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diff --git a/Documentation/driver-api/soundwire/summary.rst b/Documentation/driver-api/soundwire/summary.rst new file mode 100644 index 000000000..01dcb954f --- /dev/null +++ b/Documentation/driver-api/soundwire/summary.rst @@ -0,0 +1,208 @@ +=========================== +SoundWire Subsystem Summary +=========================== + +SoundWire is a new interface ratified in 2015 by the MIPI Alliance. +SoundWire is used for transporting data typically related to audio +functions. SoundWire interface is optimized to integrate audio devices in +mobile or mobile inspired systems. + +SoundWire is a 2-pin multi-drop interface with data and clock line. It +facilitates development of low cost, efficient, high performance systems. +Broad level key features of SoundWire interface include: + + (1) Transporting all of payload data channels, control information, and setup + commands over a single two-pin interface. + + (2) Lower clock frequency, and hence lower power consumption, by use of DDR + (Dual Data Rate) data transmission. + + (3) Clock scaling and optional multiple data lanes to give wide flexibility + in data rate to match system requirements. + + (4) Device status monitoring, including interrupt-style alerts to the Master. + +The SoundWire protocol supports up to eleven Slave interfaces. All the +interfaces share the common Bus containing data and clock line. Each of the +Slaves can support up to 14 Data Ports. 13 Data Ports are dedicated to audio +transport. Data Port0 is dedicated to transport of Bulk control information, +each of the audio Data Ports (1..14) can support up to 8 Channels in +transmit or receiving mode (typically fixed direction but configurable +direction is enabled by the specification). Bandwidth restrictions to +~19.2..24.576Mbits/s don't however allow for 11*13*8 channels to be +transmitted simultaneously. + +Below figure shows an example of connectivity between a SoundWire Master and +two Slave devices. :: + + +---------------+ +---------------+ + | | Clock Signal | | + | Master |-------+-------------------------------| Slave | + | Interface | | Data Signal | Interface 1 | + | |-------|-------+-----------------------| | + +---------------+ | | +---------------+ + | | + | | + | | + +--+-------+--+ + | | + | Slave | + | Interface 2 | + | | + +-------------+ + + +Terminology +=========== + +The MIPI SoundWire specification uses the term 'device' to refer to a Master +or Slave interface, which of course can be confusing. In this summary and +code we use the term interface only to refer to the hardware. We follow the +Linux device model by mapping each Slave interface connected on the bus as a +device managed by a specific driver. The Linux SoundWire subsystem provides +a framework to implement a SoundWire Slave driver with an API allowing +3rd-party vendors to enable implementation-defined functionality while +common setup/configuration tasks are handled by the bus. + +Bus: +Implements SoundWire Linux Bus which handles the SoundWire protocol. +Programs all the MIPI-defined Slave registers. Represents a SoundWire +Master. Multiple instances of Bus may be present in a system. + +Slave: +Registers as SoundWire Slave device (Linux Device). Multiple Slave devices +can register to a Bus instance. + +Slave driver: +Driver controlling the Slave device. MIPI-specified registers are controlled +directly by the Bus (and transmitted through the Master driver/interface). +Any implementation-defined Slave register is controlled by Slave driver. In +practice, it is expected that the Slave driver relies on regmap and does not +request direct register access. + +Programming interfaces (SoundWire Master interface Driver) +========================================================== + +SoundWire Bus supports programming interfaces for the SoundWire Master +implementation and SoundWire Slave devices. All the code uses the "sdw" +prefix commonly used by SoC designers and 3rd party vendors. + +Each of the SoundWire Master interfaces needs to be registered to the Bus. +Bus implements API to read standard Master MIPI properties and also provides +callback in Master ops for Master driver to implement its own functions that +provides capabilities information. DT support is not implemented at this +time but should be trivial to add since capabilities are enabled with the +``device_property_`` API. + +The Master interface along with the Master interface capabilities are +registered based on board file, DT or ACPI. + +Following is the Bus API to register the SoundWire Bus: + +.. code-block:: c + + int sdw_bus_master_add(struct sdw_bus *bus, + struct device *parent, + struct fwnode_handle) + { + sdw_master_device_add(bus, parent, fwnode); + + mutex_init(&bus->lock); + INIT_LIST_HEAD(&bus->slaves); + + /* Check ACPI for Slave devices */ + sdw_acpi_find_slaves(bus); + + /* Check DT for Slave devices */ + sdw_of_find_slaves(bus); + + return 0; + } + +This will initialize sdw_bus object for Master device. "sdw_master_ops" and +"sdw_master_port_ops" callback functions are provided to the Bus. + +"sdw_master_ops" is used by Bus to control the Bus in the hardware specific +way. It includes Bus control functions such as sending the SoundWire +read/write messages on Bus, setting up clock frequency & Stream +Synchronization Point (SSP). The "sdw_master_ops" structure abstracts the +hardware details of the Master from the Bus. + +"sdw_master_port_ops" is used by Bus to setup the Port parameters of the +Master interface Port. Master interface Port register map is not defined by +MIPI specification, so Bus calls the "sdw_master_port_ops" callback +function to do Port operations like "Port Prepare", "Port Transport params +set", "Port enable and disable". The implementation of the Master driver can +then perform hardware-specific configurations. + +Programming interfaces (SoundWire Slave Driver) +=============================================== + +The MIPI specification requires each Slave interface to expose a unique +48-bit identifier, stored in 6 read-only dev_id registers. This dev_id +identifier contains vendor and part information, as well as a field enabling +to differentiate between identical components. An additional class field is +currently unused. Slave driver is written for a specific vendor and part +identifier, Bus enumerates the Slave device based on these two ids. +Slave device and driver match is done based on these two ids . Probe +of the Slave driver is called by Bus on successful match between device and +driver id. A parent/child relationship is enforced between Master and Slave +devices (the logical representation is aligned with the physical +connectivity). + +The information on Master/Slave dependencies is stored in platform data, +board-file, ACPI or DT. The MIPI Software specification defines additional +link_id parameters for controllers that have multiple Master interfaces. The +dev_id registers are only unique in the scope of a link, and the link_id +unique in the scope of a controller. Both dev_id and link_id are not +necessarily unique at the system level but the parent/child information is +used to avoid ambiguity. + +.. code-block:: c + + static const struct sdw_device_id slave_id[] = { + SDW_SLAVE_ENTRY(0x025d, 0x700, 0), + {}, + }; + MODULE_DEVICE_TABLE(sdw, slave_id); + + static struct sdw_driver slave_sdw_driver = { + .driver = { + .name = "slave_xxx", + .pm = &slave_runtime_pm, + }, + .probe = slave_sdw_probe, + .remove = slave_sdw_remove, + .ops = &slave_slave_ops, + .id_table = slave_id, + }; + + +For capabilities, Bus implements API to read standard Slave MIPI properties +and also provides callback in Slave ops for Slave driver to implement own +function that provides capabilities information. Bus needs to know a set of +Slave capabilities to program Slave registers and to control the Bus +reconfigurations. + +Future enhancements to be done +============================== + + (1) Bulk Register Access (BRA) transfers. + + + (2) Multiple data lane support. + +Links +===== + +SoundWire MIPI specification 1.1 is available at: +https://members.mipi.org/wg/All-Members/document/70290 + +SoundWire MIPI DisCo (Discovery and Configuration) specification is +available at: +https://www.mipi.org/specifications/mipi-disco-soundwire + +(publicly accessible with registration or directly accessible to MIPI +members) + +MIPI Alliance Manufacturer ID Page: mid.mipi.org |