On Sat, 28 Jun 2014 16:45:03 +0300
Jani Nikula <jani.nikula@xxxxxxxxx> wrote:
> >> +/* Scale user_level in range [0..user_max] to [0..hw_max], clamping the result
> >> + * to [hw_min..hw_max]. */
> >> +static inline u32 clamp_user_to_hw(struct intel_connector *connector,
> >> + u32 user_level, u32 user_max)
> >> +{
> >> + struct intel_panel *panel = &connector->panel;
> >> + u32 hw_level;
> >> +
> >> + hw_level = scale(user_level, 0, user_max, 0, panel->backlight.max);
> >> + hw_level = clamp(hw_level, panel->backlight.min, panel->backlight.max);
> >> +
> >> + return hw_level;
> >> +}
> >
> > I like the direction here, but does this mean some user values will
> > potentially be no-ops? E.g. the low 10 values or something would all
> > map to backlight.min depending on the min?
>
> This patch doesn't really change the fact. For a max backlight of, say,
> 100000, we're bound to not have user perceivable difference between
> consecutive levels. I agree we should have a fixed, limited range here.
>
> This also depends on the backlight modulation frequency, see my earlier
> message about this: http://mid.gmane.org/87iooecg1e.fsf@xxxxxxxxx
>
> > I just want to make sure every value we expose to userspace is
> > meaningful, and somehow equidistant from the values next to it, so we
> > have nice, smooth backlight transitions, and fades look good (on that
> > front, is 256 enough? or should we have a scaled range up to 1024 or
> > so?)
>
> Probably even fewer than that is enough.
>
> But you bring up another requirement, equidistant - do you mean in terms
> of luminance? Almost certainly a linear mapping to the duty cycle is not
> going to give you a linear luminance! It's possible to define a curve
> for this in the acpi opregion; patches to implement this are welcome. ;)
Ok, I guess both of these are projects for future patches. For making
the brightness levels evenly spaced, yeah I guess we'd need to apply a
rough approximation of a luminosity function (doesn't look like a
simple exponential curve, oh well).
Jesse
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