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SubjectRe: [PATCH] ARM: dts: rockchip: Use interpolated brightness tables for veyron
On Wed, Oct 02, 2019 at 03:23:54PM -0700, Doug Anderson wrote:
> Hi,
>
> On Tue, Oct 1, 2019 at 4:07 PM Matthias Kaehlcke <mka@chromium.org> wrote:
> > --- a/arch/arm/boot/dts/rk3288-veyron-minnie.dts
> > +++ b/arch/arm/boot/dts/rk3288-veyron-minnie.dts
> > @@ -39,39 +39,8 @@
> >
> > &backlight {
> > /* Minnie panel PWM must be >= 1%, so start non-zero brightness at 3 */
> > - brightness-levels = <
> > - 0 3 4 5 6 7
> > - 8 9 10 11 12 13 14 15
> > - 16 17 18 19 20 21 22 23
> > - 24 25 26 27 28 29 30 31
> > - 32 33 34 35 36 37 38 39
> > - 40 41 42 43 44 45 46 47
> > - 48 49 50 51 52 53 54 55
> > - 56 57 58 59 60 61 62 63
> > - 64 65 66 67 68 69 70 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 99 100 101 102 103
> > - 104 105 106 107 108 109 110 111
> > - 112 113 114 115 116 117 118 119
> > - 120 121 122 123 124 125 126 127
> > - 128 129 130 131 132 133 134 135
> > - 136 137 138 139 140 141 142 143
> > - 144 145 146 147 148 149 150 151
> > - 152 153 154 155 156 157 158 159
> > - 160 161 162 163 164 165 166 167
> > - 168 169 170 171 172 173 174 175
> > - 176 177 178 179 180 181 182 183
> > - 184 185 186 187 188 189 190 191
> > - 192 193 194 195 196 197 198 199
> > - 200 201 202 203 204 205 206 207
> > - 208 209 210 211 212 213 214 215
> > - 216 217 218 219 220 221 222 223
> > - 224 225 226 227 228 229 230 231
> > - 232 233 234 235 236 237 238 239
> > - 240 241 242 243 244 245 246 247
> > - 248 249 250 251 252 253 254 255>;
> > + brightness-levels = <3 255>;
> > + num-interpolated-steps = <251>;
>
> I _think_ you want:
>
> brightness-levels = <0 3 255>;
> num-interpolated-steps = <252>;
>
> Specifically:
>
> * It seems like you're intending to keep everything the same and just
> have a more compact representation, right?

Ideally yes, I thought we were missing 1 level due to the 0 step being
missing, but it's actually 2, since I interpreted 'num-interpolated-steps'
as the number between two values in the table, however it is this number +1.

> Looking through the values in '/sys/class/backlight/backlight' on
> minnie shows differences before and after your patch.
>
> * I think you want brightness of 0 to match to PWM level 0.

For level 0 that was actually given, due to

pwm_backlight_update_status()
{
..
if (brightness > 0) {
...
} else
pwm_backlight_power_off(pb);
...
}

but we're slightly off for the rest of the levels.

> * If I put in printouts in the code with your table, I see:
>
> pwm-backlight backlight: new number of brightness levels: 252
> pwm-backlight backlight: i=0, j=0, lc=0, value=3
> pwm-backlight backlight: i=0, j=1, lc=1, value=4
> ...
> pwm-backlight backlight: i=0, j=250, lc=250, value=253
> pwm-backlight backlight: lc=251, data->levels[i]=255
>
> ...as you can see, you end up missing assigning a value of 254.

Thanks for investigating. With 'num-interpolated-steps' increased
by one this is fixed, though we are still missing one level at the
beginning of the table. I didn't expect 'brightness-levels = <0 3 255>'
to work, since there are less than 252/251 integer numbers between 0
and 3, but the code actually accounts for that case and just interprets
it as a single step, which is what we want.

Long story short: you are right, we want

brightness-levels = <0 3 255>;
num-interpolated-steps = <252>;

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