drm/vc4: 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 VC4 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
those list_splice() calls by list_splice_init() ones 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 vc4_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]>
Cc: Eric Anholt <[e-mail]>
Fixes: 05193dc38197 ("drm/bridge: Make the bridge chain a double-linked list")
Signed-off-by: Boris Brezillon <[e-mail]>
Reviewed-by: Andrzej Hajda <[e-mail]>
Acked-by: Eric Anholt <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/vc4/vc4_dsi.c

···
 	struct vc4_dsi_encoder *vc4_encoder = to_vc4_dsi_encoder(encoder);
 	struct vc4_dsi *dsi = vc4_encoder->dsi;
 	struct device *dev = &dsi->pdev->dev;
+	struct drm_bridge *iter;
 
-	drm_bridge_chain_disable(dsi->bridge);
+	list_for_each_entry_reverse(iter, &dsi->bridge_chain, chain_node) {
+		if (iter->funcs->disable)
+			iter->funcs->disable(iter);
+	}
+
 	vc4_dsi_ulps(dsi, true);
-	drm_bridge_chain_post_disable(dsi->bridge);
+
+	list_for_each_entry_from(iter, &dsi->bridge_chain, chain_node) {
+		if (iter->funcs->post_disable)
+			iter->funcs->post_disable(iter);
+	}
 
 	clk_disable_unprepare(dsi->pll_phy_clock);
 	clk_disable_unprepare(dsi->escape_clock);
···
 	struct vc4_dsi *dsi = vc4_encoder->dsi;
 	struct device *dev = &dsi->pdev->dev;
 	bool debug_dump_regs = false;
+	struct drm_bridge *iter;
 	unsigned long hs_clock;
 	u32 ui_ns;
 	/* Minimum LP state duration in escape clock cycles. */
···
 
 	vc4_dsi_ulps(dsi, false);
 
-	drm_bridge_chain_pre_enable(dsi->bridge);
+	list_for_each_entry_reverse(iter, &dsi->bridge_chain, chain_node) {
+		if (iter->funcs->pre_enable)
+			iter->funcs->pre_enable(iter);
+	}
 
 	if (dsi->mode_flags & MIPI_DSI_MODE_VIDEO) {
 		DSI_PORT_WRITE(DISP0_CTRL,
···
 			       DSI_DISP0_ENABLE);
 	}
 
-	drm_bridge_chain_enable(dsi->bridge);
+	list_for_each_entry(iter, &dsi->bridge_chain, chain_node) {
+		if (iter->funcs->enable)
+			iter->funcs->enable(iter);
+	}
 
 	if (debug_dump_regs) {
 		struct drm_printer p = drm_info_printer(&dsi->pdev->dev);
···
 	 * from our driver, since we need to sequence them within the
 	 * encoder's enable/disable paths.
 	 */
-	list_splice(&dsi->encoder->bridge_chain, &dsi->bridge_chain);
+	list_splice_init(&dsi->encoder->bridge_chain, &dsi->bridge_chain);
 
 	if (dsi->port == 0)
 		vc4_debugfs_add_regset32(drm, "dsi0_regs", &dsi->regset);
···
 	 * Restore the bridge_chain so the bridge detach procedure can happen
 	 * normally.
 	 */
-	list_splice(&dsi->bridge_chain, &dsi->encoder->bridge_chain);
+	list_splice_init(&dsi->bridge_chain, &dsi->encoder->bridge_chain);
 	vc4_dsi_encoder_destroy(dsi->encoder);
 
 	if (dsi->port == 1)