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SubjectRe: [PATCH 2/5] drm/i915: Assume 100% brightness when not in DPCD control mode
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On Fri, 22 Nov 2019, Lyude Paul <lyude@redhat.com> 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.
>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> ---
> .../gpu/drm/i915/display/intel_dp_aux_backlight.c | 15 +++++++++++++++
> 1 file changed, 15 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..0bf8772bc7bb 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 +59,23 @@ 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 control_reg;
> u16 level = 0;
>
> + if (drm_dp_dpcd_readb(&intel_dp->aux, DP_EDP_DISPLAY_CONTROL_REGISTER,

Shouldn't that be DP_EDP_BACKLIGHT_MODE_SET_REGISTER instead?

> + &control_reg) != 1) {
> + DRM_DEBUG_KMS("Failed to read the DPCD register 0x%x\n",
> + DP_EDP_DISPLAY_CONTROL_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 (!(control_reg & DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD))
> + return connector->panel.backlight.max;

It's not just a bit, I think you need to check (control_reg & mask) ==
value.

BR,
Jani.

> +
> 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",

--
Jani Nikula, Intel Open Source Graphics Center

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