Hi all,
After reviewing the adm1031 and it87 drivers auto pwm interface
implementations, I think that the auto pwm interface that was debated a
few weeks ago could be reworked a bit.
My main motivation is that I believe that these chips (and this is
probably true for others with the same function as well) offer complex
possibilities most people don't care about. When first designing the
auto pwm interface, I had in mind that it should cover everything the
chips could do. This resulted in a complex interface with (too) many
files.
Now I believe that the interface could be simplified to reflect the
users needs instead. There are two different things that the user can
configure: the trip points (pwm value, temperature value couples) and
the fan channel/temperature channel matching. Both areas could be
simplified.
Temperature channel selection
-----------------------------
The interface was designed so that more than one channel can be
associated with one given fan. I proposed that because the ADM1031 can
do that. Question is: does the user need this? I doubt it.
The drawback of the current interface is that you never know which
values are supported by the chip. Reading from the files is fine, it
reflects the current state in a standard way. But writing is not, which
makes the interface bad IMHO.
Can't we just support one channel per fan? 0 would still mean disabled,
1-N for the channel number. This is easier to understand for the user,
and I'd expect it to be enough for almost all users.
Trip points
-----------
The idea of defining trip points came from the IT87xx hardware
implementation. The ADM1031 has a completely different approach. Let's
have some ASCII art :)
This is the ADM1031 model:
PWM
^
100%| __________
| ,'
| ,'
| ,'
| ,'
| ,'
| ,'
min | ____ ,'
| | |
| | |
0%|___________|____|
|
|----------------------------------------->temperature
^ ^ ^
off min max
(Toff = Tmin - 5, this is an hysteresis so that fan doesn't start/stop
repeatedly it T is too near from Tmin)
All in all there are only three values that have to be defined: Tmin,
Tmax and PWMmin. The chip computes the fan speed between Tmin and Tmax
as a weigthed average between PWMmin and 100%.
Now, this is the IT87xx model:
PWM
^
100%| ______
| |
high| ____|
| |
med | _____|
| |
| |
low | ___________|
| | |
| | |
0%|___________|____|
|
|----------------------------------------->temperature
^ ^ ^ ^ ^
off low medium high full
Here, there are predefined PWM values and the chip sticks to these,
there is no averaging done. A total of 8 values have to be set.
I wonder if this is really necessary to expose these eight registers to
the user. I think that all the user wants to be able to configure is:
* Temperature at which the fan starts spinning, associated PWM, and
associated hysteresis temperature.
* Critical temperature, at which the fan must run at full speed.
All in all it corresponds to what the ADM1031 has.
Thus, I would propose that we do the same for the it87 driver. Values
for non-exposed registers would be computed automatically using weighted
averaging.
Does it sound OK, or is the fine grained trip points selection
considered useful and shouldn't be dropped?
Comments welcome.
Thanks.
--
Jean Delvare
http://khali.linux-fr.org/
