| ============================ | |
| Linux kernel SLIMbus support | |
| ============================ | |
| Overview | |
| ======== | |
| What is SLIMbus? | |
| ---------------- | |
| SLIMbus (Serial Low Power Interchip Media Bus) is a specification developed by | |
| MIPI (Mobile Industry Processor Interface) alliance. The bus uses master/slave | |
| configuration, and is a 2-wire multi-drop implementation (clock, and data). | |
| Currently, SLIMbus is used to interface between application processors of SoCs | |
| (System-on-Chip) and peripheral components (typically codec). SLIMbus uses | |
| Time-Division-Multiplexing to accommodate multiple data channels, and | |
| a control channel. | |
| The control channel is used for various control functions such as bus | |
| management, configuration and status updates. These messages can be unicast (e.g. | |
| reading/writing device specific values), or multicast (e.g. data channel | |
| reconfiguration sequence is a broadcast message announced to all devices) | |
| A data channel is used for data-transfer between 2 SLIMbus devices. Data | |
| channel uses dedicated ports on the device. | |
| Hardware description: | |
| --------------------- | |
| SLIMbus specification has different types of device classifications based on | |
| their capabilities. | |
| A manager device is responsible for enumeration, configuration, and dynamic | |
| channel allocation. Every bus has 1 active manager. | |
| A generic device is a device providing application functionality (e.g. codec). | |
| Framer device is responsible for clocking the bus, and transmitting frame-sync | |
| and framing information on the bus. | |
| Each SLIMbus component has an interface device for monitoring physical layer. | |
| Typically each SoC contains SLIMbus component having 1 manager, 1 framer device, | |
| 1 generic device (for data channel support), and 1 interface device. | |
| External peripheral SLIMbus component usually has 1 generic device (for | |
| functionality/data channel support), and an associated interface device. | |
| The generic device's registers are mapped as 'value elements' so that they can | |
| be written/read using SLIMbus control channel exchanging control/status type of | |
| information. | |
| In case there are multiple framer devices on the same bus, manager device is | |
| responsible to select the active-framer for clocking the bus. | |
| Per specification, SLIMbus uses "clock gears" to do power management based on | |
| current frequency and bandwidth requirements. There are 10 clock gears and each | |
| gear changes the SLIMbus frequency to be twice its previous gear. | |
| Each device has a 6-byte enumeration-address and the manager assigns every | |
| device with a 1-byte logical address after the devices report presence on the | |
| bus. | |
| Software description: | |
| --------------------- | |
| There are 2 types of SLIMbus drivers: | |
| slim_controller represents a 'controller' for SLIMbus. This driver should | |
| implement duties needed by the SoC (manager device, associated | |
| interface device for monitoring the layers and reporting errors, default | |
| framer device). | |
| slim_device represents the 'generic device/component' for SLIMbus, and a | |
| slim_driver should implement driver for that slim_device. | |
| Device notifications to the driver: | |
| ----------------------------------- | |
| Since SLIMbus devices have mechanisms for reporting their presence, the | |
| framework allows drivers to bind when corresponding devices report their | |
| presence on the bus. | |
| However, it is possible that the driver needs to be probed | |
| first so that it can enable corresponding SLIMbus device (e.g. power it up and/or | |
| take it out of reset). To support that behavior, the framework allows drivers | |
| to probe first as well (e.g. using standard DeviceTree compatibility field). | |
| This creates the necessity for the driver to know when the device is functional | |
| (i.e. reported present). device_up callback is used for that reason when the | |
| device reports present and is assigned a logical address by the controller. | |
| Similarly, SLIMbus devices 'report absent' when they go down. A 'device_down' | |
| callback notifies the driver when the device reports absent and its logical | |
| address assignment is invalidated by the controller. | |
| Another notification "boot_device" is used to notify the slim_driver when | |
| controller resets the bus. This notification allows the driver to take necessary | |
| steps to boot the device so that it's functional after the bus has been reset. | |
| Driver and Controller APIs: | |
| --------------------------- | |
| .. kernel-doc:: include/linux/slimbus.h | |
| :internal: | |
| .. kernel-doc:: drivers/slimbus/slimbus.h | |
| :internal: | |
| .. kernel-doc:: drivers/slimbus/core.c | |
| :export: | |
| Clock-pause: | |
| ------------ | |
| SLIMbus mandates that a reconfiguration sequence (known as clock-pause) be | |
| broadcast to all active devices on the bus before the bus can enter low-power | |
| mode. Controller uses this sequence when it decides to enter low-power mode so | |
| that corresponding clocks and/or power-rails can be turned off to save power. | |
| Clock-pause is exited by waking up framer device (if controller driver initiates | |
| exiting low power mode), or by toggling the data line (if a slave device wants | |
| to initiate it). | |
| Clock-pause APIs: | |
| ~~~~~~~~~~~~~~~~~ | |
| .. kernel-doc:: drivers/slimbus/sched.c | |
| :export: | |
| Messaging: | |
| ---------- | |
| The framework supports regmap and read/write apis to exchange control-information | |
| with a SLIMbus device. APIs can be synchronous or asynchronous. | |
| The header file <linux/slimbus.h> has more documentation about messaging APIs. | |
| Messaging APIs: | |
| ~~~~~~~~~~~~~~~ | |
| .. kernel-doc:: drivers/slimbus/messaging.c | |
| :export: |