Re: [v2] drm/i915: Assume 100% brightness when not in DPCD control mode

[Perr Yuan <pyuan@redhat.com>]

On 12/3/19 5:42 PM, Lyude Paul wrote:
> Currently we always determine the initial panel brightness level by
> simply reading the value from DP_EDP_BACKLIGHT_BRIGHTNESS_MSB/LSB. This
> seems wrong though, because if the panel is not currently in DPCD
> control mode there's not really any reason why there would be any
> brightness value programmed in the first place.
> 
> This appears to be the case on the Lenovo ThinkPad X1 Extreme 2nd
> Generation, where the default value in these registers is always 0 on
> boot despite the fact the panel runs at max brightness by default.
> Getting the initial brightness value correct here is important as well,
> since the panel on this laptop doesn't behave well if it's ever put into
> DPCD control mode while the brightness level is programmed to 0.
> 
> So, let's fix this by checking what the current backlight control mode
> is before reading the brightness level. If it's in DPCD control mode, we
> return the programmed brightness level. Otherwise we assume 100%
> brightness and return the highest possible brightness level. This also
> prevents us from accidentally programming a brightness level of 0.
> 
> This is one of the many fixes that gets backlight controls working on
> the ThinkPad X1 Extreme 2nd Generation with optional 4K AMOLED screen.
> 
> Changes since v1:
> * s/DP_EDP_DISPLAY_CONTROL_REGISTER/DP_EDP_BACKLIGHT_MODE_SET_REGISTER/
>    - Jani
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> ---
>   .../drm/i915/display/intel_dp_aux_backlight.c   | 17 +++++++++++++++++
>   1 file changed, 17 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> index fad470553cf9..4d467e7d29eb 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> @@ -59,8 +59,25 @@ static u32 intel_dp_aux_get_backlight(struct intel_connector *connector)
>   {
>   	struct intel_dp *intel_dp = enc_to_intel_dp(&connector->encoder->base);
>   	u8 read_val[2] = { 0x0 };
> +	u8 mode_reg;
>   	u16 level = 0;
>   
> +	if (drm_dp_dpcd_readb(&intel_dp->aux,
> +			      DP_EDP_BACKLIGHT_MODE_SET_REGISTER,
> +			      &mode_reg) != 1) {
> +		DRM_DEBUG_KMS("Failed to read the DPCD register 0x%x\n",
> +			      DP_EDP_BACKLIGHT_MODE_SET_REGISTER);
> +		return 0;
> +	}
> +
> +	/*
> +	 * If we're not in DPCD control mode yet, the programmed brightness
> +	 * value is meaningless and we should assume max brightness
> +	 */
> +	if ((mode_reg & DP_EDP_BACKLIGHT_CONTROL_MODE_MASK) !=
> +	    DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD)
> +		return connector->panel.backlight.max;
> +
>   	if (drm_dp_dpcd_read(&intel_dp->aux, DP_EDP_BACKLIGHT_BRIGHTNESS_MSB,
>   			     &read_val, sizeof(read_val)) < 0) {
>   		DRM_DEBUG_KMS("Failed to read DPCD register 0x%x\n",
> 

Tested-by:Perry Yuan <pyuan@redhat.com>