[PATCH v4 2/2] drm/panel: Add device_link from panel device to drm device

[Jyri Sarha]

Add device_link from panel device (supplier) to drm device (consumer)
when drm_panel_attach() is called. This patch should protect the
master drm driver if an attached panel driver unbinds while it is in
use. The device_link should make sure the drm device is unbound before
the panel driver becomes unavailable.

The device_link is removed when drm_panel_detach() is called. The
drm_panel_detach() should be called by the consumer DRM driver, not the
panel driver, otherwise both drivers are racing to delete the same link.

Signed-off-by: Jyri Sarha <jsarha at ti.com>
Reviewed-by: Eric Anholt <eric at anholt.net>
---
 drivers/gpu/drm/drm_panel.c | 10 ++++++++++
 include/drm/drm_panel.h     |  1 +
 2 files changed, 11 insertions(+)

diff --git a/drivers/gpu/drm/drm_panel.c b/drivers/gpu/drm/drm_panel.c
index 71e4075..965530a 100644
--- a/drivers/gpu/drm/drm_panel.c
+++ b/drivers/gpu/drm/drm_panel.c
@@ -24,6 +24,7 @@
 #include <linux/err.h>
 #include <linux/module.h>
 
+#include <drm/drm_device.h>
 #include <drm/drm_crtc.h>
 #include <drm/drm_panel.h>
 
@@ -104,6 +105,13 @@ int drm_panel_attach(struct drm_panel *panel, struct drm_connector *connector)
 	if (panel->connector)
 		return -EBUSY;
 
+	panel->link = device_link_add(connector->dev->dev, panel->dev, 0);
+	if (!panel->link) {
+		dev_err(panel->dev, "failed to link panel to %s\n",
+			dev_name(connector->dev->dev));
+		return -EINVAL;
+	}
+
 	panel->connector = connector;
 	panel->drm = connector->dev;
 
@@ -125,6 +133,8 @@ EXPORT_SYMBOL(drm_panel_attach);
  */
 int drm_panel_detach(struct drm_panel *panel)
 {
+	device_link_del(panel->link);
+
 	panel->connector = NULL;
 	panel->drm = NULL;
 
diff --git a/include/drm/drm_panel.h b/include/drm/drm_panel.h
index 14ac240..26a1b5f 100644
--- a/include/drm/drm_panel.h
+++ b/include/drm/drm_panel.h
@@ -89,6 +89,7 @@ struct drm_panel {
 	struct drm_device *drm;
 	struct drm_connector *connector;
 	struct device *dev;
+	struct device_link *link;
 
 	const struct drm_panel_funcs *funcs;
 
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