1==============================
2How to instantiate I2C devices
3==============================
4
5Unlike PCI or USB devices, I2C devices are not enumerated at the hardware
6level. Instead, the software must know which devices are connected on each
7I2C bus segment, and what address these devices are using. For this
8reason, the kernel code must instantiate I2C devices explicitly. There are
9several ways to achieve this, depending on the context and requirements.
10
11
12Method 1: Declare the I2C devices statically
13--------------------------------------------
14
15This method is appropriate when the I2C bus is a system bus as is the case
16for many embedded systems. On such systems, each I2C bus has a number which
17is known in advance. It is thus possible to pre-declare the I2C devices
18which live on this bus.
19
20This information is provided to the kernel in a different way on different
21architectures: device tree, ACPI or board files.
22
23When the I2C bus in question is registered, the I2C devices will be
24instantiated automatically by i2c-core. The devices will be automatically
25unbound and destroyed when the I2C bus they sit on goes away (if ever).
26
27
28Declare the I2C devices via devicetree
29^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
30
31On platforms using devicetree, the declaration of I2C devices is done in
32subnodes of the master controller.
33
34Example::
35
36	i2c1: i2c@400a0000 {
37		/* ... master properties skipped ... */
38		clock-frequency = <100000>;
39
40		flash@50 {
41			compatible = "atmel,24c256";
42			reg = <0x50>;
43		};
44
45		pca9532: gpio@60 {
46			compatible = "nxp,pca9532";
47			gpio-controller;
48			#gpio-cells = <2>;
49			reg = <0x60>;
50		};
51	};
52
53Here, two devices are attached to the bus using a speed of 100kHz. For
54additional properties which might be needed to set up the device, please refer
55to its devicetree documentation in Documentation/devicetree/bindings/.
56
57
58Declare the I2C devices via ACPI
59^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
60
61ACPI can also describe I2C devices. There is special documentation for this
62which is currently located at Documentation/firmware-guide/acpi/enumeration.rst.
63
64
65Declare the I2C devices in board files
66^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
67
68In many embedded architectures, devicetree has replaced the old hardware
69description based on board files, but the latter are still used in old
70code. Instantiating I2C devices via board files is done with an array of
71struct i2c_board_info which is registered by calling
72i2c_register_board_info().
73
74Example (from omap2 h4)::
75
76  static struct i2c_board_info h4_i2c_board_info[] __initdata = {
77	{
78		I2C_BOARD_INFO("isp1301_omap", 0x2d),
79		.irq		= OMAP_GPIO_IRQ(125),
80	},
81	{	/* EEPROM on mainboard */
82		I2C_BOARD_INFO("24c01", 0x52),
83		.platform_data	= &m24c01,
84	},
85	{	/* EEPROM on cpu card */
86		I2C_BOARD_INFO("24c01", 0x57),
87		.platform_data	= &m24c01,
88	},
89  };
90
91  static void __init omap_h4_init(void)
92  {
93	(...)
94	i2c_register_board_info(1, h4_i2c_board_info,
95			ARRAY_SIZE(h4_i2c_board_info));
96	(...)
97  }
98
99The above code declares 3 devices on I2C bus 1, including their respective
100addresses and custom data needed by their drivers.
101
102
103Method 2: Instantiate the devices explicitly
104--------------------------------------------
105
106This method is appropriate when a larger device uses an I2C bus for
107internal communication. A typical case is TV adapters. These can have a
108tuner, a video decoder, an audio decoder, etc. usually connected to the
109main chip by the means of an I2C bus. You won't know the number of the I2C
110bus in advance, so the method 1 described above can't be used. Instead,
111you can instantiate your I2C devices explicitly. This is done by filling
112a struct i2c_board_info and calling i2c_new_client_device().
113
114Example (from the sfe4001 network driver)::
115
116  static struct i2c_board_info sfe4001_hwmon_info = {
117	I2C_BOARD_INFO("max6647", 0x4e),
118  };
119
120  int sfe4001_init(struct efx_nic *efx)
121  {
122	(...)
123	efx->board_info.hwmon_client =
124		i2c_new_client_device(&efx->i2c_adap, &sfe4001_hwmon_info);
125
126	(...)
127  }
128
129The above code instantiates 1 I2C device on the I2C bus which is on the
130network adapter in question.
131
132A variant of this is when you don't know for sure if an I2C device is
133present or not (for example for an optional feature which is not present
134on cheap variants of a board but you have no way to tell them apart), or
135it may have different addresses from one board to the next (manufacturer
136changing its design without notice). In this case, you can call
137i2c_new_scanned_device() instead of i2c_new_client_device().
138
139Example (from the nxp OHCI driver)::
140
141  static const unsigned short normal_i2c[] = { 0x2c, 0x2d, I2C_CLIENT_END };
142
143  static int usb_hcd_nxp_probe(struct platform_device *pdev)
144  {
145	(...)
146	struct i2c_adapter *i2c_adap;
147	struct i2c_board_info i2c_info;
148
149	(...)
150	i2c_adap = i2c_get_adapter(2);
151	memset(&i2c_info, 0, sizeof(struct i2c_board_info));
152	strscpy(i2c_info.type, "isp1301_nxp", sizeof(i2c_info.type));
153	isp1301_i2c_client = i2c_new_scanned_device(i2c_adap, &i2c_info,
154						    normal_i2c, NULL);
155	i2c_put_adapter(i2c_adap);
156	(...)
157  }
158
159The above code instantiates up to 1 I2C device on the I2C bus which is on
160the OHCI adapter in question. It first tries at address 0x2c, if nothing
161is found there it tries address 0x2d, and if still nothing is found, it
162simply gives up.
163
164The driver which instantiated the I2C device is responsible for destroying
165it on cleanup. This is done by calling i2c_unregister_device() on the
166pointer that was earlier returned by i2c_new_client_device() or
167i2c_new_scanned_device().
168
169
170Method 3: Probe an I2C bus for certain devices
171----------------------------------------------
172
173Sometimes you do not have enough information about an I2C device, not even
174to call i2c_new_scanned_device(). The typical case is hardware monitoring
175chips on PC mainboards. There are several dozen models, which can live
176at 25 different addresses. Given the huge number of mainboards out there,
177it is next to impossible to build an exhaustive list of the hardware
178monitoring chips being used. Fortunately, most of these chips have
179manufacturer and device ID registers, so they can be identified by
180probing.
181
182In that case, I2C devices are neither declared nor instantiated
183explicitly. Instead, i2c-core will probe for such devices as soon as their
184drivers are loaded, and if any is found, an I2C device will be
185instantiated automatically. In order to prevent any misbehavior of this
186mechanism, the following restrictions apply:
187
188* The I2C device driver must implement the detect() method, which
189  identifies a supported device by reading from arbitrary registers.
190* Only buses which are likely to have a supported device and agree to be
191  probed, will be probed. For example this avoids probing for hardware
192  monitoring chips on a TV adapter.
193
194Example:
195See lm90_driver and lm90_detect() in drivers/hwmon/lm90.c
196
197I2C devices instantiated as a result of such a successful probe will be
198destroyed automatically when the driver which detected them is removed,
199or when the underlying I2C bus is itself destroyed, whichever happens
200first.
201
202Those of you familiar with the I2C subsystem of 2.4 kernels and early 2.6
203kernels will find out that this method 3 is essentially similar to what
204was done there. Two significant differences are:
205
206* Probing is only one way to instantiate I2C devices now, while it was the
207  only way back then. Where possible, methods 1 and 2 should be preferred.
208  Method 3 should only be used when there is no other way, as it can have
209  undesirable side effects.
210* I2C buses must now explicitly say which I2C driver classes can probe
211  them (by the means of the class bitfield), while all I2C buses were
212  probed by default back then. The default is an empty class which means
213  that no probing happens. The purpose of the class bitfield is to limit
214  the aforementioned undesirable side effects.
215
216Once again, method 3 should be avoided wherever possible. Explicit device
217instantiation (methods 1 and 2) is much preferred for it is safer and
218faster.
219
220
221Method 4: Instantiate from user-space
222-------------------------------------
223
224In general, the kernel should know which I2C devices are connected and
225what addresses they live at. However, in certain cases, it does not, so a
226sysfs interface was added to let the user provide the information. This
227interface is made of 2 attribute files which are created in every I2C bus
228directory: ``new_device`` and ``delete_device``. Both files are write
229only and you must write the right parameters to them in order to properly
230instantiate, respectively delete, an I2C device.
231
232File ``new_device`` takes 2 parameters: the name of the I2C device (a
233string) and the address of the I2C device (a number, typically expressed
234in hexadecimal starting with 0x, but can also be expressed in decimal.)
235
236File ``delete_device`` takes a single parameter: the address of the I2C
237device. As no two devices can live at the same address on a given I2C
238segment, the address is sufficient to uniquely identify the device to be
239deleted.
240
241Example::
242
243  # echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device
244
245While this interface should only be used when in-kernel device declaration
246can't be done, there is a variety of cases where it can be helpful:
247
248* The I2C driver usually detects devices (method 3 above) but the bus
249  segment your device lives on doesn't have the proper class bit set and
250  thus detection doesn't trigger.
251* The I2C driver usually detects devices, but your device lives at an
252  unexpected address.
253* The I2C driver usually detects devices, but your device is not detected,
254  either because the detection routine is too strict, or because your
255  device is not officially supported yet but you know it is compatible.
256* You are developing a driver on a test board, where you soldered the I2C
257  device yourself.
258
259This interface is a replacement for the force_* module parameters some I2C
260drivers implement. Being implemented in i2c-core rather than in each
261device driver individually, it is much more efficient, and also has the
262advantage that you do not have to reload the driver to change a setting.
263You can also instantiate the device before the driver is loaded or even
264available, and you don't need to know what driver the device needs.
265