w1_therm: adding resolution sysfs entry

commit 308bdb94

 upstream.

Adding resolution sysfs entry (RW) to get or set the device resolution
Write values are managed as follow:
	* '9..12': resolution to set in bit
	* Anything else: do nothing
Read values are :
	* '9..12': device resolution in bit
	* '-xx': xx is kernel error when reading the resolution

Only supported devices will show the sysfs entry. A new family has been
created for DS18S20 devices as they do not implement resolution feature.

The resolution of each device is check when the device is
discover by the bus master, in 'w1_therm_add_slave(struct w1_slave *)'.
The status is stored in the device structure w1_therm_family_data so
that the driver always knows the resolution of each device, which could
be used later to determine the required conversion duration (resolution
dependent).

The resolution is re evaluate each time a user read or write the sysfs
entry.

To avoid looping through the w1_therm_families at run time, the pointer
'specific_functions' is set up to the correct 'w1_therm_family_converter'
when the slave is added (which mean when it is discovered by the master).
This initialization is done by a helper function
'device_family(struct w1_slave *sl)', and a dedicated macro
'SLAVE_SPECIFIC_FUNC(sl)' allow the access to the specific function of the
slave device.

'read_scratchpad' and 'write_scratchpad' are the hardware functions to
access the device RAM, as per protocol specification.

It cancel the former 'precision' functions, which was only set and never
read (so not stored in the device struct).

Updating Documentation/ABI/testing/sysfs-driver-w1_therm accordingly.

Signed-off-by: Akira Shimahara <akira215corp@gmail.com>
Link: https://lore.kernel.org/r/20200511203708.410649-1-akira215corp@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>