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diff --git a/Documentation/nvmem/nvmem.txt b/Documentation/nvmem/nvmem.txt new file mode 100644 index 000000000..8d8d8f58f --- /dev/null +++ b/Documentation/nvmem/nvmem.txt @@ -0,0 +1,152 @@ + 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. This +framework is based on regmap, so that most of the abstraction available in +regmap can be reused, across multiple types of buses. + +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 qfprom case: + +static struct nvmem_config econfig = { + .name = "qfprom", + .owner = THIS_MODULE, +}; + +static int qfprom_probe(struct platform_device *pdev) +{ + ... + econfig.dev = &pdev->dev; + nvmem = nvmem_register(&econfig); + ... +} + +It is mandatory that the NVMEM provider has a regmap associated with its +struct device. Failure to do would return error code from nvmem_register(). + +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); +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 |