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

[Dmitry Baryshkov]

On Fri, Apr 10, 2026 at 05:34:04PM +0800, Yongxing Mou wrote:
> From: Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx>
> 
> 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@xxxxxxxxxxx>
> Signed-off-by: Yongxing Mou <yongxing.mou@xxxxxxxxxxxxxxxx>
> ---
>  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);

For 100 ms you should be using msleep() instead. But where is that
timeout coming from?

> +	}
> +
> +	ret = drm_dp_dpcd_write_byte(dp->aux, DP_MSTM_CTRL,
> +				     DP_MST_EN | DP_UP_REQ_EN | DP_UPSTREAM_IS_SRC);

Isn't it too early to enable MST? (I don't remember this part of the
standard).

> +	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
> 

-- 
With best wishes
Dmitry