So if I understand this correctly, it sounds like that some Pixelbooks boot up
with DP_EDP_BACKLIGHT_BRIGHTNESS_MSB set to a non-zero value, without the
panel actually having DPCD backlight controls enabled?
If I'm understanding that correctly, then this patch looks good to me:
Reviewed-by: Lyude Paul <lyude@xxxxxxxxxx>
On Thu, 2020-09-17 at 20:28 -0400, Sean Paul wrote:
> From: Sean Paul <seanpaul@xxxxxxxxxxxx>
>
> In commit 79946723092b ("drm/i915: Assume 100% brightness when not in
> DPCD control mode"), we fixed the brightness level when DPCD control was
> not active to max brightness. This is as good as we can guess since most
> backlights go on full when uncontrolled.
>
> However in doing so we changed the semantics of the initial
> 'backlight.enabled' value. At least on Pixelbooks, they were relying
> on the brightness level in DP_EDP_BACKLIGHT_BRIGHTNESS_MSB to be 0 on
> boot such that enabled would be false. This causes the device to be
> enabled when the brightness is set. Without this, brightness control
> doesn't work. So by changing brightness to max, we also flipped enabled
> to be true on boot.
>
> To fix this, make enabled a function of brightness and backlight control
> mechanism.
>
> Fixes: 79946723092b ("drm/i915: Assume 100% brightness when not in DPCD
> control mode")
> Cc: Lyude Paul <lyude@xxxxxxxxxx>
> Cc: Jani Nikula <jani.nikula@xxxxxxxxx>
> Cc: Juha-Pekka Heikkila <juhapekka.heikkila@xxxxxxxxx>
> Cc: "Ville Syrjälä" <ville.syrjala@xxxxxxxxxxxxxxx>
> Cc: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
> Cc: Kevin Chowski <chowski@xxxxxxxxxxxx>>
> Signed-off-by: Sean Paul <seanpaul@xxxxxxxxxxxx>
> ---
> .../drm/i915/display/intel_dp_aux_backlight.c | 31 ++++++++++++-------
> 1 file changed, 20 insertions(+), 11 deletions(-)
>
> 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 acbd7eb66cbe..036f504ac7db 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> @@ -52,17 +52,11 @@ static void set_aux_backlight_enable(struct intel_dp
> *intel_dp, bool enable)
> }
> }
>
> -/*
> - * Read the current backlight value from DPCD register(s) based
> - * on if 8-bit(MSB) or 16-bit(MSB and LSB) values are supported
> - */
> -static u32 intel_dp_aux_get_backlight(struct intel_connector *connector)
> +static bool intel_dp_aux_backlight_dpcd_mode(struct intel_connector
> *connector)
> {
> struct intel_dp *intel_dp = intel_attached_dp(connector);
> struct drm_i915_private *i915 = dp_to_i915(intel_dp);
> - 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,
> @@ -70,15 +64,29 @@ static u32 intel_dp_aux_get_backlight(struct
> intel_connector *connector)
> drm_dbg_kms(&i915->drm,
> "Failed to read the DPCD register 0x%x\n",
> DP_EDP_BACKLIGHT_MODE_SET_REGISTER);
> - return 0;
> + return false;
> }
>
> + return (mode_reg & DP_EDP_BACKLIGHT_CONTROL_MODE_MASK) ==
> + DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
> +}
> +
> +/*
> + * Read the current backlight value from DPCD register(s) based
> + * on if 8-bit(MSB) or 16-bit(MSB and LSB) values are supported
> + */
> +static u32 intel_dp_aux_get_backlight(struct intel_connector *connector)
> +{
> + struct intel_dp *intel_dp = intel_attached_dp(connector);
> + struct drm_i915_private *i915 = dp_to_i915(intel_dp);
> + u8 read_val[2] = { 0x0 };
> + u16 level = 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)
> + if (!intel_dp_aux_backlight_dpcd_mode(connector))
> return connector->panel.backlight.max;
>
> if (drm_dp_dpcd_read(&intel_dp->aux, DP_EDP_BACKLIGHT_BRIGHTNESS_MSB,
> @@ -319,7 +327,8 @@ static int intel_dp_aux_setup_backlight(struct
> intel_connector *connector,
>
> panel->backlight.min = 0;
> panel->backlight.level = intel_dp_aux_get_backlight(connector);
> - panel->backlight.enabled = panel->backlight.level != 0;
> + panel->backlight.enabled = intel_dp_aux_backlight_dpcd_mode(connector)
> &&
> + panel->backlight.level != 0;
>
> return 0;
> }
--
Cheers,
Lyude Paul (she/her)
Software Engineer at Red Hat
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