1 ===========================
2 SoundWire Subsystem Summary
3 ===========================
4
5 SoundWire is a new interface ratified in 2015 by the MIPI Alliance.
6 SoundWire is used for transporting data typically related to audio
7 functions. SoundWire interface is optimized to integrate audio devices in
8 mobile or mobile inspired systems.
9
10 SoundWire is a 2-pin multi-drop interface with data and clock line. It
11 facilitates development of low cost, efficient, high performance systems.
12 Broad level key features of SoundWire interface include:
13
14 (1) Transporting all of payload data channels, control information, and setup
15 commands over a single two-pin interface.
16
17 (2) Lower clock frequency, and hence lower power consumption, by use of DDR
18 (Dual Data Rate) data transmission.
19
20 (3) Clock scaling and optional multiple data lanes to give wide flexibility
21 in data rate to match system requirements.
22
23 (4) Device status monitoring, including interrupt-style alerts to the Master.
24
25 The SoundWire protocol supports up to eleven Slave interfaces. All the
26 interfaces share the common Bus containing data and clock line. Each of the
27 Slaves can support up to 14 Data Ports. 13 Data Ports are dedicated to audio
28 transport. Data Port0 is dedicated to transport of Bulk control information,
29 each of the audio Data Ports (1..14) can support up to 8 Channels in
30 transmit or receiving mode (typically fixed direction but configurable
31 direction is enabled by the specification). Bandwidth restrictions to
32 ~19.2..24.576Mbits/s don't however allow for 11*13*8 channels to be
33 transmitted simultaneously.
34
35 Below figure shows an example of connectivity between a SoundWire Master and
36 two Slave devices. ::
37
38 +---------------+ +---------------+
39 | | Clock Signal | |
40 | Master |-------+-------------------------------| Slave |
41 | Interface | | Data Signal | Interface 1 |
42 | |-------|-------+-----------------------| |
43 +---------------+ | | +---------------+
44 | |
45 | |
46 | |
47 +--+-------+--+
48 | |
49 | Slave |
50 | Interface 2 |
51 | |
52 +-------------+
53
54
55 Terminology
56 ===========
57
58 The MIPI SoundWire specification uses the term 'device' to refer to a Master
59 or Slave interface, which of course can be confusing. In this summary and
60 code we use the term interface only to refer to the hardware. We follow the
61 Linux device model by mapping each Slave interface connected on the bus as a
62 device managed by a specific driver. The Linux SoundWire subsystem provides
63 a framework to implement a SoundWire Slave driver with an API allowing
64 3rd-party vendors to enable implementation-defined functionality while
65 common setup/configuration tasks are handled by the bus.
66
67 Bus:
68 Implements SoundWire Linux Bus which handles the SoundWire protocol.
69 Programs all the MIPI-defined Slave registers. Represents a SoundWire
70 Master. Multiple instances of Bus may be present in a system.
71
72 Slave:
73 Registers as SoundWire Slave device (Linux Device). Multiple Slave devices
74 can register to a Bus instance.
75
76 Slave driver:
77 Driver controlling the Slave device. MIPI-specified registers are controlled
78 directly by the Bus (and transmitted through the Master driver/interface).
79 Any implementation-defined Slave register is controlled by Slave driver. In
80 practice, it is expected that the Slave driver relies on regmap and does not
81 request direct register access.
82
83 Programming interfaces (SoundWire Master interface Driver)
84 ==========================================================
85
86 SoundWire Bus supports programming interfaces for the SoundWire Master
87 implementation and SoundWire Slave devices. All the code uses the "sdw"
88 prefix commonly used by SoC designers and 3rd party vendors.
89
90 Each of the SoundWire Master interfaces needs to be registered to the Bus.
91 Bus implements API to read standard Master MIPI properties and also provides
92 callback in Master ops for Master driver to implement its own functions that
93 provides capabilities information. DT support is not implemented at this
94 time but should be trivial to add since capabilities are enabled with the
95 ``device_property_`` API.
96
97 The Master interface along with the Master interface capabilities are
98 registered based on board file, DT or ACPI.
99
100 Following is the Bus API to register the SoundWire Bus:
101
102 .. code-block:: c
103
104 int sdw_bus_master_add(struct sdw_bus *bus,
105 struct device *parent,
106 struct fwnode_handle)
107 {
108 sdw_master_device_add(bus, parent, fwnode);
109
110 mutex_init(&bus->lock);
111 INIT_LIST_HEAD(&bus->slaves);
112
113 /* Check ACPI for Slave devices */
114 sdw_acpi_find_slaves(bus);
115
116 /* Check DT for Slave devices */
117 sdw_of_find_slaves(bus);
118
119 return 0;
120 }
121
122 This will initialize sdw_bus object for Master device. "sdw_master_ops" and
123 "sdw_master_port_ops" callback functions are provided to the Bus.
124
125 "sdw_master_ops" is used by Bus to control the Bus in the hardware specific
126 way. It includes Bus control functions such as sending the SoundWire
127 read/write messages on Bus, setting up clock frequency & Stream
128 Synchronization Point (SSP). The "sdw_master_ops" structure abstracts the
129 hardware details of the Master from the Bus.
130
131 "sdw_master_port_ops" is used by Bus to setup the Port parameters of the
132 Master interface Port. Master interface Port register map is not defined by
133 MIPI specification, so Bus calls the "sdw_master_port_ops" callback
134 function to do Port operations like "Port Prepare", "Port Transport params
135 set", "Port enable and disable". The implementation of the Master driver can
136 then perform hardware-specific configurations.
137
138 Programming interfaces (SoundWire Slave Driver)
139 ===============================================
140
141 The MIPI specification requires each Slave interface to expose a unique
142 48-bit identifier, stored in 6 read-only dev_id registers. This dev_id
143 identifier contains vendor and part information, as well as a field enabling
144 to differentiate between identical components. An additional class field is
145 currently unused. Slave driver is written for a specific vendor and part
146 identifier, Bus enumerates the Slave device based on these two ids.
147 Slave device and driver match is done based on these two ids . Probe
148 of the Slave driver is called by Bus on successful match between device and
149 driver id. A parent/child relationship is enforced between Master and Slave
150 devices (the logical representation is aligned with the physical
151 connectivity).
152
153 The information on Master/Slave dependencies is stored in platform data,
154 board-file, ACPI or DT. The MIPI Software specification defines additional
155 link_id parameters for controllers that have multiple Master interfaces. The
156 dev_id registers are only unique in the scope of a link, and the link_id
157 unique in the scope of a controller. Both dev_id and link_id are not
158 necessarily unique at the system level but the parent/child information is
159 used to avoid ambiguity.
160
161 .. code-block:: c
162
163 static const struct sdw_device_id slave_id[] = {
164 SDW_SLAVE_ENTRY(0x025d, 0x700, 0),
165 {},
166 };
167 MODULE_DEVICE_TABLE(sdw, slave_id);
168
169 static struct sdw_driver slave_sdw_driver = {
170 .driver = {
171 .name = "slave_xxx",
172 .pm = &slave_runtime_pm,
173 },
174 .probe = slave_sdw_probe,
175 .remove = slave_sdw_remove,
176 .ops = &slave_slave_ops,
177 .id_table = slave_id,
178 };
179
180
181 For capabilities, Bus implements API to read standard Slave MIPI properties
182 and also provides callback in Slave ops for Slave driver to implement own
183 function that provides capabilities information. Bus needs to know a set of
184 Slave capabilities to program Slave registers and to control the Bus
185 reconfigurations.
186
187 Future enhancements to be done
188 ==============================
189
190 (1) Bulk Register Access (BRA) transfers.
191
192
193 (2) Multiple data lane support.
194
195 Links
196 =====
197
198 SoundWire MIPI specification 1.1 is available at:
199 https://members.mipi.org/wg/All-Members/document/70290
200
201 SoundWire MIPI DisCo (Discovery and Configuration) specification is
202 available at:
203 https://www.mipi.org/specifications/mipi-disco-soundwire
204
205 (publicly accessible with registration or directly accessible to MIPI
206 members)
207
208 MIPI Alliance Manufacturer ID Page: mid.mipi.org
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.