[PATCH v3] drm/probe-helper: Honour connector->force in drm_helper_probe_detect()

[Paul Menzel <pmenzel@molgen.mpg.de>]

With `video=DP-2:d drm.debug=0x04` the connectors are correctly forced off
at init time:

    [9.742468] [drm] forcing DP-2 connector off

but intel_dp_detect() is still called immediately after:

    [9.908738] [drm:intel_dp_detect] [CONNECTOR:130:DP-2]
    [9.912982] [drm:intel_hotplug_detect_connector] [CONNECTOR:130:DP-2] status updated from unknown to unknown (epoch counter 0->1)

drm_helper_probe_single_connector_modes() already short-circuits when
connector->force is set, returning the forced status without calling any
detect callbacks.  drm_helper_probe_detect() however has no such guard, so
callers that go through it directly – such as i915’s
intel_hotplug_detect_connector() – still run the full hardware probe (AUX
DPCD read, GMBus DDC transaction, ...) even when the user has forced a
connector off via the `video=` command-line parameter.

Add the same connector->force check for forced off connectors at the top of
drm_helper_probe_detect(), so that the hardware probe is skipped regardless
of the entry point.  This makes video=<connector>:d effective at boot time,
not just for userspace GETCONNECTOR ioctls.

DRM_FORCE_ON and DRM_FORCE_ON_DIGITAL are intentionally not short-
circuited here: for those cases the detect callback must still run to
initialise sink capabilities such as DPCD link rate and lane count.

Tested on a Dell XPS 13 9360 (Intel Kaby Lake-U GT2 [HD Graphics 620]
[8086:5916]). With `video=DP-2:d drm.debug=0x04, Linux logs:

    [   11.701243] [drm] forcing DP-2 connector off
    [...]
    [   12.102009] i915 0000:00:02.0: [drm:drm_helper_probe_detect [drm_kms_helper]] [CONNECTOR:130:DP-2] forced off, skipping detect

v2: Do not return early for DRM_FORCE_ON and DRM_FORCE_ON_DIGITAL (Jani,
gemini/gemini-3.1-pro-preview)

v3: Correct if condition

Assisted-by: Claude Sonnet 4.6
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
---
 drivers/gpu/drm/drm_probe_helper.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c
index d4dc8cb45bce..1e15cb42d50b 100644
--- a/drivers/gpu/drm/drm_probe_helper.c
+++ b/drivers/gpu/drm/drm_probe_helper.c
@@ -401,6 +401,12 @@ drm_helper_probe_detect(struct drm_connector *connector,
 	struct drm_device *dev = connector->dev;
 	int ret;
 
+	if (connector->force == DRM_FORCE_OFF) {
+		drm_dbg_kms(dev, "[CONNECTOR:%d:%s] forced off, skipping detect\n",
+			    connector->base.id, connector->name);
+		return connector_status_disconnected;
+	}
+
 	if (!ctx)
 		return drm_helper_probe_detect_ctx(connector, force);
 
-- 
2.53.0