Documentation: i2c: Document that client auto-detection is a legacy mechanism

Class-based client auto-detection has been considered a legacy mechanism
for 10 yrs now. See commit 0c17617 ("i2c: add deprecation warning
for class based instantiation"). Change the documentation of how to
write an i2c client accordingly.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>

Author: hkallweit

Documentation/i2c/writing-clients.rst

@@ -48,10 +48,6 @@ driver model device node, and its I2C address.
 	.id_table	= foo_idtable,
 	.probe		= foo_probe,
 	.remove		= foo_remove,
-	/* if device autodetection is needed: */
-	.class		= I2C_CLASS_SOMETHING,
-	.detect		= foo_detect,
-	.address_list	= normal_i2c,
 
 	.shutdown	= foo_shutdown,	/* optional */
 	.command	= foo_command,	/* optional, deprecated */
@@ -203,37 +199,17 @@ reference for later use.
 Device Detection
 ----------------
 
-Sometimes you do not know in advance which I2C devices are connected to
-a given I2C bus.  This is for example the case of hardware monitoring
-devices on a PC's SMBus.  In that case, you may want to let your driver
-detect supported devices automatically.  This is how the legacy model
-was working, and is now available as an extension to the standard
-driver model.
-
-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.  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
+The device detection mechanism comes with a number of disadvantages.
+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.  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.
+operation by another chip.  For these reasons, device detection is
+considered a legacy mechanism and shouldn't be used in new code.
 
 
 Device Deletion