When the panel-driver is build as a module it currently fails hard as the
panel cannot be probed directly:

dw_mipi_dsi_bind()
  __dw_mipi_dsi_probe()
    creates dsi bus
    creates panel device
    triggers panel module load
    panel not probed (module not loaded or panel probe slow)
  drm_bridge_attach
    fails with -EINVAL due to empty panel_bridge

Additionally the panel probing can run concurrently with dsi bringup
making it possible that the panel can already be found but dsi-attach
hasn't finished running.

The newly added function provides the ability for glue drivers to
check if a dsi device was actually attached and also protects
the attach part to prevent concurrency issues from panel-assignment
and drm_bridge_create.

Using that check glue drivers are able to for example defer probe/bind
in the case that the panel is not completely set up yet.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
---
 drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c | 25 +++++++++++++++++++
 include/drm/bridge/dw_mipi_dsi.h              |  1 +
 2 files changed, 26 insertions(+)

diff --git a/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c b/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c
index bb4aeca5c0f9..88fed22ff3f6 100644
--- a/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c
+++ b/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c

@@ -12,6 +12,7 @@
 #include <linux/component.h>
 #include <linux/iopoll.h>
 #include <linux/module.h>
+#include <linux/mutex.h>
 #include <linux/of_device.h>
 #include <linux/pm_runtime.h>
 #include <linux/reset.h>

@@ -219,6 +220,7 @@ struct dw_mipi_dsi {
 	struct drm_bridge bridge;
 	struct mipi_dsi_host dsi_host;
 	struct drm_bridge *panel_bridge;
+	struct mutex panel_mutex;
 	struct device *dev;
 	void __iomem *base;
 

@@ -296,10 +298,14 @@ static int dw_mipi_dsi_host_attach(struct mipi_dsi_host *host,
 			return PTR_ERR(bridge);
 	}
 
+	mutex_lock(&dsi->panel_mutex);
+
 	dsi->panel_bridge = bridge;
 
 	drm_bridge_add(&dsi->bridge);
 
+	mutex_unlock(&dsi->panel_mutex);
+
 	return 0;
 }
 

@@ -308,13 +314,30 @@ static int dw_mipi_dsi_host_detach(struct mipi_dsi_host *host,
 {
 	struct dw_mipi_dsi *dsi = host_to_dsi(host);
 
+	mutex_lock(&dsi->panel_mutex);
+
+	dsi->panel_bridge = NULL;
 	drm_of_panel_bridge_remove(host->dev->of_node, 1, 0);
 
 	drm_bridge_remove(&dsi->bridge);
 
+	mutex_unlock(&dsi->panel_mutex);
+
 	return 0;
 }
 
+bool dw_mipi_dsi_device_attached(struct dw_mipi_dsi *dsi)
+{
+	bool output;
+
+	mutex_lock(&dsi->panel_mutex);
+	output = !!dsi->panel_bridge;
+	mutex_unlock(&dsi->panel_mutex);
+
+	return output;
+}
+EXPORT_SYMBOL_GPL(dw_mipi_dsi_device_attached);

+
 static void dw_mipi_message_config(struct dw_mipi_dsi *dsi,
 				   const struct mipi_dsi_msg *msg)
 {

@@ -867,6 +890,8 @@ __dw_mipi_dsi_probe(struct platform_device *pdev,
 	dsi->dev = dev;
 	dsi->plat_data = plat_data;
 
+	mutex_init(&dsi->panel_mutex);
+
 	if (!plat_data->phy_ops->init || !plat_data->phy_ops->get_lane_mbps) {
 		DRM_ERROR("Phy not properly configured\n");
 		return ERR_PTR(-ENODEV);

diff --git a/include/drm/bridge/dw_mipi_dsi.h b/include/drm/bridge/dw_mipi_dsi.h
index 6d7f8eb5d9f2..131ff2569ed4 100644
--- a/include/drm/bridge/dw_mipi_dsi.h
+++ b/include/drm/bridge/dw_mipi_dsi.h

@@ -31,6 +31,7 @@ struct dw_mipi_dsi_plat_data {
 	void *priv_data;
 };
 
+bool dw_mipi_dsi_device_attached(struct dw_mipi_dsi *dsi);
 struct dw_mipi_dsi *dw_mipi_dsi_probe(struct platform_device *pdev,
 				      const struct dw_mipi_dsi_plat_data
 				      *plat_data);