Documentation/i2c/smbus-protocol - kernel/msm-4.9 - Gitiles

 SMBus Protocol Summary
 ======================
 The following is a summary of the SMBus protocol. It applies to
 all revisions of the protocol (1.0, 1.1, and 2.0).
 Certain protocol features which are not supported by
 this package are briefly described at the end of this document.

 Some adapters understand only the SMBus (System Management Bus) protocol,
 which is a subset from the I2C protocol. Fortunately, many devices use
 only the same subset, which makes it possible to put them on an SMBus.
 If you write a driver for some I2C device, please try to use the SMBus
 commands if at all possible (if the device uses only that subset of the
 I2C protocol). This makes it possible to use the device driver on both
 SMBus adapters and I2C adapters (the SMBus command set is automatically
 translated to I2C on I2C adapters, but plain I2C commands can not be
 handled at all on most pure SMBus adapters).

 Below is a list of SMBus commands.

 Key to symbols
 ==============

 S     (1 bit) : Start bit
 P     (1 bit) : Stop bit
 Rd/Wr (1 bit) : Read/Write bit. Rd equals 1, Wr equals 0.
 A, NA (1 bit) : Accept and reverse accept bit.
 Addr  (7 bits): I2C 7 bit address. Note that this can be expanded as usual to
                 get a 10 bit I2C address.
 Comm  (8 bits): Command byte, a data byte which often selects a register on
                 the device.
 Data  (8 bits): A plain data byte. Sometimes, I write DataLow, DataHigh
                 for 16 bit data.
 Count (8 bits): A data byte containing the length of a block operation.

 [..]: Data sent by I2C device, as opposed to data sent by the host adapter.


 SMBus Write Quick
 =================

 This sends a single bit to the device, at the place of the Rd/Wr bit.
 There is no equivalent Read Quick command.

 A Addr Rd/Wr [A] P


 SMBus Read Byte
 ===============

 This reads a single byte from a device, without specifying a device
 register. Some devices are so simple that this interface is enough; for
 others, it is a shorthand if you want to read the same register as in
 the previous SMBus command.

 S Addr Rd [A] [Data] NA P


 SMBus Write Byte
 ================

 This is the reverse of Read Byte: it sends a single byte to a device.
 See Read Byte for more information.

 S Addr Wr [A] Data [A] P


 SMBus Read Byte Data
 ====================

 This reads a single byte from a device, from a designated register.
 The register is specified through the Comm byte.

 S Addr Wr [A] Comm [A] S Addr Rd [A] [Data] NA P


 SMBus Read Word Data
 ====================

 This command is very like Read Byte Data; again, data is read from a
 device, from a designated register that is specified through the Comm
 byte. But this time, the data is a complete word (16 bits).

 S Addr Wr [A] Comm [A] S Addr Rd [A] [DataLow] A [DataHigh] NA P


 SMBus Write Byte Data
 =====================

 This writes a single byte to a device, to a designated register. The
 register is specified through the Comm byte. This is the opposite of
 the Read Byte Data command.

 S Addr Wr [A] Comm [A] Data [A] P


 SMBus Write Word Data
 =====================

 This is the opposite operation of the Read Word Data command. 16 bits
 of data is read from a device, from a designated register that is
 specified through the Comm byte.

 S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A] P


 SMBus Process Call
 ==================

 This command selects a device register (through the Comm byte), sends
 16 bits of data to it, and reads 16 bits of data in return.

 S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A]
                              S Addr Rd [A] [DataLow] A [DataHigh] NA P


 SMBus Block Read
 ================

 This command reads a block of up to 32 bytes from a device, from a
 designated register that is specified through the Comm byte. The amount
 of data is specified by the device in the Count byte.

 S Addr Wr [A] Comm [A]
            S Addr Rd [A] [Count] A [Data] A [Data] A ... A [Data] NA P


 SMBus Block Write
 =================

 The opposite of the Block Read command, this writes up to 32 bytes to
 a device, to a designated register that is specified through the
 Comm byte. The amount of data is specified in the Count byte.

 S Addr Wr [A] Comm [A] Count [A] Data [A] Data [A] ... [A] Data [A] P


 SMBus Block Process Call
 ========================

 SMBus Block Process Call was introduced in Revision 2.0 of the specification.

 This command selects a device register (through the Comm byte), sends
 1 to 31 bytes of data to it, and reads 1 to 31 bytes of data in return.

 S Addr Wr [A] Comm [A] Count [A] Data [A] ...
                              S Addr Rd [A] [Count] A [Data] ... A P


 SMBus Host Notify
 =================

 This command is sent from a SMBus device acting as a master to the
 SMBus host acting as a slave.
 It is the same form as Write Word, with the command code replaced by the
 alerting device's address.

 [S] [HostAddr] [Wr] A [DevAddr] A [DataLow] A [DataHigh] A [P]


 Packet Error Checking (PEC)
 ===========================
 Packet Error Checking was introduced in Revision 1.1 of the specification.

 PEC adds a CRC-8 error-checking byte to all transfers.


 Address Resolution Protocol (ARP)
 =================================
 The Address Resolution Protocol was introduced in Revision 2.0 of
 the specification. It is a higher-layer protocol which uses the
 messages above.

 ARP adds device enumeration and dynamic address assignment to
 the protocol. All ARP communications use slave address 0x61 and
 require PEC checksums.


 I2C Block Transactions
 ======================
 The following I2C block transactions are supported by the
 SMBus layer and are described here for completeness.
 I2C block transactions do not limit the number of bytes transferred
 but the SMBus layer places a limit of 32 bytes.


 I2C Block Read
 ==============

 This command reads a block of bytes from a device, from a
 designated register that is specified through the Comm byte.

 S Addr Wr [A] Comm [A]
            S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P


 I2C Block Read (2 Comm bytes)
 =============================

 This command reads a block of bytes from a device, from a
 designated register that is specified through the two Comm bytes.

 S Addr Wr [A] Comm1 [A] Comm2 [A]
            S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P


 I2C Block Write
 ===============

 The opposite of the Block Read command, this writes bytes to
 a device, to a designated register that is specified through the
 Comm byte. Note that command lengths of 0, 2, or more bytes are
 supported as they are indistinguishable from data.

 S Addr Wr [A] Comm [A] Data [A] Data [A] ... [A] Data [A] P
	SMBus Protocol Summary
	======================
	The following is a summary of the SMBus protocol. It applies to
	all revisions of the protocol (1.0, 1.1, and 2.0).
	Certain protocol features which are not supported by
	this package are briefly described at the end of this document.

	Some adapters understand only the SMBus (System Management Bus) protocol,
	which is a subset from the I2C protocol. Fortunately, many devices use
	only the same subset, which makes it possible to put them on an SMBus.
	If you write a driver for some I2C device, please try to use the SMBus
	commands if at all possible (if the device uses only that subset of the
	I2C protocol). This makes it possible to use the device driver on both
	SMBus adapters and I2C adapters (the SMBus command set is automatically
	translated to I2C on I2C adapters, but plain I2C commands can not be
	handled at all on most pure SMBus adapters).

	Below is a list of SMBus commands.

	Key to symbols
	==============

	S (1 bit) : Start bit
	P (1 bit) : Stop bit
	Rd/Wr (1 bit) : Read/Write bit. Rd equals 1, Wr equals 0.
	A, NA (1 bit) : Accept and reverse accept bit.
	Addr (7 bits): I2C 7 bit address. Note that this can be expanded as usual to
	get a 10 bit I2C address.
	Comm (8 bits): Command byte, a data byte which often selects a register on
	the device.
	Data (8 bits): A plain data byte. Sometimes, I write DataLow, DataHigh
	for 16 bit data.
	Count (8 bits): A data byte containing the length of a block operation.

	[..]: Data sent by I2C device, as opposed to data sent by the host adapter.


	SMBus Write Quick
	=================

	This sends a single bit to the device, at the place of the Rd/Wr bit.
	There is no equivalent Read Quick command.

	A Addr Rd/Wr [A] P


	SMBus Read Byte
	===============

	This reads a single byte from a device, without specifying a device
	register. Some devices are so simple that this interface is enough; for
	others, it is a shorthand if you want to read the same register as in
	the previous SMBus command.

	S Addr Rd [A] [Data] NA P


	SMBus Write Byte
	================

	This is the reverse of Read Byte: it sends a single byte to a device.
	See Read Byte for more information.

	S Addr Wr [A] Data [A] P


	SMBus Read Byte Data
	====================

	This reads a single byte from a device, from a designated register.
	The register is specified through the Comm byte.

	S Addr Wr [A] Comm [A] S Addr Rd [A] [Data] NA P


	SMBus Read Word Data
	====================

	This command is very like Read Byte Data; again, data is read from a
	device, from a designated register that is specified through the Comm
	byte. But this time, the data is a complete word (16 bits).

	S Addr Wr [A] Comm [A] S Addr Rd [A] [DataLow] A [DataHigh] NA P


	SMBus Write Byte Data
	=====================

	This writes a single byte to a device, to a designated register. The
	register is specified through the Comm byte. This is the opposite of
	the Read Byte Data command.

	S Addr Wr [A] Comm [A] Data [A] P


	SMBus Write Word Data
	=====================

	This is the opposite operation of the Read Word Data command. 16 bits
	of data is read from a device, from a designated register that is
	specified through the Comm byte.

	S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A] P


	SMBus Process Call
	==================

	This command selects a device register (through the Comm byte), sends
	16 bits of data to it, and reads 16 bits of data in return.

	S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A]
	S Addr Rd [A] [DataLow] A [DataHigh] NA P


	SMBus Block Read
	================

	This command reads a block of up to 32 bytes from a device, from a
	designated register that is specified through the Comm byte. The amount
	of data is specified by the device in the Count byte.

	S Addr Wr [A] Comm [A]
	S Addr Rd [A] [Count] A [Data] A [Data] A ... A [Data] NA P


	SMBus Block Write
	=================

	The opposite of the Block Read command, this writes up to 32 bytes to
	a device, to a designated register that is specified through the
	Comm byte. The amount of data is specified in the Count byte.

	S Addr Wr [A] Comm [A] Count [A] Data [A] Data [A] ... [A] Data [A] P


	SMBus Block Process Call
	========================

	SMBus Block Process Call was introduced in Revision 2.0 of the specification.

	This command selects a device register (through the Comm byte), sends
	1 to 31 bytes of data to it, and reads 1 to 31 bytes of data in return.

	S Addr Wr [A] Comm [A] Count [A] Data [A] ...
	S Addr Rd [A] [Count] A [Data] ... A P


	SMBus Host Notify
	=================

	This command is sent from a SMBus device acting as a master to the
	SMBus host acting as a slave.
	It is the same form as Write Word, with the command code replaced by the
	alerting device's address.

	[S] [HostAddr] [Wr] A [DevAddr] A [DataLow] A [DataHigh] A [P]


	Packet Error Checking (PEC)
	===========================
	Packet Error Checking was introduced in Revision 1.1 of the specification.

	PEC adds a CRC-8 error-checking byte to all transfers.


	Address Resolution Protocol (ARP)
	=================================
	The Address Resolution Protocol was introduced in Revision 2.0 of
	the specification. It is a higher-layer protocol which uses the
	messages above.

	ARP adds device enumeration and dynamic address assignment to
	the protocol. All ARP communications use slave address 0x61 and
	require PEC checksums.


	I2C Block Transactions
	======================
	The following I2C block transactions are supported by the
	SMBus layer and are described here for completeness.
	I2C block transactions do not limit the number of bytes transferred
	but the SMBus layer places a limit of 32 bytes.


	I2C Block Read
	==============

	This command reads a block of bytes from a device, from a
	designated register that is specified through the Comm byte.

	S Addr Wr [A] Comm [A]
	S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P


	I2C Block Read (2 Comm bytes)
	=============================

	This command reads a block of bytes from a device, from a
	designated register that is specified through the two Comm bytes.

	S Addr Wr [A] Comm1 [A] Comm2 [A]
	S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P


	I2C Block Write
	===============

	The opposite of the Block Read command, this writes bytes to
	a device, to a designated register that is specified through the
	Comm byte. Note that command lengths of 0, 2, or more bytes are
	supported as they are indistinguishable from data.

	S Addr Wr [A] Comm [A] Data [A] Data [A] ... [A] Data [A] P