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

[Yongxing Mou <yongxing.mou@oss.qualcomm.com>]

On 4/12/2026 8:15 AM, Dmitry Baryshkov wrote:
> On Fri, Apr 10, 2026 at 05:34:04PM +0800, Yongxing Mou wrote:
>> 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);
> 
> For 100 ms you should be using msleep() instead. But where is that
> timeout coming from?
> 
Will switch to msleep(100).

The 100 ms is an empirical workaround carried over from downstream — 
some sinks don't exit MST immediately after writing DP_MSTM_CTRL = 0, 
and re-enabling MST too quickly broke topology probe. It's not a DP spec 
requirement, and other drivers (drm_dp_mst core, i915, amdgpu, nouveau) 
don't have an equivalent delay.

>> +	}
>> +
>> +	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).
> 
No, this follows the DP 1.4a MST spec. DP_MSTM_CTRL must be set before 
topology discovery. Topology discovery uses AUX sideband messages, which 
don’t require link training.
So we enable MST first, then discover topology, and only do link 
training and payload allocation later in atomic_enable().
>> +	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
>>
>