drm/amd/display: Get backlight from PWM if DMCU is not initialized

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>

drivers/gpu/drm/amd/display/dc/core/dc_link.c

@@ -2586,13 +2586,21 @@ static struct abm *get_abm_from_stream_res(const struct dc_link *link)
 
 int dc_link_get_backlight_level(const struct dc_link *link)
 {
-
 	struct abm *abm = get_abm_from_stream_res(link);
+	struct panel_cntl *panel_cntl = link->panel_cntl;
+	struct dc  *dc = link->ctx->dc;
+	struct dmcu *dmcu = dc->res_pool->dmcu;
+	bool fw_set_brightness = true;
 
-	if (abm == NULL || abm->funcs->get_current_backlight == NULL)
-		return DC_ERROR_UNEXPECTED;
+	if (dmcu)
+		fw_set_brightness = dmcu->funcs->is_dmcu_initialized(dmcu);
 
-	return (int) abm->funcs->get_current_backlight(abm);
+	if (!fw_set_brightness && panel_cntl->funcs->get_current_backlight)
+		return panel_cntl->funcs->get_current_backlight(panel_cntl);
+	else if (abm != NULL && abm->funcs->get_current_backlight != NULL)
+		return (int) abm->funcs->get_current_backlight(abm);
+	else
+		return DC_ERROR_UNEXPECTED;
 }
 
 int dc_link_get_target_backlight_pwm(const struct dc_link *link)

drivers/gpu/drm/amd/display/dc/dce/dce_panel_cntl.c

@@ -49,7 +49,6 @@
 static unsigned int dce_get_16_bit_backlight_from_pwm(struct panel_cntl *panel_cntl)
 {
 	uint64_t current_backlight;
-	uint32_t round_result;
 	uint32_t bl_period, bl_int_count;
 	uint32_t bl_pwm, fractional_duty_cycle_en;
 	uint32_t bl_period_mask, bl_pwm_mask;
@@ -84,15 +83,6 @@ static unsigned int dce_get_16_bit_backlight_from_pwm(struct panel_cntl *panel_c
 	current_backlight = div_u64(current_backlight, bl_period);
 	current_backlight = (current_backlight + 1) >> 1;
 
-	current_backlight = (uint64_t)(current_backlight) * bl_period;
-
-	round_result = (uint32_t)(current_backlight & 0xFFFFFFFF);
-
-	round_result = (round_result >> (bl_int_count-1)) & 1;
-
-	current_backlight >>= bl_int_count;
-	current_backlight += round_result;
-
 	return (uint32_t)(current_backlight);
 }