Re: [RFC 0/2] backlight: pwm_bl: support linear brightness to human eye

[Daniel Thompson]

On 05/09/17 12:05, Daniel Thompson wrote:
> On 04/09/17 16:35, Enric Balletbo i Serra wrote:
> > Dear all,
> >
> > This patch series is a first RFC to know your opinion about implement
> > support to create brightness levels tables dinamically. I tried to argue
> > in every patch the specific reasons we think this can be interesting, to
> > sumup, the idea behind these patches is be able to pass via device tree
> > two parameters to the driver so it can calculate the brightness levels
> > based on the CIE 1931 lightness formula, which is what actually describes
> > how we perceive light.
> >
> > I think that at least the maths involved can be improved, and I've still
> > some doubts. With current code if you create a table with a max PWM
> > value of 255 and 127 steps, the first numbers are repeated so I'm 
> > thinking > that maybe we should skip/remove the repeated values. i.e. 
> > have a table
> > like this,
> >
> > [0, 1, 2, 3 ... 235, 240, 245, 250, 255]
> >
> > instead of
> >
> > [0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2, 2, 3 ... 235, 240, 245, 250, 255]
> >
> > Well, I know there are things to improve but lets see your feedback first
> > before dedicate more time on it. The patches were tested on a couple of
> > devices but I'll test on more devices meanwhile we discuss about it.
>
> I'm not fully decided on this one but my initial reaction isn't to 
> question the concept so much as to ask why the number of levels should 
> go in the devicetree at all! We could just make brightness-levels 
> optional and get the driver to pick sane curves by default.
>
> I'm sure we can debate what "sane" means for a couple of e-mails yet but 
> in principle, given it knows the PWM max counter value, the driver 
> should be able to calculate the "right" number of steps too. If we have 
> that your core code remains but we don't have to complexify the device

... tree

Sorry. ;-)


Daniel.


> <strawman>
> Basically we prefer X (?100 like some of the Intel DRM drivers do for 
> connector properties?) steps but we reduce the number of steps if the 
> PWM is rather course and we can't get sufficiently different steps.
> </strawman>
>
> I guess the summary of what I'm saying is that if we can 
> programmatically derive brightness curves then the number of steps is 
> not really a property of the hardware and doesn't belong in devicetree.
>
>
> Daniel.