Re: [PATCH 2/5] drm/bridge_connector: preserve connector status for IRQ-only HPD events

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

On 8/18/2026 11:18 AM, Dmitry Baryshkov wrote:
> On Mon, Aug 17, 2026 at 04:02:31PM +0800, Yongxing Mou wrote:
>>
>>
>> On 7/12/2026 6:33 PM, Dmitry Baryshkov wrote:
>>> On Mon, Jun 29, 2026 at 10:48:04PM +0800, Yongxing Mou wrote:
>>>> 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.
>>>
>> Ohh yes, here need to rework.
>>>>
>>>> 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)?
>>>
>> i915, amdgpu, and nouveau don't go through the drm_bridge_hpd_notify()
>> 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?
> 
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.
>>>> Signed-off-by: Yongxing Mou <yongxing.mou@oss.qualcomm.com>
>>>> ---
>>>>    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.
>>>
>> You are right, in the SST case the bridge_connector status does follow
>> 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.
> 
Got it.
> Is it still an issue if the bridge's hpd_notify() callback determines
> 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
yes, 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.
> HPD events in the drm_sysfs_connector_hotplug_event(), making sure that
> we don't send duplicate disconnected events?
> 
I think driver should confirm it really send a hotplug when we call 
drm_sysfs_connector_hotplug_event()
> 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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