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On Carrizo/Stoney systems we set backlight through panel_cntl, i.e. directly via the PWM registers, if DMCU is not initialized. We always read it back through ABM registers which leads to a mismatch and forces atomic_commit to program the backlight each time. Instead make sure we use the same logic for backlight readback, i.e. read it from panel_cntl if DMCU is not initialized. We also need to remove some extraneous and incorrect calculations at the end of dce_get_16_bit_backlight_from_pwm. Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1666 Cc: stable@vger.kernel.org Reviewed-by: Josip Pavic <josip.pavic@amd.com> Acked-by: Mikita Lipski <mikita.lipski@amd.com> Signed-off-by: Harry Wentland <harry.wentland@amd.com> Tested-by: Daniel Wheeler <daniel.wheeler@amd.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>