[Intel-gfx] [PATCH v6] drm/i915: respect the VBT minimum backlight brightness again

[Stéphane ANCELOT]

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>
> 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>
> 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 at 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)