Re: [PATCH 1/5] drm/bridge: allow hpd_notify() to suppress connector hotplug events
From: Yongxing Mou
Date: Fri Aug 21 2026 - 03:21:04 EST
On 8/18/2026 11:01 AM, Dmitry Baryshkov wrote:
On Mon, Aug 17, 2026 at 04:01:41PM +0800, Yongxing Mou wrote:The DRM API should simply pass both long HPD IRQs and short HPD IRQs to the driver as they are, and let the driver decide how to handle them. Based on my review of the implementations from all three vendors, when long and short HPD IRQs occur simultaneously, the long HPD IRQ is always handled first, followed by the short HPD IRQ.
On 7/12/2026 6:21 PM, Dmitry Baryshkov wrote:
On Mon, Jun 29, 2026 at 10:48:03PM +0800, Yongxing Mou wrote:Hi, thanks for reviwing those patches.
The bridge connector framework currently invokes all bridge
hpd_notify() callbacks and unconditionally emits a connector hotplug
event afterwards.
However, not every HPD notification requires a userspace hotplug event.
In particular, DP MST bridges may use hpd_notify() to propagate HPD and
IRQ notifications through the bridge chain while the actual hotplug
handling is performed by the DRM DP MST core. Connector creation,
removal and userspace hotplug events are already managed by the MST
topology framework.
Allow hpd_notify() implementations to suppress the bridge connector
hotplug event by introducing a bool *send_hotplug parameter. Drivers
can clear this flag when HPD processing should not result in a
connector hotplug notification.
Why? Worst case the kernel receives another hotplug notification which
gets ignored by the driver.
Let me try to explain the motivation.
Semantically, IRQ_HPD is just an IRQ notification, not a connection state
transition, and shouldn't be turned into a userspace hotplug in the first
place. However, drm_bridge_connector_handle_hpd() currently calls
drm_kms_helper_connector_hotplug_event() unconditionally after processing
the event, so every IRQ_HPD ends up reported as a hotplug.
What if the IRQ_HPD is delivered together with the first HPD event (for
example because of the TCPM processing those events)? See the mechanism
in the displayport.c AltMode driver.
I have another thought regarding the current DRM API. Could we have the DRM layer pass only the HPD event information (long IRQ and/or short IRQ) instead of connector status, and leave all handling decisions to the driver? The DRM core would simply report LONG_HPD | SHORT_HPD, and each driver could decide how to process the event. This seems like a pattern that could be shared across different drivers.
This is just my current understanding. Please let me know if I've missed anything or got something wrong. Thanks.
The driver revalidates the actual connector state, and a hotplug event is triggered only when an actual connector state change or a link status change is detected.Second, MST IRQ_HPD is level-sticky -- as long as the ACK has not been
cleared, the IRQ keeps firing repeatedly, and MST bring-up (link training
/ MST enable handshake) itself generates a burst of IRQ_HPDs. So this is
not about "one extra hotplug", but about a burst of them within a short
window.
Ok, if it is level-sticky, it should be handled as such.
Every one of those hotplugs is delivered to userspace via udev and
prompts the compositor to re-probe the connector. In the window before
mst_active is set, that re-probe walks back into msm_dp_bridge_detect()
and performs aux/DPCD accesses, racing with the MST enable flow.
If there is a race, the path needs to have a lock, preventing concurrent
access. Otherwise, you are just shortening the window instead of solving
the problem.
The amplification also isn't limited to a single connector: on Hamoa
there are 4 connectors (3x DP + eDP), and we observe that a hotplug on
any one connector causes the compositor to re-query all 4. So this burst
Please fix the compositor, it should not need to query all 4 connectors
if the HPD event came from the single one.
of spurious IRQ_HPDs during MST enable ends up amplified across the
whole card.
How do i915, amdgpu and nouveau respond to IRQ_HPD? When do they send
the HPD event to the userspace?
You are right, I will reorganize the patches in next patchset.A NULL pointer indicates that hotplug suppression is not supported by
the caller, such as the connector detect polling path.
And nothing in this patch makes any use of it. I'd say, it's
questionable addition. Let me check other patches...
Signed-off-by: Yongxing Mou <yongxing.mou@xxxxxxxxxxxxxxxx>
---
drivers/gpu/drm/bridge/lontium-lt9611uxc.c | 3 ++-
drivers/gpu/drm/display/drm_bridge_connector.c | 15 +++++++++------
drivers/gpu/drm/meson/meson_encoder_hdmi.c | 3 ++-
drivers/gpu/drm/msm/dp/dp_display.c | 3 ++-
drivers/gpu/drm/msm/dp/dp_drm.h | 3 ++-
drivers/gpu/drm/omapdrm/dss/hdmi4.c | 3 ++-
include/drm/drm_bridge.h | 3 ++-
7 files changed, 21 insertions(+), 12 deletions(-)