[PATCH 19/25] drm/amd/display: WA to ensure MUX chip gets SUPPORTED_LINK_RATES of eDP

[Qingqing Zhuo]

From: Dale Zhao <dale.zhao at amd.com>

[Why]
Customer make a request to add this WA by driver.

Some MUX chips will power down with eDP 1.4 panel and
lose previous supported link rates(DPCD 0x010) in
customer's hybrid-GPU designs. As a result, during sleep
resuming and screen turns on from idle, link training
will be performed incorrectly and eDP will flicker or
black screen. These MUX chips need source to read DPCD
0x010 again during LKT so that it can restore supported
link rates of panel.

For driver side, supported link rate set is fetched when
link detection, no need to update but just read again
as WA.

[How]
Read DPCD 0x010 again during link training for eDP 1.4.

Signed-off-by: Dale Zhao <dale.zhao at amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu at amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo at amd.com>
---
 drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c b/drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c
index 9d1c5e1d06c0..54ac3de4e8e0 100644
--- a/drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c
+++ b/drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c
@@ -183,6 +183,16 @@ static void dpcd_set_link_settings(
 	if (link->dpcd_caps.dpcd_rev.raw >= DPCD_REV_14 &&
 			lt_settings->link_settings.use_link_rate_set == true) {
 		rate = 0;
+		/* WA for some MUX chips that will power down with eDP and lose supported
+		 * link rate set for eDP 1.4. Source reads DPCD 0x010 again to ensure
+		 * MUX chip gets link rate set back before link training.
+		 */
+		if (link->connector_signal == SIGNAL_TYPE_EDP) {
+			uint8_t supported_link_rates[16];
+
+			core_link_read_dpcd(link, DP_SUPPORTED_LINK_RATES,
+					supported_link_rates, sizeof(supported_link_rates));
+		}
 		core_link_write_dpcd(link, DP_LINK_BW_SET, &rate, 1);
 		core_link_write_dpcd(link, DP_LINK_RATE_SET,
 				&lt_settings->link_settings.link_rate_set, 1);
-- 
2.17.1