Re: [PATCH 2/5] drm/bridge_connector: preserve connector status for IRQ-only HPD events
From: Yongxing Mou
Date: Fri Aug 21 2026 - 03:20:10 EST
On 8/18/2026 11:18 AM, Dmitry Baryshkov wrote:
On Mon, Aug 17, 2026 at 04:02:31PM +0800, Yongxing Mou wrote:So my understanding is that,IRQ_HPD is primarily used to trigger internal status revalidation. A hotplug event is only generated if that processing concludes that there has been an actual connector or topology state change.
On 7/12/2026 6:33 PM, Dmitry Baryshkov wrote:
On Mon, Jun 29, 2026 at 10:48:04PM +0800, Yongxing Mou wrote:Ohh yes, here need to rework.
The bridge connector HPD handling path currently updates
connector->status for every hpd_notify() invocation.
This does not work well for IRQ-only notifications where the event being
reported is carried by extra_status and no connector status transition is
associated with it.
One example is DP MST. HPD IRQs are propagated through
drm_bridge_hpd_notify_*() so that bridge drivers can process the
notification. During MST operation, however, the SST connector attached
to the bridge connector is intentionally kept disconnected while the MST
topology manager handles all connector creation, removal and hotplug
processing.
Updating connector->status for an IRQ-only MST notification may cause
the SST connector state to oscillate between connected and disconnected
depending on the notification path. These artificial state transitions
can later be detected by the polling logic and result in unnecessary
hotplug events being generated. Userspace then re-probes connector
status, potentially triggering the same sequence again.
Then the API might need to be adjusted.
Remember, we have two usecases, which we must be able to interpret
correctly:
- The driver gets separate HPD and IRQ_HPD events.
- The driver gets HPD and IRQ_HPD at the same time.
i915, amdgpu, and nouveau don't go through the drm_bridge_hpd_notify()
Treat notifications with status == connector_status_unknown and a valid
extra_status as IRQ-only events. Forward the notification to bridge
drivers without modifying connector->status.
This keeps IRQ delivery working while leaving connector state management
to the component that actually owns it, such as the DP MST topology
framework.
How is it handled by other drivers (i915, amd, nouveau)?
bridge chain -- their DP controllers are integrated into the SoC, and
HPD interrupts are handled directly in their own encoder code.
MSM DP is different in that HPD comes from Type-C / pmic_glink altmode
via aux-hpd-bridge, so it has to go through the bridge chain, which
means this semantic needs to be extended to the bridge API.
Still, when do those drivers send the HPD event in case of IRQ_HPD? Or
is it that in their case IRQ_HPD just triggers inner logic to reread the
status registers and then the driver sends the HPD if there is any
actual change?
Got it.You are right, in the SST case the bridge_connector status does followSigned-off-by: Yongxing Mou <yongxing.mou@xxxxxxxxxxxxxxxx>
---
drivers/gpu/drm/display/drm_bridge_connector.c | 12 ++++++++++++
1 file changed, 12 insertions(+)
diff --git a/drivers/gpu/drm/display/drm_bridge_connector.c b/drivers/gpu/drm/display/drm_bridge_connector.c
index 5edca47a025f..7334d6677604 100644
--- a/drivers/gpu/drm/display/drm_bridge_connector.c
+++ b/drivers/gpu/drm/display/drm_bridge_connector.c
@@ -163,6 +163,18 @@ static void drm_bridge_connector_handle_hpd(struct drm_bridge_connector *drm_bri
struct drm_connector *connector = &drm_bridge_connector->base;
struct drm_device *dev = connector->dev;
+ /*
+ * IRQ-only notification: extra_status carries the event but
+ * status is unknown — do not overwrite connector->status.
But it's not unknown at this point. The connector status is reported
following the HPD status.
HPD / link status -- because the bridge_connector itself represents
that SST connector, so its status naturally is the link status.
The MST case has a key difference though: the SST connector must be
explicitly marked disconnected (to prevent the DRM framework from
enabling it), consistent with what i915, amdgpu and nouveau do. In
other words, once MST is enabled, the SST connector that the
bridge_connector represents no longer equates to the link status --
the real link is managed by the MST topology, and the SST connector
is just a placeholder at that point.
This is fine.
The issue is that IRQ_HPD still travels through the bridge_connector
chain and takes the old "update the SST connector's status -> emit
hotplug" path. Under MST that runs into two constraints -- and this
is exactly what this series is trying to address:
1. The SST connector represented by bridge_connector no longer stands
for the link, so its status must not be overwritten by IRQ_HPD
events.
2. IRQ_HPD needs a clean path that does not trigger a hotplug on the
bridge_connector -- the MST framework already manages hotplugs
independently.
Using connector_status_unknown as a sentinel here does feel a bit odd;
let me think about whether there is a cleaner approach.
The status here must represent the status reported by the corresponding
layer: be it DP ALtMode, the dp-connector driver handling the HPD GPIO
or the DP driver itself handling the HPD pin via the state machine.
Is it still an issue if the bridge's hpd_notify() callback determinesyes, it still an issue if each irq . The SST connector should not be reported as connected, even momentarily. When the connector status is changed from connected back to disconnected, this state transition is treated as a hotplug event and trigger hotplug, introducing unnecessary userspace polling and re-probing.
that we should not be reporting the event and drops connector->status to
'disconnected' again? Why is it an issue? Should we instead filter the
HPD events in the drm_sysfs_connector_hotplug_event(), making sure thatI think driver should confirm it really send a hotplug when we call drm_sysfs_connector_hotplug_event()
we don't send duplicate disconnected events?
What is the expected behaviour of drm_client's?- Genuine connect/disconnect: the client is woken up, re-probes and follows the new state.
- IRQ_HPD with no actual change: the client must not be woken up at all, otherwise every short pulse triggers a full re-probe of all connectors.
- IRQ_HPD where the driver does conclude that something changed: the driver escalates it explicitly into a real status notification.
- The SST connector while MST is active: permanently 'disconnected', and the client must never select it. The real outputs are the MST port connectors created by the topology manager, which get their own hotplug events from it.
+ */
+ if (status == connector_status_unknown &&
+ extra_status != DRM_CONNECTOR_NO_EXTRA_STATUS) {
+ drm_bridge_connector_hpd_notify(connector,
+ connector->status,
+ extra_status, NULL);
+ return;
+ }
+
mutex_lock(&dev->mode_config.mutex);
connector->status = status;
mutex_unlock(&dev->mode_config.mutex);
--
2.43.0
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