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i2c: Document the implementation details of the /dev interface 

I wrote this explanation to answer a question on the i2c mailing list,
and thought it would be good to have in the kernel documentation.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
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Jean Delvare 2008-10-14 17:30:05 +02:00 committed by Jean Delvare
parent fceb2d0680
commit 7c15fd1249
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Documentation/i2c/dev-interface
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@ -167,3 +167,48 @@ The above functions are all inline functions, that resolve to calls to
the i2c_smbus_access function, that on its turn calls a specific ioctl
with the data in a specific format. Read the source code if you
want to know what happens behind the screens.
Implementation details
======================
For the interested, here's the code flow which happens inside the kernel
when you use the /dev interface to I2C:
1* Your program opens /dev/i2c-N and calls ioctl() on it, as described in
section "C example" above.
2* These open() and ioctl() calls are handled by the i2c-dev kernel
driver: see i2c-dev.c:i2cdev_open() and i2c-dev.c:i2cdev_ioctl(),
respectively. You can think of i2c-dev as a generic I2C chip driver
that can be programmed from user-space.
3* Some ioctl() calls are for administrative tasks and are handled by
i2c-dev directly. Examples include I2C_SLAVE (set the address of the
device you want to access) and I2C_PEC (enable or disable SMBus error
checking on future transactions.)
4* Other ioctl() calls are converted to in-kernel function calls by
i2c-dev. Examples include I2C_FUNCS, which queries the I2C adapter
functionality using i2c.h:i2c_get_functionality(), and I2C_SMBUS, which
performs an SMBus transaction using i2c-core.c:i2c_smbus_xfer().
The i2c-dev driver is responsible for checking all the parameters that
come from user-space for validity. After this point, there is no
difference between these calls that came from user-space through i2c-dev
and calls that would have been performed by kernel I2C chip drivers
directly. This means that I2C bus drivers don't need to implement
anything special to support access from user-space.
5* These i2c-core.c/i2c.h functions are wrappers to the actual
implementation of your I2C bus driver. Each adapter must declare
callback functions implementing these standard calls.
i2c.h:i2c_get_functionality() calls i2c_adapter.algo->functionality(),
while i2c-core.c:i2c_smbus_xfer() calls either
adapter.algo->smbus_xfer() if it is implemented, or if not,
i2c-core.c:i2c_smbus_xfer_emulated() which in turn calls
i2c_adapter.algo->master_xfer().
After your I2C bus driver has processed these requests, execution runs
up the call chain, with almost no processing done, except by i2c-dev to
package the returned data, if any, in suitable format for the ioctl.