[PATCH v4 29/39] drm/msm/dp: add an API to initialize MST on sink side

Yongxing Mou
2026-04-10T09:36:22Z

From: Abhinav Kumar <quic_abhinavk@quicinc.com>

If the DP controller is capable of supporting multiple streams
then initialize the DP sink in MST mode by programming the DP_MSTM_CTRL
DPCD register to enable MST mode.

Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Yongxing Mou <yongxing.mou@oss.qualcomm.com>
---
drivers/gpu/drm/msm/dp/dp_display.c | 57 ++++++++++++++++++++++++++++++++-----
1 file changed, 50 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
index 8ae690ce2b9f..abf26951819a 100644
--- a/drivers/gpu/drm/msm/dp/dp_display.c
+++ b/drivers/gpu/drm/msm/dp/dp_display.c
@@ -14,6 +14,7 @@
#include <linux/string_choices.h>
#include <drm/display/drm_dp_aux_bus.h>
#include <drm/display/drm_hdmi_audio_helper.h>
+#include <drm/display/drm_dp_mst_helper.h>
#include <drm/drm_edid.h>

#include "msm_drv.h"
@@ -270,6 +271,40 @@ static int msm_dp_display_lttpr_init(struct msm_dp_display_private *dp, u8 *dpcd
return lttpr_count;
}

+static void msm_dp_display_mst_init(struct msm_dp_display_private *dp)
+{
+ const unsigned long clear_mstm_ctrl_timeout_us = 100000;
+ u8 old_mstm_ctrl;
+ struct msm_dp *msm_dp = &dp->msm_dp_display;
+ int ret;
+
+ /* clear sink MST state */
+ drm_dp_dpcd_read_byte(dp->aux, DP_MSTM_CTRL, &old_mstm_ctrl);
+
+ ret = drm_dp_dpcd_write_byte(dp->aux, DP_MSTM_CTRL, 0);
+ if (ret < 0) {
+ DRM_ERROR("failed to clear DP_MSTM_CTRL, ret=%d\n", ret);
+ return;
+ }
+
+ /* add extra delay if MST old state is on*/
+ if (old_mstm_ctrl) {
+ drm_dbg_dp(dp->drm_dev, "wait %luus to set DP_MSTM_CTRL set 0\n",
+ clear_mstm_ctrl_timeout_us);
+ usleep_range(clear_mstm_ctrl_timeout_us,
+ clear_mstm_ctrl_timeout_us + 1000);
+ }
+
+ ret = drm_dp_dpcd_write_byte(dp->aux, DP_MSTM_CTRL,
+ DP_MST_EN | DP_UP_REQ_EN | DP_UPSTREAM_IS_SRC);
+ if (ret < 0) {
+ DRM_ERROR("sink MST enablement failed\n");
+ return;
+ }
+
+ msm_dp->mst_active = true;
+}
+
static int msm_dp_display_process_hpd_high(struct msm_dp_display_private *dp)
{
struct drm_connector *connector = dp->msm_dp_display.connector;
@@ -288,14 +323,19 @@ static int msm_dp_display_process_hpd_high(struct msm_dp_display_private *dp)
if (rc)
goto end;

- drm_edid = drm_edid_read_ddc(connector, &dp->aux->ddc);
- drm_edid_connector_update(connector, drm_edid);
+ if (!(dp->max_stream > 1) || !drm_dp_read_mst_cap(dp->aux, dp->panel->dpcd)) {
+ drm_edid = drm_edid_read_ddc(connector, &dp->aux->ddc);
+ drm_edid_connector_update(connector, drm_edid);

- if (!drm_edid) {
- DRM_ERROR("panel edid read failed\n");
- /* check edid read fail is due to unplug */
- if (!msm_dp_aux_is_link_connected(dp->aux))
- return -ETIMEDOUT;
+ if (!drm_edid) {
+ DRM_ERROR("panel edid read failed\n");
+ /* check edid read fail is due to unplug */
+ if (!msm_dp_aux_is_link_connected(dp->aux))
+ return -ETIMEDOUT;
+ }
+
+ if (rc)
+ goto end;
}

msm_dp_link_process_request(dp->link);
@@ -317,6 +357,9 @@ static int msm_dp_display_process_hpd_high(struct msm_dp_display_private *dp)
*/
msm_dp_link_psm_config(dp->link, &dp->panel->link_info, false);

+ if (dp->max_stream > 1 && drm_dp_read_mst_cap(dp->aux, dp->panel->dpcd))
+ msm_dp_display_mst_init(dp);
+
msm_dp_link_reset_phy_params_vx_px(dp->link);

end:

--
2.43.0