168 lines
6.4 KiB
Plaintext
168 lines
6.4 KiB
Plaintext
How to instantiate I2C devices
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==============================
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Unlike PCI or USB devices, I2C devices are not enumerated at the hardware
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level. Instead, the software must know which devices are connected on each
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I2C bus segment, and what address these devices are using. For this
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reason, the kernel code must instantiate I2C devices explicitly. There are
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several ways to achieve this, depending on the context and requirements.
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Method 1: Declare the I2C devices by bus number
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-----------------------------------------------
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This method is appropriate when the I2C bus is a system bus as is the case
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for many embedded systems. On such systems, each I2C bus has a number
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which is known in advance. It is thus possible to pre-declare the I2C
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devices which live on this bus. This is done with an array of struct
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i2c_board_info which is registered by calling i2c_register_board_info().
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Example (from omap2 h4):
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static struct i2c_board_info __initdata h4_i2c_board_info[] = {
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{
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I2C_BOARD_INFO("isp1301_omap", 0x2d),
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.irq = OMAP_GPIO_IRQ(125),
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},
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{ /* EEPROM on mainboard */
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I2C_BOARD_INFO("24c01", 0x52),
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.platform_data = &m24c01,
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},
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{ /* EEPROM on cpu card */
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I2C_BOARD_INFO("24c01", 0x57),
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.platform_data = &m24c01,
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},
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};
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static void __init omap_h4_init(void)
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{
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(...)
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i2c_register_board_info(1, h4_i2c_board_info,
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ARRAY_SIZE(h4_i2c_board_info));
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(...)
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}
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The above code declares 3 devices on I2C bus 1, including their respective
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addresses and custom data needed by their drivers. When the I2C bus in
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question is registered, the I2C devices will be instantiated automatically
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by i2c-core.
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The devices will be automatically unbound and destroyed when the I2C bus
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they sit on goes away (if ever.)
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Method 2: Instantiate the devices explicitly
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--------------------------------------------
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This method is appropriate when a larger device uses an I2C bus for
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internal communication. A typical case is TV adapters. These can have a
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tuner, a video decoder, an audio decoder, etc. usually connected to the
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main chip by the means of an I2C bus. You won't know the number of the I2C
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bus in advance, so the method 1 described above can't be used. Instead,
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you can instantiate your I2C devices explicitly. This is done by filling
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a struct i2c_board_info and calling i2c_new_device().
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Example (from the sfe4001 network driver):
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static struct i2c_board_info sfe4001_hwmon_info = {
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I2C_BOARD_INFO("max6647", 0x4e),
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};
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int sfe4001_init(struct efx_nic *efx)
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{
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(...)
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efx->board_info.hwmon_client =
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i2c_new_device(&efx->i2c_adap, &sfe4001_hwmon_info);
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(...)
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}
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The above code instantiates 1 I2C device on the I2C bus which is on the
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network adapter in question.
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A variant of this is when you don't know for sure if an I2C device is
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present or not (for example for an optional feature which is not present
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on cheap variants of a board but you have no way to tell them apart), or
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it may have different addresses from one board to the next (manufacturer
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changing its design without notice). In this case, you can call
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i2c_new_probed_device() instead of i2c_new_device().
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Example (from the pnx4008 OHCI driver):
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static const unsigned short normal_i2c[] = { 0x2c, 0x2d, I2C_CLIENT_END };
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static int __devinit usb_hcd_pnx4008_probe(struct platform_device *pdev)
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{
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(...)
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struct i2c_adapter *i2c_adap;
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struct i2c_board_info i2c_info;
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(...)
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i2c_adap = i2c_get_adapter(2);
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memset(&i2c_info, 0, sizeof(struct i2c_board_info));
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strlcpy(i2c_info.name, "isp1301_pnx", I2C_NAME_SIZE);
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isp1301_i2c_client = i2c_new_probed_device(i2c_adap, &i2c_info,
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normal_i2c);
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i2c_put_adapter(i2c_adap);
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(...)
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}
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The above code instantiates up to 1 I2C device on the I2C bus which is on
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the OHCI adapter in question. It first tries at address 0x2c, if nothing
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is found there it tries address 0x2d, and if still nothing is found, it
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simply gives up.
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The driver which instantiated the I2C device is responsible for destroying
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it on cleanup. This is done by calling i2c_unregister_device() on the
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pointer that was earlier returned by i2c_new_device() or
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i2c_new_probed_device().
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Method 3: Probe an I2C bus for certain devices
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----------------------------------------------
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Sometimes you do not have enough information about an I2C device, not even
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to call i2c_new_probed_device(). The typical case is hardware monitoring
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chips on PC mainboards. There are several dozen models, which can live
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at 25 different addresses. Given the huge number of mainboards out there,
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it is next to impossible to build an exhaustive list of the hardware
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monitoring chips being used. Fortunately, most of these chips have
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manufacturer and device ID registers, so they can be identified by
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probing.
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In that case, I2C devices are neither declared nor instantiated
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explicitly. Instead, i2c-core will probe for such devices as soon as their
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drivers are loaded, and if any is found, an I2C device will be
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instantiated automatically. In order to prevent any misbehavior of this
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mechanism, the following restrictions apply:
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* The I2C device driver must implement the detect() method, which
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identifies a supported device by reading from arbitrary registers.
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* Only buses which are likely to have a supported device and agree to be
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probed, will be probed. For example this avoids probing for hardware
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monitoring chips on a TV adapter.
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Example:
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See lm90_driver and lm90_detect() in drivers/hwmon/lm90.c
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I2C devices instantiated as a result of such a successful probe will be
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destroyed automatically when the driver which detected them is removed,
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or when the underlying I2C bus is itself destroyed, whichever happens
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first.
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Those of you familiar with the i2c subsystem of 2.4 kernels and early 2.6
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kernels will find out that this method 3 is essentially similar to what
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was done there. Two significant differences are:
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* Probing is only one way to instantiate I2C devices now, while it was the
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only way back then. Where possible, methods 1 and 2 should be preferred.
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Method 3 should only be used when there is no other way, as it can have
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undesirable side effects.
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* I2C buses must now explicitly say which I2C driver classes can probe
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them (by the means of the class bitfield), while all I2C buses were
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probed by default back then. The default is an empty class which means
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that no probing happens. The purpose of the class bitfield is to limit
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the aforementioned undesirable side effects.
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Once again, method 3 should be avoided wherever possible. Explicit device
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instantiation (methods 1 and 2) is much preferred for it is safer and
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faster.
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