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

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

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.
>>
>> 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.

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.
>> +	 */
>> +	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
>>
> 


_______________________________________________
linux-amlogic mailing list
linux-amlogic@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-amlogic