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-rw-r--r--Documentation/i2c/functionality95
-rw-r--r--Documentation/i2c/smbus-protocol81
2 files changed, 98 insertions, 78 deletions
diff --git a/Documentation/i2c/functionality b/Documentation/i2c/functionality
index 60cca249e45..42c17c1fb3c 100644
--- a/Documentation/i2c/functionality
+++ b/Documentation/i2c/functionality
@@ -51,26 +51,38 @@ A few combinations of the above flags are also defined for your convenience:
the transparent emulation layer)
-ALGORITHM/ADAPTER IMPLEMENTATION
---------------------------------
+ADAPTER IMPLEMENTATION
+----------------------
-When you write a new algorithm driver, you will have to implement a
-function callback `functionality', that gets an i2c_adapter structure
-pointer as its only parameter:
+When you write a new adapter driver, you will have to implement a
+function callback `functionality'. Typical implementations are given
+below.
- struct i2c_algorithm {
- /* Many other things of course; check <linux/i2c.h>! */
- u32 (*functionality) (struct i2c_adapter *);
+A typical SMBus-only adapter would list all the SMBus transactions it
+supports. This example comes from the i2c-piix4 driver:
+
+ static u32 piix4_func(struct i2c_adapter *adapter)
+ {
+ return I2C_FUNC_SMBUS_QUICK | I2C_FUNC_SMBUS_BYTE |
+ I2C_FUNC_SMBUS_BYTE_DATA | I2C_FUNC_SMBUS_WORD_DATA |
+ I2C_FUNC_SMBUS_BLOCK_DATA;
}
-A typically implementation is given below, from i2c-algo-bit.c:
+A typical full-I2C adapter would use the following (from the i2c-pxa
+driver):
- static u32 bit_func(struct i2c_adapter *adap)
+ static u32 i2c_pxa_functionality(struct i2c_adapter *adap)
{
- return I2C_FUNC_SMBUS_EMUL | I2C_FUNC_10BIT_ADDR |
- I2C_FUNC_PROTOCOL_MANGLING;
+ return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL;
}
+I2C_FUNC_SMBUS_EMUL includes all the SMBus transactions (with the
+addition of I2C block transactions) which i2c-core can emulate using
+I2C_FUNC_I2C without any help from the adapter driver. The idea is
+to let the client drivers check for the support of SMBus functions
+without having to care whether the said functions are implemented in
+hardware by the adapter, or emulated in software by i2c-core on top
+of an I2C adapter.
CLIENT CHECKING
@@ -78,36 +90,33 @@ CLIENT CHECKING
Before a client tries to attach to an adapter, or even do tests to check
whether one of the devices it supports is present on an adapter, it should
-check whether the needed functionality is present. There are two functions
-defined which should be used instead of calling the functionality hook
-in the algorithm structure directly:
-
- /* Return the functionality mask */
- extern u32 i2c_get_functionality (struct i2c_adapter *adap);
-
- /* Return 1 if adapter supports everything we need, 0 if not. */
- extern int i2c_check_functionality (struct i2c_adapter *adap, u32 func);
+check whether the needed functionality is present. The typical way to do
+this is (from the lm75 driver):
-This is a typical way to use these functions (from the writing-clients
-document):
- int foo_detect_client(struct i2c_adapter *adapter, int address,
- unsigned short flags, int kind)
+ static int lm75_detect(...)
{
- /* Define needed variables */
-
- /* As the very first action, we check whether the adapter has the
- needed functionality: we need the SMBus read_word_data,
- write_word_data and write_byte functions in this example. */
- if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA |
- I2C_FUNC_SMBUS_WRITE_BYTE))
- goto ERROR0;
-
- /* Now we can do the real detection */
-
- ERROR0:
- /* Return an error */
+ (...)
+ if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA |
+ I2C_FUNC_SMBUS_WORD_DATA))
+ goto exit;
+ (...)
}
+Here, the lm75 driver checks if the adapter can do both SMBus byte data
+and SMBus word data transactions. If not, then the driver won't work on
+this adapter and there's no point in going on. If the check above is
+successful, then the driver knows that it can call the following
+functions: i2c_smbus_read_byte_data(), i2c_smbus_write_byte_data(),
+i2c_smbus_read_word_data() and i2c_smbus_write_word_data(). As a rule of
+thumb, the functionality constants you test for with
+i2c_check_functionality() should match exactly the i2c_smbus_* functions
+which you driver is calling.
+
+Note that the check above doesn't tell whether the functionalities are
+implemented in hardware by the underlying adapter or emulated in
+software by i2c-core. Client drivers don't have to care about this, as
+i2c-core will transparently implement SMBus transactions on top of I2C
+adapters.
CHECKING THROUGH /DEV
@@ -116,19 +125,19 @@ CHECKING THROUGH /DEV
If you try to access an adapter from a userspace program, you will have
to use the /dev interface. You will still have to check whether the
functionality you need is supported, of course. This is done using
-the I2C_FUNCS ioctl. An example, adapted from the lm_sensors i2cdetect
-program, is below:
+the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is
+below:
int file;
- if (file = open("/dev/i2c-0",O_RDWR) < 0) {
+ if (file = open("/dev/i2c-0", O_RDWR) < 0) {
/* Some kind of error handling */
exit(1);
}
- if (ioctl(file,I2C_FUNCS,&funcs) < 0) {
+ if (ioctl(file, I2C_FUNCS, &funcs) < 0) {
/* Some kind of error handling */
exit(1);
}
- if (! (funcs & I2C_FUNC_SMBUS_QUICK)) {
+ if (!(funcs & I2C_FUNC_SMBUS_QUICK)) {
/* Oops, the needed functionality (SMBus write_quick function) is
not available! */
exit(1);
diff --git a/Documentation/i2c/smbus-protocol b/Documentation/i2c/smbus-protocol
index 8a653c60d25..03f08fb491c 100644
--- a/Documentation/i2c/smbus-protocol
+++ b/Documentation/i2c/smbus-protocol
@@ -1,5 +1,6 @@
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
@@ -8,6 +9,7 @@ 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
@@ -15,7 +17,12 @@ 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.
+Below is a list of SMBus protocol operations, and the functions executing
+them. Note that the names used in the SMBus protocol specifications usually
+don't match these function names. For some of the operations which pass a
+single data byte, the functions using SMBus protocol operation names execute
+a different protocol operation entirely.
+
Key to symbols
==============
@@ -35,17 +42,16 @@ 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
-=================
+SMBus Quick Command: i2c_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
-===============
+SMBus Receive Byte: i2c_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
@@ -55,17 +61,17 @@ the previous SMBus command.
S Addr Rd [A] [Data] NA P
-SMBus Write Byte
-================
+SMBus Send Byte: i2c_smbus_write_byte()
+========================================
-This is the reverse of Read Byte: it sends a single byte to a device.
-See Read Byte for more information.
+This operation is the reverse of Receive Byte: it sends a single byte
+to a device. See Receive Byte for more information.
S Addr Wr [A] Data [A] P
-SMBus Read Byte Data
-====================
+SMBus Read Byte: i2c_smbus_read_byte_data()
+============================================
This reads a single byte from a device, from a designated register.
The register is specified through the Comm byte.
@@ -73,30 +79,30 @@ 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
-====================
+SMBus Read Word: i2c_smbus_read_word_data()
+============================================
-This command is very like Read Byte Data; again, data is read from a
+This operation is very like Read Byte; 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
-=====================
+SMBus Write Byte: i2c_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.
+the Read Byte operation.
S Addr Wr [A] Comm [A] Data [A] P
-SMBus Write Word Data
-=====================
+SMBus Write Word: i2c_smbus_write_word_data()
+==============================================
-This is the opposite operation of the Read Word Data command. 16 bits
+This is the opposite of the Read Word operation. 16 bits
of data is written to a device, to the designated register that is
specified through the Comm byte.
@@ -113,8 +119,8 @@ S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A]
S Addr Rd [A] [DataLow] A [DataHigh] NA P
-SMBus Block Read
-================
+SMBus Block Read: i2c_smbus_read_block_data()
+==============================================
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
@@ -124,8 +130,8 @@ S Addr Wr [A] Comm [A]
S Addr Rd [A] [Count] A [Data] A [Data] A ... A [Data] NA P
-SMBus Block Write
-=================
+SMBus Block Write: i2c_smbus_write_block_data()
+================================================
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
@@ -134,10 +140,11 @@ 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 Write - Block Read Process Call
+===========================================
-SMBus Block Process Call was introduced in Revision 2.0 of the specification.
+SMBus Block Write - Block Read 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.
@@ -159,13 +166,16 @@ alerting device's address.
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.
+PEC adds a CRC-8 error-checking byte to transfers using it, immediately
+before the terminating STOP.
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.
@@ -177,14 +187,17 @@ require PEC checksums.
I2C Block Transactions
======================
+
The following I2C block transactions are supported by the
SMBus layer and are described here for completeness.
+They are *NOT* defined by the SMBus specification.
+
I2C block transactions do not limit the number of bytes transferred
but the SMBus layer places a limit of 32 bytes.
-I2C Block Read
-==============
+I2C Block Read: i2c_smbus_read_i2c_block_data()
+================================================
This command reads a block of bytes from a device, from a
designated register that is specified through the Comm byte.
@@ -203,8 +216,8 @@ S Addr Wr [A] Comm1 [A] Comm2 [A]
S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P
-I2C Block Write
-===============
+I2C Block Write: i2c_smbus_write_i2c_block_data()
+==================================================
The opposite of the Block Read command, this writes bytes to
a device, to a designated register that is specified through the
@@ -212,5 +225,3 @@ 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
-
-