drm/ast: Implement polling for VGA and SIL164 connectors

Implement polling for VGA and SIL164 connectors. Set the flag
DRM_CONNECTOR_POLL_DISCONNECT for each to detect the removal of the
monitor cable. Implement struct drm_connector_helper_funcs.detect_ctx
for each type of connector by testing for EDID data.

The helper drm_connector_helper_detect_ctx() implements .detect_ctx()
on top of the connector's DDC channel. The function can be used by
other drivers as companion to drm_connector_helper_get_modes().

v6:
- change helper name to drm_connector_helper_detec_from_ddc()
  (Maxime, Sui)
v5:
- share implementation in drm_connector_helper_detect_ctx() (Maxime)
- test for DDC presence with drm_probe_ddc() (Maxime, Jani)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240325200855.21150-13-tzimmermann@suse.de

Author: tdz

drivers/gpu/drm/ast/ast_mode.c

@@ -1346,6 +1346,7 @@ static int ast_crtc_init(struct drm_device *dev)
 
 static const struct drm_connector_helper_funcs ast_vga_connector_helper_funcs = {
 	.get_modes = drm_connector_helper_get_modes,
+	.detect_ctx = drm_connector_helper_detect_from_ddc,
 };
 
 static const struct drm_connector_funcs ast_vga_connector_funcs = {
@@ -1379,7 +1380,7 @@ static int ast_vga_connector_init(struct drm_device *dev, struct drm_connector *
 	connector->interlace_allowed = 0;
 	connector->doublescan_allowed = 0;
 
-	connector->polled = DRM_CONNECTOR_POLL_CONNECT;
+	connector->polled = DRM_CONNECTOR_POLL_CONNECT | DRM_CONNECTOR_POLL_DISCONNECT;
 
 	return 0;
 }
@@ -1414,6 +1415,7 @@ static int ast_vga_output_init(struct ast_device *ast)
 
 static const struct drm_connector_helper_funcs ast_sil164_connector_helper_funcs = {
 	.get_modes = drm_connector_helper_get_modes,
+	.detect_ctx = drm_connector_helper_detect_from_ddc,
 };
 
 static const struct drm_connector_funcs ast_sil164_connector_funcs = {
@@ -1447,7 +1449,7 @@ static int ast_sil164_connector_init(struct drm_device *dev, struct drm_connecto
 	connector->interlace_allowed = 0;
 	connector->doublescan_allowed = 0;
 
-	connector->polled = DRM_CONNECTOR_POLL_CONNECT;
+	connector->polled = DRM_CONNECTOR_POLL_CONNECT | DRM_CONNECTOR_POLL_DISCONNECT;
 
 	return 0;
 }

drivers/gpu/drm/drm_probe_helper.c

@@ -1279,3 +1279,32 @@ int drm_connector_helper_tv_get_modes(struct drm_connector *connector)
 	return i;
 }
 EXPORT_SYMBOL(drm_connector_helper_tv_get_modes);
+
+/**
+ * drm_connector_helper_detect_from_ddc - Read EDID and detect connector status.
+ * @connector: The connector
+ * @ctx: Acquire context
+ * @force: Perform screen-destructive operations, if necessary
+ *
+ * Detects the connector status by reading the EDID using drm_probe_ddc(),
+ * which requires connector->ddc to be set. Returns connector_status_connected
+ * on success or connector_status_disconnected on failure.
+ *
+ * Returns:
+ * The connector status as defined by enum drm_connector_status.
+ */
+int drm_connector_helper_detect_from_ddc(struct drm_connector *connector,
+					 struct drm_modeset_acquire_ctx *ctx,
+					 bool force)
+{
+	struct i2c_adapter *ddc = connector->ddc;
+
+	if (!ddc)
+		return connector_status_unknown;
+
+	if (drm_probe_ddc(ddc))
+		return connector_status_connected;
+
+	return connector_status_disconnected;
+}
+EXPORT_SYMBOL(drm_connector_helper_detect_from_ddc);

include/drm/drm_probe_helper.h

@@ -37,4 +37,8 @@ int drm_connector_helper_get_modes_fixed(struct drm_connector *connector,
 int drm_connector_helper_get_modes(struct drm_connector *connector);
 int drm_connector_helper_tv_get_modes(struct drm_connector *connector);
 
+int drm_connector_helper_detect_from_ddc(struct drm_connector *connector,
+					 struct drm_modeset_acquire_ctx *ctx,
+					 bool force);
+
 #endif