Re: [PATCH v2 29/38] drm/msm/dp: add connector abstraction for DP MST
From: Yongxing Mou
Date: Mon Jun 16 2025 - 10:10:41 EST
On 2025/6/11 22:31, Dmitry Baryshkov wrote:
On Wed, Jun 11, 2025 at 08:06:28PM +0800, Yongxing Mou wrote:
On 2025/6/9 23:44, Dmitry Baryshkov wrote:
On Mon, Jun 09, 2025 at 08:21:48PM +0800, Yongxing Mou wrote:
From: Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx>
Add connector abstraction for the DP MST. Each MST encoder
is connected through a DRM bridge to a MST connector and each
MST connector has a DP panel abstraction attached to it.
Signed-off-by: Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx>
Signed-off-by: Yongxing Mou <quic_yongmou@xxxxxxxxxxx>
---
drivers/gpu/drm/msm/dp/dp_mst_drm.c | 515 ++++++++++++++++++++++++++++++++++++
drivers/gpu/drm/msm/dp/dp_mst_drm.h | 3 +
2 files changed, 518 insertions(+)
It generally feels liks 80% of this patch is a generic code. Please
extract generic DP MST connector and push it under drm/display. Other DP
MST drivers should be able to use it.
diff --git a/drivers/gpu/drm/msm/dp/dp_mst_drm.c b/drivers/gpu/drm/msm/dp/dp_mst_drm.c
index a3ea34ae63511db0ac920cbeebe30c4e2320b8c4..489fa46aa518ff1cc5f4769b2153fc5153c4cb41 100644
--- a/drivers/gpu/drm/msm/dp/dp_mst_drm.c
+++ b/drivers/gpu/drm/msm/dp/dp_mst_drm.c
@@ -25,8 +25,12 @@
* OF THIS SOFTWARE.
*/
+#include <drm/drm_edid.h>
+#include <drm/drm_managed.h>
#include "dp_mst_drm.h"
+#define MAX_DPCD_TRANSACTION_BYTES 16
+
static struct drm_private_state *msm_dp_mst_duplicate_bridge_state(struct drm_private_obj *obj)
{
struct msm_dp_mst_bridge_state *state;
@@ -79,6 +83,61 @@ static int msm_dp_mst_find_vcpi_slots(struct drm_dp_mst_topology_mgr *mgr, int p
return num_slots;
}
+static int msm_dp_mst_get_mst_pbn_div(struct msm_dp_panel *msm_dp_panel)
+{
+ struct msm_dp_link_info *link_info;
+
+ link_info = &msm_dp_panel->link_info;
+
+ return link_info->rate * link_info->num_lanes / 54000;
+}
+
+static int msm_dp_mst_compute_config(struct drm_atomic_state *state,
+ struct msm_dp_mst *mst, struct drm_connector *connector,
+ struct drm_display_mode *mode)
+{
+ int slots = 0, pbn;
+ struct msm_dp_mst_connector *mst_conn = to_msm_dp_mst_connector(connector);
+ int rc = 0;
+ struct drm_dp_mst_topology_state *mst_state;
+ int pbn_div;
+ struct msm_dp *dp_display = mst->msm_dp;
+ u32 bpp;
+
+ bpp = connector->display_info.bpc * 3;
+
+ pbn = drm_dp_calc_pbn_mode(mode->clock, bpp << 4);
Is this going to change if DSC is in place? Will it bring fractional BPP
here?
Actually, in this patch series, MST not support DSC. So we just don't
consider this scenario.
But you still can answer the question.
[...]
1.Emm, for my current understanding, if DSC is enabled, the BPP should
change and recaculated.
Will it bring fractional BPP here?
>>>I'm not entirely sure about this answer. I checked how other drivers
call this function, and they all use bpp << 4, so can we assume that
this way of calling it is valid?
+
+ return msm_dp_display_mode_valid(dp_display, &dp_display->connector->display_info, mode);
+}
+
+static struct drm_encoder *
+msm_dp_mst_atomic_best_encoder(struct drm_connector *connector, struct drm_atomic_state *state)
Do we need this callback? Don't we have a fixed relationship between
connectors and encoders?
This was left unanswered.
Sorry, I didn't mean to skip any questions — I just planned to reply a
bit later. Apologies for the confusion.
For this question, yes , we don't have the fixed relationship between
them. Under the current codes, the Connector selects the available
encoder and bridge in order from index 0 to 4 (up to max_streams) when
the connector's status changes to 'connected'.
+{
+ struct msm_dp_mst_connector *mst_conn = to_msm_dp_mst_connector(connector);
+ struct msm_dp *dp_display = mst_conn->msm_dp;
+ struct msm_dp_mst *mst = dp_display->msm_dp_mst;
+ struct drm_encoder *enc = NULL;
+ struct msm_dp_mst_bridge_state *bridge_state;
+ u32 i;
+ struct drm_connector_state *conn_state = drm_atomic_get_new_connector_state(state,
+ connector);
+
[...]
+ if (drm_atomic_crtc_needs_modeset(crtc_state)) {
+ if (WARN_ON(!old_conn_state->best_encoder)) {
+ rc = -EINVAL;
+ goto end;
+ }
+
+ drm_bridge = drm_bridge_chain_get_first_bridge(old_conn_state->best_encoder);
This really looks like this should be a bridge's callback.
And this one
Emm, the bridge does not implement atomic_check(). All MST-related
checks (such as drm_dp_atomic_release_time_slots,
drm_dp_mst_atomic_check, or others) are performed in the connector's
atomic_check function. I believe this is because both num_slots and pbn
are stored in the bridge, and we call this to get the drm_bridge..
+ if (WARN_ON(!drm_bridge)) {
+ rc = -EINVAL;
+ goto end;
+ }
+ bridge = to_msm_dp_mst_bridge(drm_bridge);
+
+ bridge_state = msm_dp_mst_br_priv_state(state, bridge);
+ if (IS_ERR(bridge_state)) {
+ rc = PTR_ERR(bridge_state);
+ goto end;
+ }
+
+ if (WARN_ON(bridge_state->connector != connector)) {
+ rc = -EINVAL;
+ goto end;
+ }
+
+ slots = bridge_state->num_slots;
+ if (slots > 0) {
+ rc = drm_dp_atomic_release_time_slots(state,
+ &mst->mst_mgr,
+ mst_conn->mst_port);
+ if (rc) {
+ DRM_ERROR("failed releasing %d vcpi slots %d\n", slots, rc);
+ goto end;
+ }
+ vcpi_released = true;
+ }
+
+ if (!new_conn_state->crtc) {
+ /* for cases where crtc is not disabled the slots are not
+ * freed by drm_dp_atomic_release_time_slots. this results
+ * in subsequent atomic_check failing since internal slots
+ * were freed but not the dp mst mgr's
+ */
+ bridge_state->num_slots = 0;
+ bridge_state->connector = NULL;
+ bridge_state->msm_dp_panel = NULL;
+
+ drm_dbg_dp(dp_display->drm_dev, "clear best encoder: %d\n", bridge->id);
+ }
+ }
This looks like there are several functions fused together. Please
unfuse those into small and neat code blocks.
And this :-D
Got it.. this code only do one thing, check and try to release
time_slots.. we can try to package it into small functions..
+
+mode_set:
+ if (!new_conn_state->crtc)
+ goto end;
+
+ crtc_state = drm_atomic_get_new_crtc_state(state, new_conn_state->crtc);
+
+ if (drm_atomic_crtc_needs_modeset(crtc_state) && crtc_state->active) {
Use of crtc_state->active doesn't look correct.
...
Sorry, I'm still not quite sure where the issue is. Could you please
help point it out? Thanks~~
+ if (WARN_ON(!new_conn_state->best_encoder)) {
+ rc = -EINVAL;
+ goto end;
+ }
+
+ drm_bridge = drm_bridge_chain_get_first_bridge(new_conn_state->best_encoder);
+ if (WARN_ON(!drm_bridge)) {
+ rc = -EINVAL;
+ goto end;
+ }
+ bridge = to_msm_dp_mst_bridge(drm_bridge);
+
+ bridge_state = msm_dp_mst_br_priv_state(state, bridge);
+ if (IS_ERR(bridge_state)) {
+ rc = PTR_ERR(bridge_state);
+ goto end;
+ }
+
+ if (WARN_ON(bridge_state->connector != connector)) {
+ rc = -EINVAL;
+ goto end;
+ }
Can all of this actually happen?
...
Actually not, I haven't encountered it yet. I'm not sure how to trigger
it, but it might occur under race conditions? Or we just remove it
untill some case it really happen..
+
+ /*
+ * check if vcpi slots are trying to get allocated in same phase
+ * as deallocation. If so, go to end to avoid allocation.
+ */
+ if (vcpi_released) {
+ drm_dbg_dp(dp_display->drm_dev,
+ "skipping allocation since vcpi was released in the same state\n");
+ goto end;
+ }
+
+ if (WARN_ON(bridge_state->num_slots)) {
+ rc = -EINVAL;
+ goto end;
+ }
+
+ slots = msm_dp_mst_compute_config(state, mst, connector, &crtc_state->mode);
+ if (slots < 0) {
+ rc = slots;
+ goto end;
+ }
+
+ bridge_state->num_slots = slots;
+ }
+
+end:
+ drm_dbg_dp(dp_display->drm_dev, "mst connector:%d atomic check ret %d\n",
+ connector->base.id, rc);
+ return rc;
+}
+
+static void dp_mst_connector_destroy(struct drm_connector *connector)
+{
+ struct msm_dp_mst_connector *mst_conn = to_msm_dp_mst_connector(connector);
+
+ drm_connector_cleanup(connector);
+ drm_dp_mst_put_port_malloc(mst_conn->mst_port);
+}
+
+/* DRM MST callbacks */
+static const struct drm_connector_helper_funcs msm_dp_drm_mst_connector_helper_funcs = {
+ .get_modes = msm_dp_mst_connector_get_modes,
+ .detect_ctx = msm_dp_mst_connector_detect,
+ .mode_valid = msm_dp_mst_connector_mode_valid,
+ .atomic_best_encoder = msm_dp_mst_atomic_best_encoder,
+ .atomic_check = msm_dp_mst_connector_atomic_check,
+};
+
+static const struct drm_connector_funcs msm_dp_drm_mst_connector_funcs = {
+ .reset = drm_atomic_helper_connector_reset,
+ .destroy = dp_mst_connector_destroy,
+ .fill_modes = drm_helper_probe_single_connector_modes,
+ .atomic_duplicate_state = drm_atomic_helper_connector_duplicate_state,
+ .atomic_destroy_state = drm_atomic_helper_connector_destroy_state,
+};
+
+static struct drm_connector *
+msm_dp_mst_add_connector(struct drm_dp_mst_topology_mgr *mgr,
+ struct drm_dp_mst_port *port, const char *pathprop)
+{
+ struct msm_dp_mst *dp_mst;
+ struct drm_device *dev;
+ struct msm_dp *dp_display;
+ struct msm_dp_mst_connector *mst_connector;
+ struct drm_connector *connector;
+ int rc, i;
+
+ dp_mst = container_of(mgr, struct msm_dp_mst, mst_mgr);
+
+ dp_display = dp_mst->msm_dp;
+ dev = dp_display->drm_dev;
+
+ mst_connector = devm_kzalloc(dev->dev, sizeof(*mst_connector), GFP_KERNEL);
This shows that somebody doesn't understand the reason for drmm and the
difference between devm and drmm and the lifetime of the objects. Do you
see two issues in this line?
Let me help you. Please use normal (non-managed) memory here. It is the
only correct way to allocate memory for MST connectors.
Thanks for point it.. it will lead to mem leak.. so we need to use
kzalloc()...
- Did you understand why devm is unsuitable here?
- Why drmm is also unsutable?
- What is the implication of using kzalloc() here?
For my understanding, memory allocated with devm_kzalloc is released
when the device is removed, while memory allocated with drmm_kzalloc is
released when the DRM device is unregistered. I believe this is because
the allocation and release of connectors happen during hotplug events,
which have a different lifecycle from other devices. If we use
kzalloc(), we would need to manually free the memory.
+
+ drm_modeset_lock_all(dev);
+
+ rc = drm_connector_dynamic_init(dev, &mst_connector->connector,
+ &msm_dp_drm_mst_connector_funcs,
+ DRM_MODE_CONNECTOR_DisplayPort, NULL);
+ if (rc) {
+ drm_modeset_unlock_all(dev);
+ return NULL;
+ }
+
+ mst_connector->dp_panel = msm_dp_display_get_panel(dp_display);
+ if (!mst_connector->dp_panel) {
+ DRM_ERROR("failed to get dp_panel for connector\n");
+ drm_modeset_unlock_all(dev);
+ return NULL;
+ }
+
+ mst_connector->dp_panel->connector = &mst_connector->connector;
+ mst_connector->msm_dp = dp_display;
+ connector = &mst_connector->connector;
+ drm_connector_helper_add(&mst_connector->connector, &msm_dp_drm_mst_connector_helper_funcs);
+
+ if (connector->funcs->reset)
+ connector->funcs->reset(connector);
+
+ /* add all encoders as possible encoders */
+ for (i = 0; i < dp_mst->max_streams; i++) {
+ rc = drm_connector_attach_encoder(&mst_connector->connector,
+ dp_mst->mst_bridge[i].encoder);
+ if (rc) {
+ DRM_ERROR("failed to attach encoder to connector, %d\n", rc);
+ drm_modeset_unlock_all(dev);
+ return NULL;
+ }
+ }
+
+ mst_connector->mst_port = port;
+ drm_dp_mst_get_port_malloc(mst_connector->mst_port);
+
+ drm_object_attach_property(&mst_connector->connector.base,
+ dev->mode_config.path_property, 0);
+ drm_object_attach_property(&mst_connector->connector.base,
+ dev->mode_config.tile_property, 0);
subconnector? Or do we report the subconnector only for the main DP
port?
...
Sorry, I'm not quite sure what 'subconnector' means in this context...
Could you please help explain it a bit more? From what I’ve seen in
other drivers, these two properties are registered for each MST connector.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/i915/display/intel_dp_mst.c?h=v6.16-rc2#n1618
+
+ drm_modeset_unlock_all(dev);
+