Re: [PATCH v4 29/39] drm/msm/dp: add an API to initialize MST on sink side
From: Yongxing Mou
Date: Tue Jun 16 2026 - 05:48:15 EST
On 6/16/2026 8:45 AM, Dmitry Baryshkov wrote:
On Mon, Jun 15, 2026 at 05:02:27PM +0800, Yongxing Mou wrote:Got it.
On 4/12/2026 8:15 AM, Dmitry Baryshkov wrote:
On Fri, Apr 10, 2026 at 05:34:04PM +0800, Yongxing Mou wrote:Will switch to msleep(100).
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?
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.
Comment.
This comes from the downstream implementation and was added due to issues observed with some dongles, which reportedly required a ~100 ms delay. However, it’s unclear whether such a long delay is actually necessary (e.g. if a shorter delay would be sufficient).It's not a DP spec
requirement, and other drivers (drm_dp_mst core, i915, amdgpu, nouveau)
don't have an equivalent delay.
Why?
Alternatively, we could follow nouveau and avoid adding the delay for now, and only introduce it if it turns out to be required.
No, this follows the DP 1.4a MST spec. DP_MSTM_CTRL must be set before+ }
+
+ 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).
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().
Ack, thanks.