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author | Linus Torvalds <torvalds@ppc970.osdl.org> | 2005-04-16 15:20:36 -0700 |
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committer | Linus Torvalds <torvalds@ppc970.osdl.org> | 2005-04-16 15:20:36 -0700 |
commit | 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch) | |
tree | 0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/i2c/dev-interface | |
download | kernel_samsung_smdk4412-1da177e4c3f41524e886b7f1b8a0c1fc7321cac2.zip kernel_samsung_smdk4412-1da177e4c3f41524e886b7f1b8a0c1fc7321cac2.tar.gz kernel_samsung_smdk4412-1da177e4c3f41524e886b7f1b8a0c1fc7321cac2.tar.bz2 |
Linux-2.6.12-rc2
Initial git repository build. I'm not bothering with the full history,
even though we have it. We can create a separate "historical" git
archive of that later if we want to, and in the meantime it's about
3.2GB when imported into git - space that would just make the early
git days unnecessarily complicated, when we don't have a lot of good
infrastructure for it.
Let it rip!
Diffstat (limited to 'Documentation/i2c/dev-interface')
-rw-r--r-- | Documentation/i2c/dev-interface | 146 |
1 files changed, 146 insertions, 0 deletions
diff --git a/Documentation/i2c/dev-interface b/Documentation/i2c/dev-interface new file mode 100644 index 0000000..09d6cda --- /dev/null +++ b/Documentation/i2c/dev-interface @@ -0,0 +1,146 @@ +Usually, i2c devices are controlled by a kernel driver. But it is also +possible to access all devices on an adapter from userspace, through +the /dev interface. You need to load module i2c-dev for this. + +Each registered i2c adapter gets a number, counting from 0. You can +examine /sys/class/i2c-dev/ to see what number corresponds to which adapter. +I2C device files are character device files with major device number 89 +and a minor device number corresponding to the number assigned as +explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ..., +i2c-10, ...). All 256 minor device numbers are reserved for i2c. + + +C example +========= + +So let's say you want to access an i2c adapter from a C program. The +first thing to do is `#include <linux/i2c.h>" and "#include <linux/i2c-dev.h>. +Yes, I know, you should never include kernel header files, but until glibc +knows about i2c, there is not much choice. + +Now, you have to decide which adapter you want to access. You should +inspect /sys/class/i2c-dev/ to decide this. Adapter numbers are assigned +somewhat dynamically, so you can not even assume /dev/i2c-0 is the +first adapter. + +Next thing, open the device file, as follows: + int file; + int adapter_nr = 2; /* probably dynamically determined */ + char filename[20]; + + sprintf(filename,"/dev/i2c-%d",adapter_nr); + if ((file = open(filename,O_RDWR)) < 0) { + /* ERROR HANDLING; you can check errno to see what went wrong */ + exit(1); + } + +When you have opened the device, you must specify with what device +address you want to communicate: + int addr = 0x40; /* The I2C address */ + if (ioctl(file,I2C_SLAVE,addr) < 0) { + /* ERROR HANDLING; you can check errno to see what went wrong */ + exit(1); + } + +Well, you are all set up now. You can now use SMBus commands or plain +I2C to communicate with your device. SMBus commands are preferred if +the device supports them. Both are illustrated below. + __u8 register = 0x10; /* Device register to access */ + __s32 res; + char buf[10]; + /* Using SMBus commands */ + res = i2c_smbus_read_word_data(file,register); + if (res < 0) { + /* ERROR HANDLING: i2c transaction failed */ + } else { + /* res contains the read word */ + } + /* Using I2C Write, equivalent of + i2c_smbus_write_word_data(file,register,0x6543) */ + buf[0] = register; + buf[1] = 0x43; + buf[2] = 0x65; + if ( write(file,buf,3) != 3) { + /* ERROR HANDLING: i2c transaction failed */ + } + /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */ + if (read(file,buf,1) != 1) { + /* ERROR HANDLING: i2c transaction failed */ + } else { + /* buf[0] contains the read byte */ + } + +IMPORTANT: because of the use of inline functions, you *have* to use +'-O' or some variation when you compile your program! + + +Full interface description +========================== + +The following IOCTLs are defined and fully supported +(see also i2c-dev.h and i2c.h): + +ioctl(file,I2C_SLAVE,long addr) + Change slave address. The address is passed in the 7 lower bits of the + argument (except for 10 bit addresses, passed in the 10 lower bits in this + case). + +ioctl(file,I2C_TENBIT,long select) + Selects ten bit addresses if select not equals 0, selects normal 7 bit + addresses if select equals 0. Default 0. + +ioctl(file,I2C_PEC,long select) + Selects SMBus PEC (packet error checking) generation and verification + if select not equals 0, disables if select equals 0. Default 0. + Used only for SMBus transactions. + +ioctl(file,I2C_FUNCS,unsigned long *funcs) + Gets the adapter functionality and puts it in *funcs. + +ioctl(file,I2C_RDWR,struct i2c_ioctl_rdwr_data *msgset) + + Do combined read/write transaction without stop in between. + The argument is a pointer to a struct i2c_ioctl_rdwr_data { + + struct i2c_msg *msgs; /* ptr to array of simple messages */ + int nmsgs; /* number of messages to exchange */ + } + + The msgs[] themselves contain further pointers into data buffers. + The function will write or read data to or from that buffers depending + on whether the I2C_M_RD flag is set in a particular message or not. + The slave address and whether to use ten bit address mode has to be + set in each message, overriding the values set with the above ioctl's. + + +Other values are NOT supported at this moment, except for I2C_SMBUS, +which you should never directly call; instead, use the access functions +below. + +You can do plain i2c transactions by using read(2) and write(2) calls. +You do not need to pass the address byte; instead, set it through +ioctl I2C_SLAVE before you try to access the device. + +You can do SMBus level transactions (see documentation file smbus-protocol +for details) through the following functions: + __s32 i2c_smbus_write_quick(int file, __u8 value); + __s32 i2c_smbus_read_byte(int file); + __s32 i2c_smbus_write_byte(int file, __u8 value); + __s32 i2c_smbus_read_byte_data(int file, __u8 command); + __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value); + __s32 i2c_smbus_read_word_data(int file, __u8 command); + __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value); + __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value); + __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values); + __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length, + __u8 *values); +All these transactions return -1 on failure; you can read errno to see +what happened. The 'write' transactions return 0 on success; the +'read' transactions return the read value, except for read_block, which +returns the number of values read. The block buffers need not be longer +than 32 bytes. + +The above functions are all macros, that resolve to calls to the +i2c_smbus_access function, that on its turn calls a specific ioctl +with the data in a specific format. Read the source code if you +want to know what happens behind the screens. |