This patch add basic how-to and api summary documentation for simple NVMEM framework. Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@xxxxxxxxxx> --- Documentation/nvmem/nvmem.txt | 151 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 151 insertions(+) create mode 100644 Documentation/nvmem/nvmem.txt diff --git a/Documentation/nvmem/nvmem.txt b/Documentation/nvmem/nvmem.txt new file mode 100644 index 0000000..28b3579 --- /dev/null +++ b/Documentation/nvmem/nvmem.txt @@ -0,0 +1,151 @@ + NVMEM SUBSYSTEM + Srinivas Kandagatla <srinivas.kandagatla@xxxxxxxxxx> + +This document explains the Simple NVMEM Framework along with the APIs provided, +and how-to-use. + +1. Introduction +=============== +*NVMEM* is the abbreviation for Non Volatile Memory layer. It is used to +retrieve configuration or SOC or Device specific data from a non volatile memories +like eeprom, efuses and so on. + +Up until now, 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 on driver to another, there +was a rather big abstraction leak. + +Introduction of this framework aims at solving this. 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 suppling relevant +nvmem configuration to nvmem_register(), on success core would return a valid +nvmem_device pointer. + +nvmem_unregister(nvmem) is used to unregister the already registered provider. + +For example for 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. + +NVMEM Consumers ++++++++++++++++ + +NVMEM consumers are the entities which make use of the NVMEM provider to +read/write into 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 directly 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. + +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 the consumers 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. +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/class/nvmem/*/nvmem + +ex: + +hexdump /sys/class/nvmem/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 +===================== + +The documentation for NVMEM dt binding can be found @ +Documentation/devicetree/bindings/nvmem/nvmem.txt -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe devicetree" in