nvmem_device_write(struct nvmem_device *nvmem, unsigned int offset,
134		       size_t bytes, void *buf);
135  int nvmem_device_cell_read(struct nvmem_device *nvmem,
136			   struct nvmem_cell_info *info, void *buf);
137  int nvmem_device_cell_write(struct nvmem_device *nvmem,
138			    struct nvmem_cell_info *info, void *buf);
139
140Before the consumers can read/write NVMEM directly, it should get hold
141of nvmem_controller from one of the `*nvmem_device_get()` api.
142
143The difference between these apis and cell based apis is that these apis always
144take nvmem_device as parameter.
145
1465. Releasing a reference to the NVMEM
147=====================================
148
149When a consumer no longer needs the NVMEM, it has to release the reference
150to the NVMEM it has obtained using the APIs mentioned in the above section.
151The NVMEM framework provides 2 APIs to release a reference to the NVMEM::
152
153  void nvmem_cell_put(struct nvmem_cell *cell);
154  void devm_nvmem_cell_put(struct device *dev, struct nvmem_cell *cell);
155  void nvmem_device_put(struct nvmem_device *nvmem);
156  void devm_nvmem_device_put(struct device *dev, struct nvmem_device *nvmem);
157
158Both these APIs are used to release a reference to the NVMEM and
159devm_nvmem_cell_put and devm_nvmem_device_put destroys the devres associated
160with this NVMEM.
161
162Userspace
163+++++++++
164
1656. Userspace binary interface
166==============================
167
168Userspace can read/write the raw NVMEM file located at::
169
170	/sys/bus/nvmem/devices/*/nvmem
171
172ex::
173
174  hexdump /sys/bus/nvmem/devices/qfprom0/nvmem
175
176  0000000 0000 0000 0000 0000 0000 0000 0000 0000
177  *
178  00000a0 db10 2240 0000 e000 0c00 0c00 0000 0c00
179  0000000 0000 0000 0000 0000 0000 0000 0000 0000
180  ...
181  *
182  0001000
183
1847. DeviceTree Binding
185=====================
186
187See Documentation/devicetree/bindings/nvmem/nvmem.txt
188
1898. NVMEM layouts
190================
191
192NVMEM layouts are yet another mechanism to create cells. With the device
193tree binding it is possible to specify simple cells by using an offset
194and a length. Sometimes, the cells doesn't have a static offset, but
195the content is still well defined, e.g. tag-length-values. In this case,
196the NVMEM device content has to be first parsed and the cells need to
197be added accordingly. Layouts let you read the content of the NVMEM device
198and let you add cells dynamically.
199
200Another use case for layouts is the post processing of cells. With layouts,
201it is possible to associate a custom post processing hook to a cell. It
202even possible to add this hook to cells not created by the layout itself.
203