03f1805ad0
The sysfs path to i2c adapters has changed recently, update the documentation to reflect that change. Signed-off-by: Jean Delvare <khali@linux-fr.org>
211 lines
8.6 KiB
Text
211 lines
8.6 KiB
Text
How to instantiate I2C devices
|
|
==============================
|
|
|
|
Unlike PCI or USB devices, I2C devices are not enumerated at the hardware
|
|
level. Instead, the software must know which devices are connected on each
|
|
I2C bus segment, and what address these devices are using. For this
|
|
reason, the kernel code must instantiate I2C devices explicitly. There are
|
|
several ways to achieve this, depending on the context and requirements.
|
|
|
|
|
|
Method 1: Declare the I2C devices by bus number
|
|
-----------------------------------------------
|
|
|
|
This method is appropriate when the I2C bus is a system bus as is the case
|
|
for many embedded systems. On such systems, each I2C bus has a number
|
|
which is known in advance. It is thus possible to pre-declare the I2C
|
|
devices which live on this bus. This is done with an array of struct
|
|
i2c_board_info which is registered by calling i2c_register_board_info().
|
|
|
|
Example (from omap2 h4):
|
|
|
|
static struct i2c_board_info __initdata h4_i2c_board_info[] = {
|
|
{
|
|
I2C_BOARD_INFO("isp1301_omap", 0x2d),
|
|
.irq = OMAP_GPIO_IRQ(125),
|
|
},
|
|
{ /* EEPROM on mainboard */
|
|
I2C_BOARD_INFO("24c01", 0x52),
|
|
.platform_data = &m24c01,
|
|
},
|
|
{ /* EEPROM on cpu card */
|
|
I2C_BOARD_INFO("24c01", 0x57),
|
|
.platform_data = &m24c01,
|
|
},
|
|
};
|
|
|
|
static void __init omap_h4_init(void)
|
|
{
|
|
(...)
|
|
i2c_register_board_info(1, h4_i2c_board_info,
|
|
ARRAY_SIZE(h4_i2c_board_info));
|
|
(...)
|
|
}
|
|
|
|
The above code declares 3 devices on I2C bus 1, including their respective
|
|
addresses and custom data needed by their drivers. When the I2C bus in
|
|
question is registered, the I2C devices will be instantiated automatically
|
|
by i2c-core.
|
|
|
|
The devices will be automatically unbound and destroyed when the I2C bus
|
|
they sit on goes away (if ever.)
|
|
|
|
|
|
Method 2: Instantiate the devices explicitly
|
|
--------------------------------------------
|
|
|
|
This method is appropriate when a larger device uses an I2C bus for
|
|
internal communication. A typical case is TV adapters. These can have a
|
|
tuner, a video decoder, an audio decoder, etc. usually connected to the
|
|
main chip by the means of an I2C bus. You won't know the number of the I2C
|
|
bus in advance, so the method 1 described above can't be used. Instead,
|
|
you can instantiate your I2C devices explicitly. This is done by filling
|
|
a struct i2c_board_info and calling i2c_new_device().
|
|
|
|
Example (from the sfe4001 network driver):
|
|
|
|
static struct i2c_board_info sfe4001_hwmon_info = {
|
|
I2C_BOARD_INFO("max6647", 0x4e),
|
|
};
|
|
|
|
int sfe4001_init(struct efx_nic *efx)
|
|
{
|
|
(...)
|
|
efx->board_info.hwmon_client =
|
|
i2c_new_device(&efx->i2c_adap, &sfe4001_hwmon_info);
|
|
|
|
(...)
|
|
}
|
|
|
|
The above code instantiates 1 I2C device on the I2C bus which is on the
|
|
network adapter in question.
|
|
|
|
A variant of this is when you don't know for sure if an I2C device is
|
|
present or not (for example for an optional feature which is not present
|
|
on cheap variants of a board but you have no way to tell them apart), or
|
|
it may have different addresses from one board to the next (manufacturer
|
|
changing its design without notice). In this case, you can call
|
|
i2c_new_probed_device() instead of i2c_new_device().
|
|
|
|
Example (from the pnx4008 OHCI driver):
|
|
|
|
static const unsigned short normal_i2c[] = { 0x2c, 0x2d, I2C_CLIENT_END };
|
|
|
|
static int __devinit usb_hcd_pnx4008_probe(struct platform_device *pdev)
|
|
{
|
|
(...)
|
|
struct i2c_adapter *i2c_adap;
|
|
struct i2c_board_info i2c_info;
|
|
|
|
(...)
|
|
i2c_adap = i2c_get_adapter(2);
|
|
memset(&i2c_info, 0, sizeof(struct i2c_board_info));
|
|
strlcpy(i2c_info.name, "isp1301_pnx", I2C_NAME_SIZE);
|
|
isp1301_i2c_client = i2c_new_probed_device(i2c_adap, &i2c_info,
|
|
normal_i2c);
|
|
i2c_put_adapter(i2c_adap);
|
|
(...)
|
|
}
|
|
|
|
The above code instantiates up to 1 I2C device on the I2C bus which is on
|
|
the OHCI adapter in question. It first tries at address 0x2c, if nothing
|
|
is found there it tries address 0x2d, and if still nothing is found, it
|
|
simply gives up.
|
|
|
|
The driver which instantiated the I2C device is responsible for destroying
|
|
it on cleanup. This is done by calling i2c_unregister_device() on the
|
|
pointer that was earlier returned by i2c_new_device() or
|
|
i2c_new_probed_device().
|
|
|
|
|
|
Method 3: Probe an I2C bus for certain devices
|
|
----------------------------------------------
|
|
|
|
Sometimes you do not have enough information about an I2C device, not even
|
|
to call i2c_new_probed_device(). The typical case is hardware monitoring
|
|
chips on PC mainboards. There are several dozen models, which can live
|
|
at 25 different addresses. Given the huge number of mainboards out there,
|
|
it is next to impossible to build an exhaustive list of the hardware
|
|
monitoring chips being used. Fortunately, most of these chips have
|
|
manufacturer and device ID registers, so they can be identified by
|
|
probing.
|
|
|
|
In that case, I2C devices are neither declared nor instantiated
|
|
explicitly. Instead, i2c-core will probe for such devices as soon as their
|
|
drivers are loaded, and if any is found, an I2C device will be
|
|
instantiated automatically. In order to prevent any misbehavior of this
|
|
mechanism, the following restrictions apply:
|
|
* The I2C device driver must implement the detect() method, which
|
|
identifies a supported device by reading from arbitrary registers.
|
|
* Only buses which are likely to have a supported device and agree to be
|
|
probed, will be probed. For example this avoids probing for hardware
|
|
monitoring chips on a TV adapter.
|
|
|
|
Example:
|
|
See lm90_driver and lm90_detect() in drivers/hwmon/lm90.c
|
|
|
|
I2C devices instantiated as a result of such a successful probe will be
|
|
destroyed automatically when the driver which detected them is removed,
|
|
or when the underlying I2C bus is itself destroyed, whichever happens
|
|
first.
|
|
|
|
Those of you familiar with the i2c subsystem of 2.4 kernels and early 2.6
|
|
kernels will find out that this method 3 is essentially similar to what
|
|
was done there. Two significant differences are:
|
|
* Probing is only one way to instantiate I2C devices now, while it was the
|
|
only way back then. Where possible, methods 1 and 2 should be preferred.
|
|
Method 3 should only be used when there is no other way, as it can have
|
|
undesirable side effects.
|
|
* I2C buses must now explicitly say which I2C driver classes can probe
|
|
them (by the means of the class bitfield), while all I2C buses were
|
|
probed by default back then. The default is an empty class which means
|
|
that no probing happens. The purpose of the class bitfield is to limit
|
|
the aforementioned undesirable side effects.
|
|
|
|
Once again, method 3 should be avoided wherever possible. Explicit device
|
|
instantiation (methods 1 and 2) is much preferred for it is safer and
|
|
faster.
|
|
|
|
|
|
Method 4: Instantiate from user-space
|
|
-------------------------------------
|
|
|
|
In general, the kernel should know which I2C devices are connected and
|
|
what addresses they live at. However, in certain cases, it does not, so a
|
|
sysfs interface was added to let the user provide the information. This
|
|
interface is made of 2 attribute files which are created in every I2C bus
|
|
directory: new_device and delete_device. Both files are write only and you
|
|
must write the right parameters to them in order to properly instantiate,
|
|
respectively delete, an I2C device.
|
|
|
|
File new_device takes 2 parameters: the name of the I2C device (a string)
|
|
and the address of the I2C device (a number, typically expressed in
|
|
hexadecimal starting with 0x, but can also be expressed in decimal.)
|
|
|
|
File delete_device takes a single parameter: the address of the I2C
|
|
device. As no two devices can live at the same address on a given I2C
|
|
segment, the address is sufficient to uniquely identify the device to be
|
|
deleted.
|
|
|
|
Example:
|
|
# echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device
|
|
|
|
While this interface should only be used when in-kernel device declaration
|
|
can't be done, there is a variety of cases where it can be helpful:
|
|
* The I2C driver usually detects devices (method 3 above) but the bus
|
|
segment your device lives on doesn't have the proper class bit set and
|
|
thus detection doesn't trigger.
|
|
* The I2C driver usually detects devices, but your device lives at an
|
|
unexpected address.
|
|
* The I2C driver usually detects devices, but your device is not detected,
|
|
either because the detection routine is too strict, or because your
|
|
device is not officially supported yet but you know it is compatible.
|
|
* You are developing a driver on a test board, where you soldered the I2C
|
|
device yourself.
|
|
|
|
This interface is a replacement for the force_* module parameters some I2C
|
|
drivers implement. Being implemented in i2c-core rather than in each
|
|
device driver individually, it is much more efficient, and also has the
|
|
advantage that you do not have to reload the driver to change a setting.
|
|
You can also instantiate the device before the driver is loaded or even
|
|
available, and you don't need to know what driver the device needs.
|