Re: [PATCH v4] drm/i915: Set brightness maximum to a fixed value 100.

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 11 Nov 2015, "Shih-Yuan Lee (FourDollars)" <sylee@canonical.com> wrote:
> Take Dell XPS 13 (2015) as an example. The vbt min 10 out of [0..255].
> The PWM max is 937 so the corresponding PWM min is 37 (10*937/256).
> This commit makes the sysfs brightness 1 map to the PWM brightness 37
> and 100 map to the PWM brightness 937.
>
> Signed-off-by: Shih-Yuan Lee (FourDollars) <sylee@canonical.com>
> ---
>  drivers/gpu/drm/i915/intel_panel.c | 30 ++++++++++++++++++++++++------
>  1 file changed, 24 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index a24df35..8652ce2 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -1207,11 +1207,12 @@ static int intel_backlight_device_register(struct intel_connector *connector)
>  	memset(&props, 0, sizeof(props));
>  	props.type = BACKLIGHT_RAW;
>  
> -	/*
> -	 * Note: Everything should work even if the backlight device max
> -	 * presented to the userspace is arbitrarily chosen.
> -	 */
> -	props.max_brightness = panel->backlight.max;
> +	/* Setting max to fixed 100 if the range is large enough. */
> +	if (panel->backlight.max > panel->backlight.min + 99)
> +		props.max_brightness = 100;
> +	else
> +		props.max_brightness = panel->backlight.max - panel->backlight.min;

No, the intention is to have a fixed 0..100 range no ifs no buts.

> +
>  	props.brightness = scale_hw_to_user(connector,
>  					    panel->backlight.level,
>  					    props.max_brightness);
> @@ -1414,6 +1415,8 @@ 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;
> +	u32 pwm_min;
> +	u32 pwm_step;
>  	int min;
>  
>  	WARN_ON(panel->backlight.max == 0);
> @@ -1432,7 +1435,22 @@ static u32 get_backlight_min_vbt(struct intel_connector *connector)
>  	}
>  
>  	/* vbt value is a coefficient in range [0..255] */
> -	return scale(min, 0, 255, 0, panel->backlight.max);
> +	pwm_min = scale(min, 0, 255, 0, panel->backlight.max);
> +
> +	/* Calculate the PWM step */
> +	if (panel->backlight.max > pwm_min + 99)
> +		pwm_step = scale(1, 0, 99, 0, panel->backlight.max - pwm_min);
> +	else
> +		pwm_step = 1;
> +
> +	/*
> +	 * Because sysfs brightness 0 is used to turn off the backlight, we need step
> +	 * down a little bit to make sysfs brightness 1 match the lowest brightness.
> +	 */
> +	if (pwm_min >= pwm_step)
> +		pwm_min -= pwm_step;
> +
> +	return pwm_min;

This is getting tricky, and you assume too much.

Currently, 0 means off if dev_priv->vbt.backlight.min_brightness == 0 or
the panel is eDP. LVDS panels with non-zero min brightness aren't
switched off with 0 brightness.

Pardon my bluntness, but I'm starting to feel like you just want to get
a patch in. Take my word for it, backlight is not where you should get
started.

BR,
Jani.



>  }
>  
>  static int lpt_setup_backlight(struct intel_connector *connector, enum pipe unused)

-- 
Jani Nikula, Intel Open Source Technology Center
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