[PATCH v6 5/9] Documentation: nvmem: add nvmem api level and how-to doc

[Srinivas Kandagatla <srinivas.kandagatla@linaro.org>]

This patch add basic how-to and api summary documentation for simple
NVMEM framework.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
---
 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@linaro.org>
+
+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