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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: 0fb03890d182 ("drm/i915/backlight: Check if VESA backlight is possible")
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Link: https://patch.msgid.link/20260407030710.1440046-1-suraj.kandpal@intel.com
(cherry picked from commit 102d44b3a8fad96e94e9ccd0579986c14a1f2f75)
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>

authored by

Suraj Kandpal and committed by
Tvrtko Ursulin ed8be780 83e8d8bb

+7 -2
+7 -2
drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
··· 615 615 int ret; 616 616 u8 bit_min, bit_max; 617 617 618 - if (!(intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP)) 619 - return true; 618 + /* 619 + * Since we only support Fully AUX Based VESA Backlight interface make sure 620 + * backlight enable is possible via AUX along with backlight adjustment 621 + */ 622 + if (!(intel_dp->edp_dpcd[1] & DP_EDP_BACKLIGHT_AUX_ENABLE_CAP && 623 + intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP)) 624 + return false; 620 625 621 626 ret = drm_dp_dpcd_read_byte(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &bit_min); 622 627 if (ret < 0)