[PATCH 01/14] Documentation: Add SoundWire summary

[Vinod Koul]

From: Sanyog Kale <sanyog.r.kale@xxxxxxxxx>

SoundWire is a new Linux bus which implements a new MIPI bus protocol
'SoundWire'. The summary of SoundWire bus and registration APIs is
documented in the 'summary' file.

Signed-off-by: Sanyog Kale <sanyog.r.kale@xxxxxxxxx>
Signed-off-by: Hardik T Shah <hardik.t.shah@xxxxxxxxx>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxxxxxxxx>
Signed-off-by: Vinod Koul <vinod.koul@xxxxxxxxx>
---
 Documentation/soundwire/summary.txt | 192 ++++++++++++++++++++++++++++++++++++
 1 file changed, 192 insertions(+)
 create mode 100644 Documentation/soundwire/summary.txt

diff --git a/Documentation/soundwire/summary.txt b/Documentation/soundwire/summary.txt
new file mode 100644
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+SoundWire
+===========
+
+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.
+It programs all the MIPI defined Slave registers. It represents a SoundWire
+Master. There can be multiple instances of Bus maybe 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 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.
+
+int sdw_add_bus_master(struct sdw_bus *bus)
+{
+        if (!bus->dev)
+                return -ENODEV;
+
+        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 the specific 48-bit
+identifier, Bus enumerates the Slave device based on the 48-bit identifier.
+Slave device and driver match is done based on this 48-bit identifier. Probe
+of the Slave driver is called by Bus on successful match between device and
+driver id. A parent/child relationship is enforced between Slave and Master
+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 an
+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.
+
+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)
-- 
2.7.4