[Intel-gfx] [PATCH 2/2] drm/i915: respect the VBT minimum backlight brightness

[Jesse Barnes]

On Mon, 30 Jun 2014 08:05:34 -0700
Jesse Barnes <jesse.barnes at intel.com> wrote:

> On Sat, 28 Jun 2014 16:45:03 +0300
> Jani Nikula <jani.nikula at intel.com> 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@intel.com
> > 
> > > 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).

Oh and r-b for this patch. :)

Thanks,
-- 
Jesse Barnes, Intel Open Source Technology Center