Re: [PATCH v6] drm/i915: respect the VBT minimum backlight brightness again

["Stéphane ANCELOT" <sancelot@free.fr>]

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On 12/11/2015 09:28, Shih-Yuan Lee (FourDollars) wrote:
> I think the first step is to identify which backlight interface is 
> able to actually change the brightness.
> Usually there are two backlight interfaces under 
> /sys/class/backlight/, i.e. /sys/class/backlight/acpi_video0 and 
> /sys/class/backlight/intel_backlight.
> However you may see other different backlight interfaces under 
> /sys/class/backlight/. It depends on what hardware you are using.
> Executing `cat max_brightness` under those folders can see the max 
> brightness and then you can execute `echo $(($(cat max_brightness)/2)) 
> | sudo tee brightness` to adjust it to about 50% brightness.
>
only /sys/class/backlight/intel_backlight is present.
max_brightness is 937

these commands have no effect on display brightness:
echo 0 >brightness
echo 468 >brightness
echo 937 >brightness

Regards,
Steph


> On Thu, Nov 12, 2015 at 4:08 PM, Stéphane ANCELOT <sancelot@free.fr 
> <mailto:sancelot@free.fr>> wrote:
>
>     Hi,
>     I have seen you were working on brightness. Right, but this is
>     useful only if many worlwide users would be able to activate it.
>     I have never been able to make it working on my intel systemes
>     (core i5 - 4300u , core i3 ...)
>     Very difficult to check where the problem is coming from, altough
>     you have either an intel_backlight or acpi_backlight folder in the
>     sys directory
>     Regarding many many posts on the web, this sounds tricky and
>     difficult to know why it has not worked, or simply the
>     documentation is lacking ....
>
>     Regards,
>     S.Ancelot
>
>
>     On 12/11/2015 06:43, Shih-Yuan Lee (FourDollars) wrote:
>
>         There was a wonderful period after
>
>         commit 6dda730e55f412a6dfb181cae6784822ba463847
>         Author: Jani Nikula <jani.nikula at intel.com <http://intel.com>>
>         Date:   Tue Jun 24 18:27:40 2014 +0300
>
>              drm/i915: respect the VBT minimum backlight brightness
>
>         The backlight class 0 brightness means the PWM min and it does
>         not turn
>         off the backlight. After kernel 3.18, the backlight class 0
>         brightness
>         is used to turn off the backlight and the PWM min is missing.
>
>         Because of
>
>         commit e6755fb78e8f20ecadf2a4080084121336624ad9
>         Author: Jani Nikula <jani.nikula at intel.com <http://intel.com>>
>         Date:   Tue Aug 12 17:11:42 2014 +0300
>
>              drm/i915: switch off backlight for backlight class 0
>         brightness
>
>         Use "VBT backlight PWM modulation frequency 200 Hz, active
>         high, min
>         brightness 10, level 255" as an example. It means the VBT min
>         is 10
>         out of [0..255] and the PWM max is 937 from other place so the
>         corresponding PWM min should be 37 (10 * 937 / 255).
>
>         When we set backlight class 0 brightness, the backlight is
>         turned off.
>         Althought the PWM value is 37 when the backlight is off but it is
>         useless. When we set set backlight class 1 brightness, the
>         backlight is
>         on but the PWM value is 38 and it doesn't match the
>         corresponding PWM
>         min of the VBT minimum backlight.
>
>         And it has another minor issue that there are some backlight class
>         brightness values are mapped to the same PWM value because the
>         backlight
>         class brightness range is larger than the valid PWM brightness
>         range.
>
>         The backlight class brightness range [0..937]
>         The valid PWM brightness range [37..937]
>
>         This commit makes that backlight class 1 brightness means the
>         PWM min
>         and backlight class max_brightness means the PWM max, and the
>         ranges
>         become
>
>         The backlight class brightness range [0..901]
>         The valid PWM brightness range [36..937]
>
>         That's no backlight class brightness value mapped to the same
>         PWM value.
>
>         Signed-off-by: Shih-Yuan Lee (FourDollars)
>         <sylee@canonical.com <mailto:sylee@canonical.com>>
>         ---
>           drivers/gpu/drm/i915/intel_panel.c | 26
>         ++++++++++++++++++--------
>           1 file changed, 18 insertions(+), 8 deletions(-)
>
>         diff --git a/drivers/gpu/drm/i915/intel_panel.c
>         b/drivers/gpu/drm/i915/intel_panel.c
>         index b05c6d9..8efa199 100644
>         --- a/drivers/gpu/drm/i915/intel_panel.c
>         +++ b/drivers/gpu/drm/i915/intel_panel.c
>         @@ -1194,10 +1194,9 @@ static int
>         intel_backlight_device_register(struct intel_connector *connector)
>                 props.type = BACKLIGHT_RAW;
>                 /*
>         -        * Note: Everything should work even if the backlight
>         device max
>         -        * presented to the userspace is arbitrarily chosen.
>         +        * Expose the whole valid PWM brightness range to the
>         backlight class.
>                  */
>         -       props.max_brightness = panel->backlight.max;
>         +       props.max_brightness = panel->backlight.max -
>         panel->backlight.min;
>                 props.brightness = scale_hw_to_user(connector,
>         panel->backlight.level,
>         props.max_brightness);
>         @@ -1400,7 +1399,8 @@ static u32 get_backlight_min_vbt(struct
>         intel_connector *connector)
>                 struct drm_device *dev = connector->base.dev;
>                 struct drm_i915_private *dev_priv = dev->dev_private;
>                 struct intel_panel *panel = &connector->panel;
>         -       int min;
>         +       int vbt_min;
>         +       u32 pwm_min;
>                 WARN_ON(panel->backlight.max == 0);
>           @@ -1411,14 +1411,24 @@ static u32
>         get_backlight_min_vbt(struct intel_connector *connector)
>                  * against this by letting the minimum be at most
>         (arbitrarily chosen)
>                  * 25% of the max.
>                  */
>         -       min = clamp_t(int,
>         dev_priv->vbt.backlight.min_brightness, 0, 64);
>         -       if (min != dev_priv->vbt.backlight.min_brightness) {
>         +       vbt_min = clamp_t(int,
>         dev_priv->vbt.backlight.min_brightness, 0, 64);
>         +       if (vbt_min != dev_priv->vbt.backlight.min_brightness) {
>                         DRM_DEBUG_KMS("clamping VBT min backlight
>         %d/255 to %d/255\n",
>         -  dev_priv->vbt.backlight.min_brightness, min);
>         +  dev_priv->vbt.backlight.min_brightness, vbt_min);
>                 }
>                 /* vbt value is a coefficient in range [0..255] */
>         -       return scale(min, 0, 255, 0, panel->backlight.max);
>         +       pwm_min = scale(vbt_min, 0, 255, 0, panel->backlight.max);
>         +
>         +       /*
>         +        * Because backlight class brightness 0 is used to
>         turn off the backlight, we
>         +        * need to step down a little bit here to make
>         backlight class brightness 1
>         +        * match the real PWM min.
>         +        */
>         +       if (pwm_min > 0)
>         +               return pwm_min - 1;
>         +       else
>         +               return 0;
>           }
>             static int lpt_setup_backlight(struct intel_connector
>         *connector, enum pipe unused)
>
>
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>


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