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Subject[PATCH v2 3/7] docs: driver-api: Add I3C documentation
Add the I3C documentation describing the protocol, the master driver API
and the device driver API.

Signed-off-by: Boris Brezillon <>
Changes in v2:
- Moved out of patch "i3c: Add core I3C infrastructure"
- Add link to the I3C spec
- Move rst files in Documentation/driver-api/i3c/
Documentation/driver-api/i3c/ | 10 +
Documentation/driver-api/i3c/device-driver-api.rst | 7 +
Documentation/driver-api/i3c/index.rst | 9 +
Documentation/driver-api/i3c/master-driver-api.rst | 8 +
Documentation/driver-api/i3c/protocol.rst | 201 +++++++++++++++++++++
Documentation/driver-api/index.rst | 1 +
6 files changed, 236 insertions(+)
create mode 100644 Documentation/driver-api/i3c/
create mode 100644 Documentation/driver-api/i3c/device-driver-api.rst
create mode 100644 Documentation/driver-api/i3c/index.rst
create mode 100644 Documentation/driver-api/i3c/master-driver-api.rst
create mode 100644 Documentation/driver-api/i3c/protocol.rst

diff --git a/Documentation/driver-api/i3c/ b/Documentation/driver-api/i3c/
new file mode 100644
index 000000000000..5a20832d59a7
--- /dev/null
+++ b/Documentation/driver-api/i3c/
@@ -0,0 +1,10 @@
+# -*- coding: utf-8; mode: python -*-
+project = "Linux I3C Subsystem"
+latex_documents = [
+ ('index', 'i3c.tex', project,
+ 'The kernel development community', 'manual'),
diff --git a/Documentation/driver-api/i3c/device-driver-api.rst b/Documentation/driver-api/i3c/device-driver-api.rst
new file mode 100644
index 000000000000..63c843f148a6
--- /dev/null
+++ b/Documentation/driver-api/i3c/device-driver-api.rst
@@ -0,0 +1,7 @@
+I3C device driver API
+.. kernel-doc:: include/linux/i3c/device.h
+.. kernel-doc:: drivers/i3c/device.c
diff --git a/Documentation/driver-api/i3c/index.rst b/Documentation/driver-api/i3c/index.rst
new file mode 100644
index 000000000000..9c439220439d
--- /dev/null
+++ b/Documentation/driver-api/i3c/index.rst
@@ -0,0 +1,9 @@
+I3C subsystem
+.. toctree::
+ protocol
+ device-driver-api
+ master-driver-api
diff --git a/Documentation/driver-api/i3c/master-driver-api.rst b/Documentation/driver-api/i3c/master-driver-api.rst
new file mode 100644
index 000000000000..017e7711cdf7
--- /dev/null
+++ b/Documentation/driver-api/i3c/master-driver-api.rst
@@ -0,0 +1,8 @@
+I3C master controller driver API
+.. kernel-doc:: drivers/i3c/master.c
+.. kernel-doc:: include/linux/i3c/master.h
diff --git a/Documentation/driver-api/i3c/protocol.rst b/Documentation/driver-api/i3c/protocol.rst
new file mode 100644
index 000000000000..891c4f50c472
--- /dev/null
+++ b/Documentation/driver-api/i3c/protocol.rst
@@ -0,0 +1,201 @@
+I3C protocol
+This chapter will focus on aspects that matter to software developers. For
+everything hardware related (like how things are transmitted on the bus, how
+collisions are prevented, ...) please have a look at the I3C specification.
+This document is just a brief introduction to the I3C protocol and the concepts
+it brings on the table. If you need more information, please refer to the MIPI
+I3C specification (can be downloaded here
+The I3C (pronounced 'eye-three-see') is a MIPI standardized protocol designed
+to overcome I2C limitations (limited speed, external signals needed for
+interrupts, no automatic detection of the devices connected to the bus, ...)
+while remaining power-efficient.
+I3C Bus
+An I3C bus is made of several I3C devices and possibly some I2C devices as
+well, but let's focus on I3C devices for now.
+An I3C device on the I3C bus can have one of the following roles:
+* Master: the device is driving the bus. It's the one in charge of initiating
+ transactions or deciding who is allowed to talk on the bus (slave generated
+ events are possible in I3C, see below).
+* Slave: the device acts as a slave, and is not able to send frames to another
+ slave on the bus. The device can still send events to the master on
+ its own initiative if the master allowed it.
+I3C is a multi-master protocol, so there might be several masters on a bus,
+though only one device can act as a master at a given time. In order to gain
+bus ownership, a master has to follow a specific procedure.
+Each device on the I3C bus has to be assigned a dynamic address to be able to
+communicate. Until this is done, the device should only respond to a limited
+set of commands. If it has a static address (also called legacy I2C address),
+the device can reply to I2C transfers.
+In addition to these per-device addresses, the protocol defines a broadcast
+address in order to address all devices on the bus.
+Once a dynamic address has been assigned to a device, this address will be used
+for any direct communication with the device. Note that even after being
+assigned a dynamic address, the device should still process broadcast messages.
+I3C Device discovery
+The I3C protocol defines a mechanism to automatically discover devices present
+on the bus, their capabilities and the functionalities they provide. In this
+regard I3C is closer to a discoverable bus like USB than it is to I2C or SPI.
+The discovery mechanism is called DAA (Dynamic Address Assignment), because it
+not only discovers devices but also assigns them a dynamic address.
+During DAA, each I3C device reports 3 important things:
+* BCR: Bus Characteristic Register. This 8-bit register describes the device bus
+ related capabilities
+* DCR: Device Characteristic Register. This 8-bit register describes the
+ functionalities provided by the device
+* Provisional ID: A 48-bit unique identifier. On a given bus there should be no
+ Provisional ID collision, otherwise the discovery mechanism may fail.
+I3C slave events
+The I3C protocol allows slaves to generate events on their own, and thus allows
+them to take temporary control of the bus.
+This mechanism is called IBI for In Band Interrupts, and as stated in the name,
+it allows devices to generate interrupts without requiring an external signal.
+During DAA, each device on the bus has been assigned an address, and this
+address will serve as a priority identifier to determine who wins if 2 different
+devices are generating an interrupt at the same moment on the bus (the lower the
+dynamic address the higher the priority).
+Masters are allowed to inhibit interrupts if they want to. This inhibition
+request can be broadcasted (applies to all devices) or sent to a specific
+I3C Hot-Join
+The Hot-Join mechanism is similart to USB hotplug. This mechanism allows
+slaves to join the bus after it has been initialized by the master.
+This covers the following use cases:
+* the device is not powered when the bus is probed
+* the device is hotplugged on the bus through an extension board
+This mechanism is relying on slave events to inform the master that a new
+device joined the bus and is waiting for a dynamic address.
+The master is then free to address the request as it wishes: ignore it or
+assign a dynamic address to the slave.
+I3C transfer types
+If you omit SMBus (which is just a standardization on how to access registers
+exposed by I2C devices), I2C has only one transfer type.
+I3C defines 3 different classes of transfer in addition to I2C transfers which
+are here for backward compatibility with I2C devices.
+I3C CCC commands
+CCC (Common Command Code) commands are meant to be used for anything that is
+related to bus management and all features that are common to a set of devices.
+CCC commands contain an 8-bit CCC id describing the command that is executed.
+The MSB of this id specifies whether this is a broadcast command (bit7 = 0) or a
+unicast one (bit7 = 1).
+The command ID can be followed by a payload. Depending on the command, this
+payload is either sent by the master sending the command (write CCC command),
+or sent by the slave receiving the command (read CCC command). Of course, read
+accesses only apply to unicast commands.
+Note that, when sending a CCC command to a specific device, the device address
+is passed in the first byte of the payload.
+The payload length is not explicitly passed on the bus, and should be extracted
+from the CCC id.
+Note that vendors can use a dedicated range of CCC ids for their own commands
+(0x61-0x7f and 0xe0-0xef).
+I3C Private SDR transfers
+Private SDR (Single Data Rate) transfers should be used for anything that is
+device specific and does not require high transfer speed.
+It is the equivalent of I2C transfers but in the I3C world. Each transfer is
+passed the device address (dynamic address assigned during DAA), a payload
+and a direction.
+The only difference with I2C is that the transfer is much faster (typical SCL
+frequency is 12.5MHz).
+I3C Private HDR commands
+HDR commands should be used for anything that is device specific and requires
+high transfer speed.
+The first thing attached to an HDR command is the HDR mode. There are currently
+3 different modes defined by the I3C specification (refer to the specification
+for more details):
+* HDR-DDR: Double Data Rate mode
+* HDR-TSP: Ternary Symbol Pure. Only usable on busses with no I2C devices
+* HDR-TSL: Ternary Symbol Legacy. Usable on busses with I2C devices
+When sending an HDR command, the whole bus has to enter HDR mode, which is done
+using a broadcast CCC command.
+Once the bus has entered a specific HDR mode, the master sends the HDR command.
+An HDR command is made of:
+* one 16-bits command word
+* N 16-bits data words
+Those words may be wrapped with specific preambles/post-ambles which depend on
+the chosen HDR mode and are detailed here (see the specification for more
+The 16-bits command word is made of:
+* bit[15]: direction bit, read is 1 write is 0
+* bit[14:8]: command code. Identifies the command being executed, the amount of
+ data words and their meaning
+* bit[7:1]: I3C address of the device this command is addressed to
+* bit[0]: reserved/parity-bit
+Backward compatibility with I2C devices
+The I3C protocol has been designed to be backward compatible with I2C devices.
+This backward compatibility allows one to connect a mix of I2C and I3C devices
+on the same bus, though, in order to be really efficient, I2C devices should
+be equipped with 50 ns spike filters.
+I2C devices can't be discovered like I3C ones and have to be statically
+declared. In order to let the master know what these devices are capable of
+(both in terms of bus related limitations and functionalities), the software
+has to provide some information, which is done through the LVR (Legacy I2C
+Virtual Register).
diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index d17a9876b473..8896d3868ae1 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -30,6 +30,7 @@ available subsections can be seen below.
+ i3c/index
 \ /
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