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Linux/Documentation/i2c/instantiating-devices

  1 How to instantiate I2C devices
  2 ==============================
  3 
  4 Unlike PCI or USB devices, I2C devices are not enumerated at the hardware
  5 level. Instead, the software must know which devices are connected on each
  6 I2C bus segment, and what address these devices are using. For this
  7 reason, the kernel code must instantiate I2C devices explicitly. There are
  8 several ways to achieve this, depending on the context and requirements.
  9 
 10 
 11 Method 1a: Declare the I2C devices by bus number
 12 ------------------------------------------------
 13 
 14 This method is appropriate when the I2C bus is a system bus as is the case
 15 for many embedded systems. On such systems, each I2C bus has a number
 16 which is known in advance. It is thus possible to pre-declare the I2C
 17 devices which live on this bus. This is done with an array of struct
 18 i2c_board_info which is registered by calling i2c_register_board_info().
 19 
 20 Example (from omap2 h4):
 21 
 22 static struct i2c_board_info h4_i2c_board_info[] __initdata = {
 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 
 37 static 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 
 45 The above code declares 3 devices on I2C bus 1, including their respective
 46 addresses and custom data needed by their drivers. When the I2C bus in
 47 question is registered, the I2C devices will be instantiated automatically
 48 by i2c-core.
 49 
 50 The devices will be automatically unbound and destroyed when the I2C bus
 51 they sit on goes away (if ever.)
 52 
 53 
 54 Method 1b: Declare the I2C devices via devicetree
 55 -------------------------------------------------
 56 
 57 This method has the same implications as method 1a. The declaration of I2C
 58 devices is here done via devicetree as subnodes of the master controller.
 59 
 60 Example:
 61 
 62         i2c1: i2c@400a0000 {
 63                 /* ... master properties skipped ... */
 64                 clock-frequency = <100000>;
 65 
 66                 flash@50 {
 67                         compatible = "atmel,24c256";
 68                         reg = <0x50>;
 69                 };
 70 
 71                 pca9532: gpio@60 {
 72                         compatible = "nxp,pca9532";
 73                         gpio-controller;
 74                         #gpio-cells = <2>;
 75                         reg = <0x60>;
 76                 };
 77         };
 78 
 79 Here, two devices are attached to the bus using a speed of 100kHz. For
 80 additional properties which might be needed to set up the device, please refer
 81 to its devicetree documentation in Documentation/devicetree/bindings/.
 82 
 83 
 84 Method 1c: Declare the I2C devices via ACPI
 85 -------------------------------------------
 86 
 87 ACPI can also describe I2C devices. There is special documentation for this
 88 which is currently located at Documentation/acpi/enumeration.txt.
 89 
 90 
 91 Method 2: Instantiate the devices explicitly
 92 --------------------------------------------
 93 
 94 This method is appropriate when a larger device uses an I2C bus for
 95 internal communication. A typical case is TV adapters. These can have a
 96 tuner, a video decoder, an audio decoder, etc. usually connected to the
 97 main chip by the means of an I2C bus. You won't know the number of the I2C
 98 bus in advance, so the method 1 described above can't be used. Instead,
 99 you can instantiate your I2C devices explicitly. This is done by filling
100 a struct i2c_board_info and calling i2c_new_device().
101 
102 Example (from the sfe4001 network driver):
103 
104 static struct i2c_board_info sfe4001_hwmon_info = {
105         I2C_BOARD_INFO("max6647", 0x4e),
106 };
107 
108 int sfe4001_init(struct efx_nic *efx)
109 {
110         (...)
111         efx->board_info.hwmon_client =
112                 i2c_new_device(&efx->i2c_adap, &sfe4001_hwmon_info);
113 
114         (...)
115 }
116 
117 The above code instantiates 1 I2C device on the I2C bus which is on the
118 network adapter in question.
119 
120 A variant of this is when you don't know for sure if an I2C device is
121 present or not (for example for an optional feature which is not present
122 on cheap variants of a board but you have no way to tell them apart), or
123 it may have different addresses from one board to the next (manufacturer
124 changing its design without notice). In this case, you can call
125 i2c_new_probed_device() instead of i2c_new_device().
126 
127 Example (from the nxp OHCI driver):
128 
129 static const unsigned short normal_i2c[] = { 0x2c, 0x2d, I2C_CLIENT_END };
130 
131 static int usb_hcd_nxp_probe(struct platform_device *pdev)
132 {
133         (...)
134         struct i2c_adapter *i2c_adap;
135         struct i2c_board_info i2c_info;
136 
137         (...)
138         i2c_adap = i2c_get_adapter(2);
139         memset(&i2c_info, 0, sizeof(struct i2c_board_info));
140         strlcpy(i2c_info.type, "isp1301_nxp", I2C_NAME_SIZE);
141         isp1301_i2c_client = i2c_new_probed_device(i2c_adap, &i2c_info,
142                                                    normal_i2c, NULL);
143         i2c_put_adapter(i2c_adap);
144         (...)
145 }
146 
147 The above code instantiates up to 1 I2C device on the I2C bus which is on
148 the OHCI adapter in question. It first tries at address 0x2c, if nothing
149 is found there it tries address 0x2d, and if still nothing is found, it
150 simply gives up.
151 
152 The driver which instantiated the I2C device is responsible for destroying
153 it on cleanup. This is done by calling i2c_unregister_device() on the
154 pointer that was earlier returned by i2c_new_device() or
155 i2c_new_probed_device().
156 
157 
158 Method 3: Probe an I2C bus for certain devices
159 ----------------------------------------------
160 
161 Sometimes you do not have enough information about an I2C device, not even
162 to call i2c_new_probed_device(). The typical case is hardware monitoring
163 chips on PC mainboards. There are several dozen models, which can live
164 at 25 different addresses. Given the huge number of mainboards out there,
165 it is next to impossible to build an exhaustive list of the hardware
166 monitoring chips being used. Fortunately, most of these chips have
167 manufacturer and device ID registers, so they can be identified by
168 probing.
169 
170 In that case, I2C devices are neither declared nor instantiated
171 explicitly. Instead, i2c-core will probe for such devices as soon as their
172 drivers are loaded, and if any is found, an I2C device will be
173 instantiated automatically. In order to prevent any misbehavior of this
174 mechanism, the following restrictions apply:
175 * The I2C device driver must implement the detect() method, which
176   identifies a supported device by reading from arbitrary registers.
177 * Only buses which are likely to have a supported device and agree to be
178   probed, will be probed. For example this avoids probing for hardware
179   monitoring chips on a TV adapter.
180 
181 Example:
182 See lm90_driver and lm90_detect() in drivers/hwmon/lm90.c
183 
184 I2C devices instantiated as a result of such a successful probe will be
185 destroyed automatically when the driver which detected them is removed,
186 or when the underlying I2C bus is itself destroyed, whichever happens
187 first.
188 
189 Those of you familiar with the i2c subsystem of 2.4 kernels and early 2.6
190 kernels will find out that this method 3 is essentially similar to what
191 was done there. Two significant differences are:
192 * Probing is only one way to instantiate I2C devices now, while it was the
193   only way back then. Where possible, methods 1 and 2 should be preferred.
194   Method 3 should only be used when there is no other way, as it can have
195   undesirable side effects.
196 * I2C buses must now explicitly say which I2C driver classes can probe
197   them (by the means of the class bitfield), while all I2C buses were
198   probed by default back then. The default is an empty class which means
199   that no probing happens. The purpose of the class bitfield is to limit
200   the aforementioned undesirable side effects.
201 
202 Once again, method 3 should be avoided wherever possible. Explicit device
203 instantiation (methods 1 and 2) is much preferred for it is safer and
204 faster.
205 
206 
207 Method 4: Instantiate from user-space
208 -------------------------------------
209 
210 In general, the kernel should know which I2C devices are connected and
211 what addresses they live at. However, in certain cases, it does not, so a
212 sysfs interface was added to let the user provide the information. This
213 interface is made of 2 attribute files which are created in every I2C bus
214 directory: new_device and delete_device. Both files are write only and you
215 must write the right parameters to them in order to properly instantiate,
216 respectively delete, an I2C device.
217 
218 File new_device takes 2 parameters: the name of the I2C device (a string)
219 and the address of the I2C device (a number, typically expressed in
220 hexadecimal starting with 0x, but can also be expressed in decimal.)
221 
222 File delete_device takes a single parameter: the address of the I2C
223 device. As no two devices can live at the same address on a given I2C
224 segment, the address is sufficient to uniquely identify the device to be
225 deleted.
226 
227 Example:
228 # echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device
229 
230 While this interface should only be used when in-kernel device declaration
231 can't be done, there is a variety of cases where it can be helpful:
232 * The I2C driver usually detects devices (method 3 above) but the bus
233   segment your device lives on doesn't have the proper class bit set and
234   thus detection doesn't trigger.
235 * The I2C driver usually detects devices, but your device lives at an
236   unexpected address.
237 * The I2C driver usually detects devices, but your device is not detected,
238   either because the detection routine is too strict, or because your
239   device is not officially supported yet but you know it is compatible.
240 * You are developing a driver on a test board, where you soldered the I2C
241   device yourself.
242 
243 This interface is a replacement for the force_* module parameters some I2C
244 drivers implement. Being implemented in i2c-core rather than in each
245 device driver individually, it is much more efficient, and also has the
246 advantage that you do not have to reload the driver to change a setting.
247 You can also instantiate the device before the driver is loaded or even
248 available, and you don't need to know what driver the device needs.

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