drm/exynos: dsi: Fix bridge chain handling

Commit 05193dc38197 ("drm/bridge: Make the bridge chain a double-linked
list") patched the bridge chain logic to use a double-linked list instead
of a single-linked list. This change induced changes to the Exynos driver
which was manually resetting the encoder->bridge element to NULL to
control the enable/disable sequence of the bridge chain. During this
conversion, 2 bugs were introduced:

1/ list_splice() was used to move chain elements to our own internal
chain, but list_splice() does not reset the source list to an empty
state, leading to unexpected bridge hook calls when
drm_bridge_chain_xxx() helpers were called by the core. Replacing
the list_splice() call by list_splice_init() fixes this problem.

2/ drm_bridge_chain_xxx() helpers operate on the
bridge->encoder->bridge_chain list, which is now empty. When the
helper uses list_for_each_entry_reverse() we end up with no operation
done which is not what we want. But that's even worse when the helper
uses list_for_each_entry_from(), because in that case we end up in
an infinite loop searching for the list head element which is no
longer encoder->bridge_chain but exynos_dsi->bridge_chain. To address
that problem we stop using the bridge chain helpers and call the
hooks directly.

Reported-by: Marek Szyprowski <[e-mail]>
Fixes: 05193dc38197 ("drm/bridge: Make the bridge chain a double-linked list")
Signed-off-by: Boris Brezillon <[e-mail]>
Tested-by: Marek Szyprowski <[e-mail]>
Reviewed-by: Andrzej Hajda <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/exynos/exynos_drm_dsi.c

···
 static void exynos_dsi_enable(struct drm_encoder *encoder)
 {
 	struct exynos_dsi *dsi = encoder_to_dsi(encoder);
+	struct drm_bridge *iter;
 	int ret;
 
 	if (dsi->state & DSIM_STATE_ENABLED)
···
 		if (ret < 0)
 			goto err_put_sync;
 	} else {
-		drm_bridge_chain_pre_enable(dsi->out_bridge);
+		list_for_each_entry_reverse(iter, &dsi->bridge_chain,
+					    chain_node) {
+			if (iter->funcs->pre_enable)
+				iter->funcs->pre_enable(iter);
+		}
 	}
 
 	exynos_dsi_set_display_mode(dsi);
···
 		if (ret < 0)
 			goto err_display_disable;
 	} else {
-		drm_bridge_chain_enable(dsi->out_bridge);
+		list_for_each_entry(iter, &dsi->bridge_chain, chain_node) {
+			if (iter->funcs->enable)
+				iter->funcs->enable(iter);
+		}
 	}
 
 	dsi->state |= DSIM_STATE_VIDOUT_AVAILABLE;
···
 static void exynos_dsi_disable(struct drm_encoder *encoder)
 {
 	struct exynos_dsi *dsi = encoder_to_dsi(encoder);
+	struct drm_bridge *iter;
 
 	if (!(dsi->state & DSIM_STATE_ENABLED))
 		return;
···
 	dsi->state &= ~DSIM_STATE_VIDOUT_AVAILABLE;
 
 	drm_panel_disable(dsi->panel);
-	drm_bridge_chain_disable(dsi->out_bridge);
+
+	list_for_each_entry_reverse(iter, &dsi->bridge_chain, chain_node) {
+		if (iter->funcs->disable)
+			iter->funcs->disable(iter);
+	}
+
 	exynos_dsi_set_display_enable(dsi, false);
 	drm_panel_unprepare(dsi->panel);
-	drm_bridge_chain_post_disable(dsi->out_bridge);
+
+	list_for_each_entry(iter, &dsi->bridge_chain, chain_node) {
+		if (iter->funcs->post_disable)
+			iter->funcs->post_disable(iter);
+	}
+
 	dsi->state &= ~DSIM_STATE_ENABLED;
 	pm_runtime_put_sync(dsi->dev);
 }
···
 	if (out_bridge) {
 		drm_bridge_attach(encoder, out_bridge, NULL);
 		dsi->out_bridge = out_bridge;
-		list_splice(&encoder->bridge_chain, &dsi->bridge_chain);
+		list_splice_init(&encoder->bridge_chain, &dsi->bridge_chain);
 	} else {
 		int ret = exynos_dsi_create_connector(encoder);