Re: Re: [PATCH v4 3/3] ARM: dts: sun7i: Add support for the Ainol AW1 tablet

Brüns, Stefan
2018-05-19T00:26:14Z

On Freitag, 18. Mai 2018 09:14:36 CEST Maxime Ripard wrote:
> On Mon, May 14, 2018 at 10:36:08PM +0200, Paul Kocialkowski wrote:
> > > > + backlight: backlight {
> > > > + compatible = "pwm-backlight";
> > > > + pwms = <&pwm 0 50000 PWM_POLARITY_INVERTED>;
> > > > + brightness-levels = < 0 1 1 1 1 2 2 2
> > > > + 2 3 3 3 3 4 4 4
> > > > + 5 5 5 6 6 6 7 7
> > > > + 8 8 8 9 9 9 10 10
> > > > + 10 11 11 12 12 12 13 13
> > > > + 14 14 14 15 15 16 16 17
> > > > + 17 17 18 18 19 19 20 20
> > > > + 21 21 21 22 22 23 23 24
> > > > + 24 25 25 26 26 27 27 28
> > > > + 28 29 30 30 31 31 32 32
> > > > + 33 33 34 35 35 36 36 37
> > > > + 38 38 39 39 40 41 41 42
> > > > + 43 43 44 44 45 46 47 47
> > > > + 48 49 49 50 51 51 52 53
> > > > + 54 54 55 56 57 57 58 59
> > > > + 60 61 61 62 63 64 65 65
> > > > + 66 67 68 69 70 71 71 72
> > > > + 73 74 75 76 77 78 79 80
> > > > + 81 82 83 84 85 86 87 88
> > > > + 89 90 91 92 93 94 95 96
> > > > + 97 98 99 101 102 103 104 105
> > > > + 106 108 109 110 111 112 114 115
> > > > + 116 117 119 120 121 123 124 125
> > > > + 127 128 129 131 132 133 135 136
> > > > + 138 139 141 142 144 145 147 148
> > > > + 150 151 153 154 156 157 159 161
> > > > + 162 164 166 167 169 171 173 174
> > > > + 176 178 180 181 183 185 187 189
> > > > + 191 192 194 196 198 200 202 204
> > > > + 206 208 210 212 214 216 219 221
> > > > + 223 225 227 229 232 234 236 238
> > > > + 241 242 244 246 248 250 253 255>;
> > >
> > > You kind of overdid it here :)
> > >
> > > What I meant to say before was that if you have 10 elements (and you
> > > really should have something in that magnitude) each step should
> > > increase the perceived brightness by 10%.
> >
> > Mhh I think 10 elements would fall too short to really depict the curve
> > with appropriate precision. Given the usual size for brightness cursors
> > in e.g. gnome-shell, it feels like a bigger number would be more
> > appropriate. Let's make it to 100 with values from 0 to 255!
> >
> > > In this particular case, I really think having something close to <0 4
> > > 8 16 32 64 128 255> would be enough.
> > >
> > > And in general, that kind of odd looking table without any more
> > > context is just screaming for a comment :)
> >
> > Noted, I will explain the idea, but probably without the exact formula
> > that's really a nasty hack written down on a piece of paper sitting in
> > my garbage at this point.
>
> So no one will ever be able to understand where this sequence comes
> from (yourself-in-two-years included). That sounds like a pretty bad
> idea.
>
> Maxime

The following formula yields practically the same table:

out = ceil(255 * (0.245 * in/255 + 0.755 * pow(in/255, 2.6) ))

Maximum error: 4, maximum relative error: 0.33

Kind regards,

Stefan