Re: [PATCH RESEND v5 15/25] drm/msm/dp: add an API to initialize MST on sink side

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

On 7/13/2026 5:35 AM, Dmitry Baryshkov wrote:
> On Mon, Jun 29, 2026 at 10:14:36PM +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 | 48 +++++++++++++++++++++++++++++++------
>>   1 file changed, 41 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
>> index 5786e598a406..c3be656f10ee 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 +271,31 @@ 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)
>> +{
>> +	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;
>> +	}
> 
> Is it not enough to write the new value? Please add a small comment,
> why (reference DP standard in the commit message, please).
> 
Got it.
>> +
>> +	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 +314,19 @@ 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;
> 
> This also is going to break DP. If you are using dp->max_streams as a
> determining factor, rearrange commits so that max_streams are parseed
> and set only after the rest of the commits are in place (and the driver
> is ready for DP MST).
> 
Sorry, that was an oversight. Those two lines shouldn't be there. The 
patch wasn't clean enough, and I'll remove them in the next revision.
>>   	}
>>   
>>   	msm_dp_link_process_request(dp->link);
>> @@ -317,6 +348,9 @@ 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
>>
>