[PATCH v2 23/24] drm/amd/display: Read before writing Backlight Mode Set Register

[Roman.Li at amd.com]

From: Iswara Nagulendran <iswara.nagulendran at amd.com>

[HOW&WHY]
Reading the value from
DP_EDP_BACKLIGHT_MODE_SET_REGISTER, DPCD 0x721
before setting the
BP_EDP_PANEL_LUMINANC_CONTROL_ENABLE bit
to ensure there are no accidental overwrites.

Reviewed-by: Sreeja Golui <sreeja.golui at amd.com>
Reviewed-by: Harry Vanzylldejong <harry.vanzylldejong at amd.com>
Acked-by: Roman Li <roman.li at amd.com>
Signed-off-by: Iswara Nagulendran <iswara.nagulendran at amd.com>
---
 .../amd/display/dc/link/protocols/link_edp_panel_control.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/display/dc/link/protocols/link_edp_panel_control.c b/drivers/gpu/drm/amd/display/dc/link/protocols/link_edp_panel_control.c
index 86f97ddcc595..e32a7974a4bc 100644
--- a/drivers/gpu/drm/amd/display/dc/link/protocols/link_edp_panel_control.c
+++ b/drivers/gpu/drm/amd/display/dc/link/protocols/link_edp_panel_control.c
@@ -182,7 +182,7 @@ bool edp_set_backlight_level_nits(struct dc_link *link,
 			&backlight_control, 1) != DC_OK)
 			return false;
 	} else {
-		const uint8_t backlight_enable = DP_EDP_PANEL_LUMINANCE_CONTROL_ENABLE;
+		uint8_t backlight_enable = 0;
 		struct target_luminance_value *target_luminance = NULL;
 
 		//if target luminance value is greater than 24 bits, clip the value to 24 bits
@@ -191,6 +191,11 @@ bool edp_set_backlight_level_nits(struct dc_link *link,
 
 		target_luminance = (struct target_luminance_value *)&backlight_millinits;
 
+		core_link_read_dpcd(link, DP_EDP_BACKLIGHT_MODE_SET_REGISTER,
+			&backlight_enable, sizeof(uint8_t));
+
+		backlight_enable |= DP_EDP_PANEL_LUMINANCE_CONTROL_ENABLE;
+
 		if (core_link_write_dpcd(link, DP_EDP_BACKLIGHT_MODE_SET_REGISTER,
 			&backlight_enable,
 			sizeof(backlight_enable)) != DC_OK)
-- 
2.34.1