hwmon: (pmbus) Add fan control support

hwmon: (pmbus) Add fan control support

Expose fanX_target, pwmX and pwmX_enable hwmon sysfs attributes.

Fans in a PMBus device are driven by the configuration of two registers,
FAN_CONFIG_x_y and FAN_COMMAND_x: FAN_CONFIG_x_y dictates how the fan
and the tacho operate (if installed), while FAN_COMMAND_x sets the
desired fan rate. The unit of FAN_COMMAND_x is dependent on the
operational fan mode, RPM or PWM percent duty, as determined by the
corresponding configuration in FAN_CONFIG_x_y.

The mapping of fanX_target, pwmX and pwmX_enable onto FAN_CONFIG_x_y and
FAN_COMMAND_x is implemented with the addition of virtual registers to
facilitate the necessary side-effects of each access:

1. PMBUS_VIRT_FAN_TARGET_x
2. PMBUS_VIRT_PWM_x
3. PMBUS_VIRT_PWM_ENABLE_x

Some complexity arises with the fanX_target and pwmX attributes both mapping
onto FAN_COMMAND_x: There is no general mapping between PWM percent duty and
RPM, so we can't display values in either attribute in terms of the other
(which in my mind is the intuitive, if impossible, behaviour). This problem
also affects the pwmX_enable attribute which allows userspace to switch between
full speed, manual PWM and a number of automatic control modes, possibly
including a switch to RPM behaviour (e.g. automatically adjusting PWM duty to
reach a RPM target, the behaviour of fanX_target).

The next most intuitive behaviour is for fanX_target and pwmX to simply be
independent, to retain their most recently set value even if that value is not
active on the hardware (due to switching to the alternative control mode). This
property of retaining the value independent of the hardware state has useful
results for both userspace and the kernel: Userspace always sees a sensible
value in the attribute (the last thing it was set to, as opposed to 0 or
receiving an error on read), and the kernel can use the attributes as a value
cache. This latter point eases the implementation of pwmX_enable, which can
look up the associated pmbus_sensor object, take its cached value and apply it
to hardware on changing control mode. This ensures we will not arbitrarily set
a PWM value as an RPM value or vice versa, and we can assume that the RPM or
PWM value set was sensible at least at some point in the past.

Finally, the DIRECT mode coefficients of some controllers is different between
RPM and PWM percent duty control modes, so PSC_PWM is introduced to capture the
necessary coefficients. As pmbus core had no PWM support previously PSC_FAN
continues to be used to capture the RPM DIRECT coefficients, but in order to
avoid falsely applying RPM scaling to PWM values I have introduced the
PMBUS_HAVE_PWM12 and PMB_BUS_HAVE_PWM34 feature bits. These feature bits allow
drivers to explicitly declare PWM support in order to have the attributes
exposed.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>