drm/i915/backlight: Fix VESA backlight possible check condition

VESA backlight enable is possible when
BACKLIGHT_AUX_ENABLE_CAPABLE is true via AUX command or when
BACKLIGHT_PIN_ENABLE_CAPABLE is true via eDP connector pin.
Similarly, backlight brightness adjustment can be
done via AUX-based control or PWM pin-based control.
It means there can be three configurations:
1) Full AUX-based: Enable and adjustment both via AUX. We currently
   support this (apart from the AUX luminance-based backlight control).
2) Hybrid: Enable via the BL_ENABLE pin, adjustment via either AUX or
   PWM.
3) Fully PWM pin-based: Enable via the BL_ENABLE pin, adjustment via
   PWM.
Since that only 1 is supported as of now we need to make
sure we do not try to manipulate backlight when
BACKLIGHT_AUX_ENABLE_CAPABLE is not set.
Also fix return value when condition is not fulfilled.

Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/15907
Fixes: 0fb0389 ("drm/i915/backlight: Check if VESA backlight is possible")
Signed-off-by: Suraj Kandpal <[email protected]>
Tested-by: Ville Syrjälä <[email protected]>
Reviewed-by: Ankit Nautiyal <[email protected]>
Link: https://patch.msgid.link/[email protected]
(cherry picked from commit 102d44b3a8fad96e94e9ccd0579986c14a1f2f75)
Signed-off-by: Tvrtko Ursulin <[email protected]>

Author: surajk8

drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c

@@ -615,8 +615,13 @@ check_if_vesa_backlight_possible(struct intel_dp *intel_dp)
 	int ret;
 	u8 bit_min, bit_max;
 
-	if (!(intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP))
-		return true;
+	/*
+	 * Since we only support Fully AUX Based VESA Backlight interface make sure
+	 * backlight enable is possible via AUX along with backlight adjustment
+	 */
+	if (!(intel_dp->edp_dpcd[1] & DP_EDP_BACKLIGHT_AUX_ENABLE_CAP &&
+	      intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP))
+		return false;
 
 	ret = drm_dp_dpcd_read_byte(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &bit_min);
 	if (ret < 0)