i2c: Document the different ways to instantiate i2c devices

On popular demand, here comes some documentation about how to
instantiate i2c devices in the new (standard) i2c device driver
binding model.

I have also clarified how the class bitfield lets driver authors
control which buses are probed in the auto-detect case, and warned
more loudly against the abuse of this method.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Michael Lawnick <nospam_lawnick@gmx.de>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>

1 files changed, 15 insertions, 4 deletions

diff --git a/Documentation/i2c/writing-clients b/Documentation/i2c/writing-clients
index 6b9af7d479c2..c1a06f989cf7 100644
--- a/Documentation/i2c/writing-clients
+++ b/Documentation/i2c/writing-clients
@@ -207,15 +207,26 @@ You simply have to define a detect callback which will attempt to
 identify supported devices (returning 0 for supported ones and -ENODEV
 for unsupported ones), a list of addresses to probe, and a device type
 (or class) so that only I2C buses which may have that type of device
-connected (and not otherwise enumerated) will be probed.  The i2c
-core will then call you back as needed and will instantiate a device
-for you for every successful detection.
+connected (and not otherwise enumerated) will be probed.  For example,
+a driver for a hardware monitoring chip for which auto-detection is
+needed would set its class to I2C_CLASS_HWMON, and only I2C adapters
+with a class including I2C_CLASS_HWMON would be probed by this driver.
+Note that the absence of matching classes does not prevent the use of
+a device of that type on the given I2C adapter.  All it prevents is
+auto-detection; explicit instantiation of devices is still possible.
 
 Note that this mechanism is purely optional and not suitable for all
 devices.  You need some reliable way to identify the supported devices
 (typically using device-specific, dedicated identification registers),
 otherwise misdetections are likely to occur and things can get wrong
-quickly.
+quickly.  Keep in mind that the I2C protocol doesn't include any
+standard way to detect the presence of a chip at a given address, let
+alone a standard way to identify devices.  Even worse is the lack of
+semantics associated to bus transfers, which means that the same
+transfer can be seen as a read operation by a chip and as a write
+operation by another chip.  For these reasons, explicit device
+instantiation should always be preferred to auto-detection where
+possible.
 
 
 Device Deletion