325 lines
9.8 KiB
ReStructuredText
325 lines
9.8 KiB
ReStructuredText
==================
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The SMBus Protocol
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==================
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The following is a summary of the SMBus protocol. It applies to
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all revisions of the protocol (1.0, 1.1, and 2.0).
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Certain protocol features which are not supported by
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this package are briefly described at the end of this document.
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Some adapters understand only the SMBus (System Management Bus) protocol,
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which is a subset from the I2C protocol. Fortunately, many devices use
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only the same subset, which makes it possible to put them on an SMBus.
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If you write a driver for some I2C device, please try to use the SMBus
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commands if at all possible (if the device uses only that subset of the
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I2C protocol). This makes it possible to use the device driver on both
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SMBus adapters and I2C adapters (the SMBus command set is automatically
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translated to I2C on I2C adapters, but plain I2C commands can not be
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handled at all on most pure SMBus adapters).
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Below is a list of SMBus protocol operations, and the functions executing
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them. Note that the names used in the SMBus protocol specifications usually
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don't match these function names. For some of the operations which pass a
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single data byte, the functions using SMBus protocol operation names execute
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a different protocol operation entirely.
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Each transaction type corresponds to a functionality flag. Before calling a
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transaction function, a device driver should always check (just once) for
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the corresponding functionality flag to ensure that the underlying I2C
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adapter supports the transaction in question. See
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Documentation/i2c/functionality.rst for the details.
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Key to symbols
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==============
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=============== =============================================================
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S Start condition
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Sr Repeated start condition, used to switch from write to
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read mode.
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P Stop condition
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Rd/Wr (1 bit) Read/Write bit. Rd equals 1, Wr equals 0.
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A, NA (1 bit) Acknowledge (ACK) and Not Acknowledge (NACK) bit
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Addr (7 bits) I2C 7 bit address. Note that this can be expanded to
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get a 10 bit I2C address.
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Comm (8 bits) Command byte, a data byte which often selects a register on
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the device.
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Data (8 bits) A plain data byte. DataLow and DataHigh represent the low and
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high byte of a 16 bit word.
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Count (8 bits) A data byte containing the length of a block operation.
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[..] Data sent by I2C device, as opposed to data sent by the host
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adapter.
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=============== =============================================================
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SMBus Quick Command
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===================
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This sends a single bit to the device, at the place of the Rd/Wr bit::
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S Addr Rd/Wr [A] P
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Functionality flag: I2C_FUNC_SMBUS_QUICK
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SMBus Receive Byte
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==================
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Implemented by i2c_smbus_read_byte()
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This reads a single byte from a device, without specifying a device
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register. Some devices are so simple that this interface is enough; for
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others, it is a shorthand if you want to read the same register as in
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the previous SMBus command::
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S Addr Rd [A] [Data] NA P
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Functionality flag: I2C_FUNC_SMBUS_READ_BYTE
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SMBus Send Byte
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===============
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Implemented by i2c_smbus_write_byte()
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This operation is the reverse of Receive Byte: it sends a single byte
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to a device. See Receive Byte for more information.
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::
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S Addr Wr [A] Data [A] P
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Functionality flag: I2C_FUNC_SMBUS_WRITE_BYTE
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SMBus Read Byte
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===============
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Implemented by i2c_smbus_read_byte_data()
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This reads a single byte from a device, from a designated register.
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The register is specified through the Comm byte::
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S Addr Wr [A] Comm [A] Sr Addr Rd [A] [Data] NA P
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Functionality flag: I2C_FUNC_SMBUS_READ_BYTE_DATA
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SMBus Read Word
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===============
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Implemented by i2c_smbus_read_word_data()
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This operation is very like Read Byte; again, data is read from a
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device, from a designated register that is specified through the Comm
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byte. But this time, the data is a complete word (16 bits)::
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S Addr Wr [A] Comm [A] Sr Addr Rd [A] [DataLow] A [DataHigh] NA P
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Functionality flag: I2C_FUNC_SMBUS_READ_WORD_DATA
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Note the convenience function i2c_smbus_read_word_swapped() is
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available for reads where the two data bytes are the other way
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around (not SMBus compliant, but very popular.)
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SMBus Write Byte
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================
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Implemented by i2c_smbus_write_byte_data()
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This writes a single byte to a device, to a designated register. The
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register is specified through the Comm byte. This is the opposite of
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the Read Byte operation.
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::
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S Addr Wr [A] Comm [A] Data [A] P
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Functionality flag: I2C_FUNC_SMBUS_WRITE_BYTE_DATA
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SMBus Write Word
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================
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Implemented by i2c_smbus_write_word_data()
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This is the opposite of the Read Word operation. 16 bits
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of data are written to a device, to the designated register that is
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specified through the Comm byte::
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S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A] P
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Functionality flag: I2C_FUNC_SMBUS_WRITE_WORD_DATA
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Note the convenience function i2c_smbus_write_word_swapped() is
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available for writes where the two data bytes are the other way
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around (not SMBus compliant, but very popular.)
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SMBus Process Call
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==================
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This command selects a device register (through the Comm byte), sends
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16 bits of data to it, and reads 16 bits of data in return::
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S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A]
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Sr Addr Rd [A] [DataLow] A [DataHigh] NA P
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Functionality flag: I2C_FUNC_SMBUS_PROC_CALL
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SMBus Block Read
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================
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Implemented by i2c_smbus_read_block_data()
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This command reads a block of up to 32 bytes from a device, from a
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designated register that is specified through the Comm byte. The amount
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of data is specified by the device in the Count byte.
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::
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S Addr Wr [A] Comm [A]
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Sr Addr Rd [A] [Count] A [Data] A [Data] A ... A [Data] NA P
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Functionality flag: I2C_FUNC_SMBUS_READ_BLOCK_DATA
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SMBus Block Write
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=================
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Implemented by i2c_smbus_write_block_data()
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The opposite of the Block Read command, this writes up to 32 bytes to
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a device, to a designated register that is specified through the
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Comm byte. The amount of data is specified in the Count byte.
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::
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S Addr Wr [A] Comm [A] Count [A] Data [A] Data [A] ... [A] Data [A] P
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Functionality flag: I2C_FUNC_SMBUS_WRITE_BLOCK_DATA
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SMBus Block Write - Block Read Process Call
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===========================================
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SMBus Block Write - Block Read Process Call was introduced in
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Revision 2.0 of the specification.
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This command selects a device register (through the Comm byte), sends
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1 to 31 bytes of data to it, and reads 1 to 31 bytes of data in return::
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S Addr Wr [A] Comm [A] Count [A] Data [A] ...
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Sr Addr Rd [A] [Count] A [Data] ... A P
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Functionality flag: I2C_FUNC_SMBUS_BLOCK_PROC_CALL
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SMBus Host Notify
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=================
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This command is sent from a SMBus device acting as a master to the
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SMBus host acting as a slave.
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It is the same form as Write Word, with the command code replaced by the
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alerting device's address.
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::
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[S] [HostAddr] [Wr] A [DevAddr] A [DataLow] A [DataHigh] A [P]
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This is implemented in the following way in the Linux kernel:
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* I2C bus drivers which support SMBus Host Notify should report
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I2C_FUNC_SMBUS_HOST_NOTIFY.
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* I2C bus drivers trigger SMBus Host Notify by a call to
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i2c_handle_smbus_host_notify().
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* I2C drivers for devices which can trigger SMBus Host Notify will have
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client->irq assigned to a Host Notify IRQ if no one else specified another.
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There is currently no way to retrieve the data parameter from the client.
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Packet Error Checking (PEC)
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===========================
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Packet Error Checking was introduced in Revision 1.1 of the specification.
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PEC adds a CRC-8 error-checking byte to transfers using it, immediately
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before the terminating STOP.
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Address Resolution Protocol (ARP)
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=================================
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The Address Resolution Protocol was introduced in Revision 2.0 of
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the specification. It is a higher-layer protocol which uses the
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messages above.
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ARP adds device enumeration and dynamic address assignment to
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the protocol. All ARP communications use slave address 0x61 and
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require PEC checksums.
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SMBus Alert
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===========
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SMBus Alert was introduced in Revision 1.0 of the specification.
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The SMBus alert protocol allows several SMBus slave devices to share a
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single interrupt pin on the SMBus master, while still allowing the master
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to know which slave triggered the interrupt.
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This is implemented the following way in the Linux kernel:
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* I2C bus drivers which support SMBus alert should call
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i2c_new_smbus_alert_device() to install SMBus alert support.
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* I2C drivers for devices which can trigger SMBus alerts should implement
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the optional alert() callback.
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I2C Block Transactions
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======================
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The following I2C block transactions are similar to the SMBus Block Read
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and Write operations, except these do not have a Count byte. They are
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supported by the SMBus layer and are described here for completeness, but
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they are *NOT* defined by the SMBus specification.
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I2C block transactions do not limit the number of bytes transferred
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but the SMBus layer places a limit of 32 bytes.
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I2C Block Read
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==============
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Implemented by i2c_smbus_read_i2c_block_data()
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This command reads a block of bytes from a device, from a
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designated register that is specified through the Comm byte::
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S Addr Wr [A] Comm [A]
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Sr Addr Rd [A] [Data] A [Data] A ... A [Data] NA P
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Functionality flag: I2C_FUNC_SMBUS_READ_I2C_BLOCK
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I2C Block Write
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===============
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Implemented by i2c_smbus_write_i2c_block_data()
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The opposite of the Block Read command, this writes bytes to
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a device, to a designated register that is specified through the
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Comm byte. Note that command lengths of 0, 2, or more bytes are
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supported as they are indistinguishable from data.
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::
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S Addr Wr [A] Comm [A] Data [A] Data [A] ... [A] Data [A] P
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Functionality flag: I2C_FUNC_SMBUS_WRITE_I2C_BLOCK
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