linux-kernel - Re: [PATCH] ARM: dts: rockchip: Use interpolated brightness tables for veyron

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Date:   Wed, 2 Oct 2019 16:44:36 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Heiko Stuebner <heiko@...ech.de>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [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@...omium.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>;