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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-11 08:27:49 +0000
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+.. SPDX-License-Identifier: GPL-2.0
+
+===============
+NVMEM Subsystem
+===============
+
+ Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
+
+This document explains the NVMEM Framework along with the APIs provided,
+and how to use it.
+
+1. Introduction
+===============
+*NVMEM* is the abbreviation for Non Volatile Memory layer. It is used to
+retrieve configuration of SOC or Device specific data from non volatile
+memories like eeprom, efuses and so on.
+
+Before this framework existed, NVMEM drivers like eeprom were stored in
+drivers/misc, where they all had to duplicate pretty much the same code to
+register a sysfs file, allow in-kernel users to access the content of the
+devices they were driving, etc.
+
+This was also a problem as far as other in-kernel users were involved, since
+the solutions used were pretty much different from one driver to another, there
+was a rather big abstraction leak.
+
+This framework aims at solve these problems. It also introduces DT
+representation for consumer devices to go get the data they require (MAC
+Addresses, SoC/Revision ID, part numbers, and so on) from the NVMEMs.
+
+NVMEM Providers
++++++++++++++++
+
+NVMEM provider refers to an entity that implements methods to initialize, read
+and write the non-volatile memory.
+
+2. Registering/Unregistering the NVMEM provider
+===============================================
+
+A NVMEM provider can register with NVMEM core by supplying relevant
+nvmem configuration to nvmem_register(), on success core would return a valid
+nvmem_device pointer.
+
+nvmem_unregister(nvmem) is used to unregister a previously registered provider.
+
+For example, a simple nvram case::
+
+ static int brcm_nvram_probe(struct platform_device *pdev)
+ {
+ struct nvmem_config config = {
+ .name = "brcm-nvram",
+ .reg_read = brcm_nvram_read,
+ };
+ ...
+ config.dev = &pdev->dev;
+ config.priv = priv;
+ config.size = resource_size(res);
+
+ devm_nvmem_register(&config);
+ }
+
+Users of board files can define and register nvmem cells using the
+nvmem_cell_table struct::
+
+ static struct nvmem_cell_info foo_nvmem_cells[] = {
+ {
+ .name = "macaddr",
+ .offset = 0x7f00,
+ .bytes = ETH_ALEN,
+ }
+ };
+
+ static struct nvmem_cell_table foo_nvmem_cell_table = {
+ .nvmem_name = "i2c-eeprom",
+ .cells = foo_nvmem_cells,
+ .ncells = ARRAY_SIZE(foo_nvmem_cells),
+ };
+
+ nvmem_add_cell_table(&foo_nvmem_cell_table);
+
+Additionally it is possible to create nvmem cell lookup entries and register
+them with the nvmem framework from machine code as shown in the example below::
+
+ static struct nvmem_cell_lookup foo_nvmem_lookup = {
+ .nvmem_name = "i2c-eeprom",
+ .cell_name = "macaddr",
+ .dev_id = "foo_mac.0",
+ .con_id = "mac-address",
+ };
+
+ nvmem_add_cell_lookups(&foo_nvmem_lookup, 1);
+
+NVMEM Consumers
++++++++++++++++
+
+NVMEM consumers are the entities which make use of the NVMEM provider to
+read from and to NVMEM.
+
+3. NVMEM cell based consumer APIs
+=================================
+
+NVMEM cells are the data entries/fields in the NVMEM.
+The NVMEM framework provides 3 APIs to read/write NVMEM cells::
+
+ struct nvmem_cell *nvmem_cell_get(struct device *dev, const char *name);
+ struct nvmem_cell *devm_nvmem_cell_get(struct device *dev, const char *name);
+
+ void nvmem_cell_put(struct nvmem_cell *cell);
+ void devm_nvmem_cell_put(struct device *dev, struct nvmem_cell *cell);
+
+ void *nvmem_cell_read(struct nvmem_cell *cell, ssize_t *len);
+ int nvmem_cell_write(struct nvmem_cell *cell, void *buf, ssize_t len);
+
+`*nvmem_cell_get()` apis will get a reference to nvmem cell for a given id,
+and nvmem_cell_read/write() can then read or write to the cell.
+Once the usage of the cell is finished the consumer should call
+`*nvmem_cell_put()` to free all the allocation memory for the cell.
+
+4. Direct NVMEM device based consumer APIs
+==========================================
+
+In some instances it is necessary to directly read/write the NVMEM.
+To facilitate such consumers NVMEM framework provides below apis::
+
+ struct nvmem_device *nvmem_device_get(struct device *dev, const char *name);
+ struct nvmem_device *devm_nvmem_device_get(struct device *dev,
+ const char *name);
+ struct nvmem_device *nvmem_device_find(void *data,
+ int (*match)(struct device *dev, const void *data));
+ void nvmem_device_put(struct nvmem_device *nvmem);
+ int nvmem_device_read(struct nvmem_device *nvmem, unsigned int offset,
+ size_t bytes, void *buf);
+ int nvmem_device_write(struct nvmem_device *nvmem, unsigned int offset,
+ size_t bytes, void *buf);
+ int nvmem_device_cell_read(struct nvmem_device *nvmem,
+ struct nvmem_cell_info *info, void *buf);
+ int nvmem_device_cell_write(struct nvmem_device *nvmem,
+ struct nvmem_cell_info *info, void *buf);
+
+Before the consumers can read/write NVMEM directly, it should get hold
+of nvmem_controller from one of the `*nvmem_device_get()` api.
+
+The difference between these apis and cell based apis is that these apis always
+take nvmem_device as parameter.
+
+5. Releasing a reference to the NVMEM
+=====================================
+
+When a consumer no longer needs the NVMEM, it has to release the reference
+to the NVMEM it has obtained using the APIs mentioned in the above section.
+The NVMEM framework provides 2 APIs to release a reference to the NVMEM::
+
+ void nvmem_cell_put(struct nvmem_cell *cell);
+ void devm_nvmem_cell_put(struct device *dev, struct nvmem_cell *cell);
+ void nvmem_device_put(struct nvmem_device *nvmem);
+ void devm_nvmem_device_put(struct device *dev, struct nvmem_device *nvmem);
+
+Both these APIs are used to release a reference to the NVMEM and
+devm_nvmem_cell_put and devm_nvmem_device_put destroys the devres associated
+with this NVMEM.
+
+Userspace
++++++++++
+
+6. Userspace binary interface
+==============================
+
+Userspace can read/write the raw NVMEM file located at::
+
+ /sys/bus/nvmem/devices/*/nvmem
+
+ex::
+
+ hexdump /sys/bus/nvmem/devices/qfprom0/nvmem
+
+ 0000000 0000 0000 0000 0000 0000 0000 0000 0000
+ *
+ 00000a0 db10 2240 0000 e000 0c00 0c00 0000 0c00
+ 0000000 0000 0000 0000 0000 0000 0000 0000 0000
+ ...
+ *
+ 0001000
+
+7. DeviceTree Binding
+=====================
+
+See Documentation/devicetree/bindings/nvmem/nvmem.txt
+
+8. NVMEM layouts
+================
+
+NVMEM layouts are yet another mechanism to create cells. With the device
+tree binding it is possible to specify simple cells by using an offset
+and a length. Sometimes, the cells doesn't have a static offset, but
+the content is still well defined, e.g. tag-length-values. In this case,
+the NVMEM device content has to be first parsed and the cells need to
+be added accordingly. Layouts let you read the content of the NVMEM device
+and let you add cells dynamically.
+
+Another use case for layouts is the post processing of cells. With layouts,
+it is possible to associate a custom post processing hook to a cell. It
+even possible to add this hook to cells not created by the layout itself.