linux/Documentation/i2c/instantiating-devices
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   1How to instantiate I2C devices
   2==============================
   3
   4Unlike PCI or USB devices, I2C devices are not enumerated at the hardware
   5level. Instead, the software must know which devices are connected on each
   6I2C bus segment, and what address these devices are using. For this
   7reason, the kernel code must instantiate I2C devices explicitly. There are
   8several ways to achieve this, depending on the context and requirements.
   9
  10
  11Method 1: Declare the I2C devices by bus number
  12-----------------------------------------------
  13
  14This method is appropriate when the I2C bus is a system bus as is the case
  15for many embedded systems. On such systems, each I2C bus has a number
  16which is known in advance. It is thus possible to pre-declare the I2C
  17devices which live on this bus. This is done with an array of struct
  18i2c_board_info which is registered by calling i2c_register_board_info().
  19
  20Example (from omap2 h4):
  21
  22static struct i2c_board_info __initdata h4_i2c_board_info[] = {
  23        {
  24                I2C_BOARD_INFO("isp1301_omap", 0x2d),
  25                .irq            = OMAP_GPIO_IRQ(125),
  26        },
  27        {       /* EEPROM on mainboard */
  28                I2C_BOARD_INFO("24c01", 0x52),
  29                .platform_data  = &m24c01,
  30        },
  31        {       /* EEPROM on cpu card */
  32                I2C_BOARD_INFO("24c01", 0x57),
  33                .platform_data  = &m24c01,
  34        },
  35};
  36
  37static void __init omap_h4_init(void)
  38{
  39        (...)
  40        i2c_register_board_info(1, h4_i2c_board_info,
  41                        ARRAY_SIZE(h4_i2c_board_info));
  42        (...)
  43}
  44
  45The above code declares 3 devices on I2C bus 1, including their respective
  46addresses and custom data needed by their drivers. When the I2C bus in
  47question is registered, the I2C devices will be instantiated automatically
  48by i2c-core.
  49
  50The devices will be automatically unbound and destroyed when the I2C bus
  51they sit on goes away (if ever.)
  52
  53
  54Method 2: Instantiate the devices explicitly
  55--------------------------------------------
  56
  57This method is appropriate when a larger device uses an I2C bus for
  58internal communication. A typical case is TV adapters. These can have a
  59tuner, a video decoder, an audio decoder, etc. usually connected to the
  60main chip by the means of an I2C bus. You won't know the number of the I2C
  61bus in advance, so the method 1 described above can't be used. Instead,
  62you can instantiate your I2C devices explicitly. This is done by filling
  63a struct i2c_board_info and calling i2c_new_device().
  64
  65Example (from the sfe4001 network driver):
  66
  67static struct i2c_board_info sfe4001_hwmon_info = {
  68        I2C_BOARD_INFO("max6647", 0x4e),
  69};
  70
  71int sfe4001_init(struct efx_nic *efx)
  72{
  73        (...)
  74        efx->board_info.hwmon_client =
  75                i2c_new_device(&efx->i2c_adap, &sfe4001_hwmon_info);
  76
  77        (...)
  78}
  79
  80The above code instantiates 1 I2C device on the I2C bus which is on the
  81network adapter in question.
  82
  83A variant of this is when you don't know for sure if an I2C device is
  84present or not (for example for an optional feature which is not present
  85on cheap variants of a board but you have no way to tell them apart), or
  86it may have different addresses from one board to the next (manufacturer
  87changing its design without notice). In this case, you can call
  88i2c_new_probed_device() instead of i2c_new_device().
  89
  90Example (from the nxp OHCI driver):
  91
  92static const unsigned short normal_i2c[] = { 0x2c, 0x2d, I2C_CLIENT_END };
  93
  94static int __devinit usb_hcd_nxp_probe(struct platform_device *pdev)
  95{
  96        (...)
  97        struct i2c_adapter *i2c_adap;
  98        struct i2c_board_info i2c_info;
  99
 100        (...)
 101        i2c_adap = i2c_get_adapter(2);
 102        memset(&i2c_info, 0, sizeof(struct i2c_board_info));
 103        strlcpy(i2c_info.type, "isp1301_nxp", I2C_NAME_SIZE);
 104        isp1301_i2c_client = i2c_new_probed_device(i2c_adap, &i2c_info,
 105                                                   normal_i2c, NULL);
 106        i2c_put_adapter(i2c_adap);
 107        (...)
 108}
 109
 110The above code instantiates up to 1 I2C device on the I2C bus which is on
 111the OHCI adapter in question. It first tries at address 0x2c, if nothing
 112is found there it tries address 0x2d, and if still nothing is found, it
 113simply gives up.
 114
 115The driver which instantiated the I2C device is responsible for destroying
 116it on cleanup. This is done by calling i2c_unregister_device() on the
 117pointer that was earlier returned by i2c_new_device() or
 118i2c_new_probed_device().
 119
 120
 121Method 3: Probe an I2C bus for certain devices
 122----------------------------------------------
 123
 124Sometimes you do not have enough information about an I2C device, not even
 125to call i2c_new_probed_device(). The typical case is hardware monitoring
 126chips on PC mainboards. There are several dozen models, which can live
 127at 25 different addresses. Given the huge number of mainboards out there,
 128it is next to impossible to build an exhaustive list of the hardware
 129monitoring chips being used. Fortunately, most of these chips have
 130manufacturer and device ID registers, so they can be identified by
 131probing.
 132
 133In that case, I2C devices are neither declared nor instantiated
 134explicitly. Instead, i2c-core will probe for such devices as soon as their
 135drivers are loaded, and if any is found, an I2C device will be
 136instantiated automatically. In order to prevent any misbehavior of this
 137mechanism, the following restrictions apply:
 138* The I2C device driver must implement the detect() method, which
 139  identifies a supported device by reading from arbitrary registers.
 140* Only buses which are likely to have a supported device and agree to be
 141  probed, will be probed. For example this avoids probing for hardware
 142  monitoring chips on a TV adapter.
 143
 144Example:
 145See lm90_driver and lm90_detect() in drivers/hwmon/lm90.c
 146
 147I2C devices instantiated as a result of such a successful probe will be
 148destroyed automatically when the driver which detected them is removed,
 149or when the underlying I2C bus is itself destroyed, whichever happens
 150first.
 151
 152Those of you familiar with the i2c subsystem of 2.4 kernels and early 2.6
 153kernels will find out that this method 3 is essentially similar to what
 154was done there. Two significant differences are:
 155* Probing is only one way to instantiate I2C devices now, while it was the
 156  only way back then. Where possible, methods 1 and 2 should be preferred.
 157  Method 3 should only be used when there is no other way, as it can have
 158  undesirable side effects.
 159* I2C buses must now explicitly say which I2C driver classes can probe
 160  them (by the means of the class bitfield), while all I2C buses were
 161  probed by default back then. The default is an empty class which means
 162  that no probing happens. The purpose of the class bitfield is to limit
 163  the aforementioned undesirable side effects.
 164
 165Once again, method 3 should be avoided wherever possible. Explicit device
 166instantiation (methods 1 and 2) is much preferred for it is safer and
 167faster.
 168
 169
 170Method 4: Instantiate from user-space
 171-------------------------------------
 172
 173In general, the kernel should know which I2C devices are connected and
 174what addresses they live at. However, in certain cases, it does not, so a
 175sysfs interface was added to let the user provide the information. This
 176interface is made of 2 attribute files which are created in every I2C bus
 177directory: new_device and delete_device. Both files are write only and you
 178must write the right parameters to them in order to properly instantiate,
 179respectively delete, an I2C device.
 180
 181File new_device takes 2 parameters: the name of the I2C device (a string)
 182and the address of the I2C device (a number, typically expressed in
 183hexadecimal starting with 0x, but can also be expressed in decimal.)
 184
 185File delete_device takes a single parameter: the address of the I2C
 186device. As no two devices can live at the same address on a given I2C
 187segment, the address is sufficient to uniquely identify the device to be
 188deleted.
 189
 190Example:
 191# echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device
 192
 193While this interface should only be used when in-kernel device declaration
 194can't be done, there is a variety of cases where it can be helpful:
 195* The I2C driver usually detects devices (method 3 above) but the bus
 196  segment your device lives on doesn't have the proper class bit set and
 197  thus detection doesn't trigger.
 198* The I2C driver usually detects devices, but your device lives at an
 199  unexpected address.
 200* The I2C driver usually detects devices, but your device is not detected,
 201  either because the detection routine is too strict, or because your
 202  device is not officially supported yet but you know it is compatible.
 203* You are developing a driver on a test board, where you soldered the I2C
 204  device yourself.
 205
 206This interface is a replacement for the force_* module parameters some I2C
 207drivers implement. Being implemented in i2c-core rather than in each
 208device driver individually, it is much more efficient, and also has the
 209advantage that you do not have to reload the driver to change a setting.
 210You can also instantiate the device before the driver is loaded or even
 211available, and you don't need to know what driver the device needs.
 212
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