[PATCH 3/3] drm/nouveau/dp: retry link check once on HPD IRQ before disconnect

[Marek Czernohous <mczernohous@gmail.com>]

On NV50/G94-family chipsets with DisplayPort, transient link glitches
can trigger an HPD IRQ where the first nouveau_dp_link_check() fails
momentarily. The driver then falls through to connector status
re-detection, treating it as a disconnect event. This causes a brief
display blackout followed by a re-plug, which is disruptive under
both X11 and Wayland.

Add a single retry with a 100ms delay before giving up on the link.
DisplayPort link training typically completes within a few
milliseconds; 100ms is generous enough to cover worst-case
re-negotiation on older hardware without adding perceptible latency
for genuine unplug events.

The retry is bounded (exactly one attempt) and only applies to the
IRQ path — plug/unplug events are not affected.

Tested on NVAC (MCP79) with DisplayPort output over extended DPMS
on/off cycles. Without this patch, occasional display flickers were
observed after DPMS resume. With this patch, all DPMS transitions are
clean.

Signed-off-by: Marek Czernohous <marek@czernohous.de>
---
 drivers/gpu/drm/nouveau/nouveau_display.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/nouveau/nouveau_display.c b/drivers/gpu/drm/nouveau/nouveau_display.c
index d71dcfc6e..b6e40794b 100644
--- a/drivers/gpu/drm/nouveau/nouveau_display.c
+++ b/drivers/gpu/drm/nouveau/nouveau_display.c
@@ -465,6 +465,10 @@ nouveau_display_hpd_work(struct work_struct *work)
 		if (bits & NVIF_CONN_EVENT_V0_IRQ) {
 			if (nouveau_dp_link_check(nv_connector))
 				continue;
+			/* Retry once after 100ms for transient DP glitches (NV50/G94) */
+			msleep(100);
+			if (nouveau_dp_link_check(nv_connector))
+				continue;
 		}
 
 		connector->status = drm_helper_probe_detect(connector, NULL, false);
-- 
2.52.0