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