linux-kernel - [PATCH v2 10/12] drm/bridge: samsung-dsim: samsung_dsim_host_attach: use a temporary variable for the next bridge

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Message-Id: <20260109-drm-bridge-alloc-getput-drm_of_find_bridge-2-v2-10-8bad3ef90b9f@bootlin.com>
Date: Fri, 09 Jan 2026 08:31:41 +0100
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
 Philippe Cornu <philippe.cornu@...com>, benjamin.gaignard@...aro.org, 
 Andrzej Hajda <andrzej.hajda@...el.com>, 
 Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, 
 Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
 Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, 
 Adrien Grassein <adrien.grassein@...il.com>, Liu Ying <victor.liu@....com>, 
 Shawn Guo <shawnguo@...nel.org>, Sascha Hauer <s.hauer@...gutronix.de>, 
 Pengutronix Kernel Team <kernel@...gutronix.de>, 
 Fabio Estevam <festevam@...il.com>, Inki Dae <inki.dae@...sung.com>, 
 Jagan Teki <jagan@...rulasolutions.com>, 
 Marek Szyprowski <m.szyprowski@...sung.com>
Cc: Hui Pu <Hui.Pu@...ealthcare.com>, 
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>, 
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
 imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org, 
 Luca Ceresoli <luca.ceresoli@...tlin.com>
Subject: [PATCH v2 10/12] drm/bridge: samsung-dsim:
 samsung_dsim_host_attach: use a temporary variable for the next bridge

In preparation to handle refcounting of the out_bridge, we need to ensure
the out_bridge pointer contains either a valid bridge pointer or NULL, not
an ERR_PTR. Otherwise calls such as drm_bridge_get/put() would try to
redeference an ERR_PTR.

As a preliminary cleanup, add a temporary local 'next_bridge' pointer and
only copy it in dsi->out_bridge as late as possible, i.e. just before
calling pdata->host_ops->attach() which uses it (only in the exynos
driver).

Not strictly needed, but for symmetry move the clearing of dsi->out_bridge
in samsung_dsim_host_detach() to after pdata->host_ops->detach().

Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>

---

Changes in v2:
- fix "pointer set too late" bug (NULL pointer deref in the exynos case)
- *not* add Acked-by: Maxime as the patch has changed
---
 drivers/gpu/drm/bridge/samsung-dsim.c | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
index eabc4c32f6ab..8dd058124e93 100644
--- a/drivers/gpu/drm/bridge/samsung-dsim.c
+++ b/drivers/gpu/drm/bridge/samsung-dsim.c
@@ -1886,6 +1886,7 @@ static int samsung_dsim_host_attach(struct mipi_dsi_host *host,
 {
 	struct samsung_dsim *dsi = host_to_dsi(host);
 	const struct samsung_dsim_plat_data *pdata = dsi->plat_data;
+	struct drm_bridge *next_bridge;
 	struct device *dev = dsi->dev;
 	struct device_node *np = dev->of_node;
 	struct device_node *remote;
@@ -1924,17 +1925,17 @@ static int samsung_dsim_host_attach(struct mipi_dsi_host *host,
 
 	panel = of_drm_find_panel(remote);
 	if (!IS_ERR(panel)) {
-		dsi->out_bridge = devm_drm_panel_bridge_add(dev, panel);
+		next_bridge = devm_drm_panel_bridge_add(dev, panel);
 	} else {
-		dsi->out_bridge = of_drm_find_bridge(remote);
-		if (!dsi->out_bridge)
-			dsi->out_bridge = ERR_PTR(-EINVAL);
+		next_bridge = of_drm_find_bridge(remote);
+		if (!next_bridge)
+			next_bridge = ERR_PTR(-EINVAL);
 	}
 
 	of_node_put(remote);
 
-	if (IS_ERR(dsi->out_bridge)) {
-		ret = PTR_ERR(dsi->out_bridge);
+	if (IS_ERR(next_bridge)) {
+		ret = PTR_ERR(next_bridge);
 		DRM_DEV_ERROR(dev, "failed to find the bridge: %d\n", ret);
 		return ret;
 	}
@@ -1958,10 +1959,13 @@ static int samsung_dsim_host_attach(struct mipi_dsi_host *host,
 			return ret;
 	}
 
+	// The next bridge can be used by host_ops->attach
+	dsi->out_bridge = next_bridge;
+
 	if (pdata->host_ops && pdata->host_ops->attach) {
 		ret = pdata->host_ops->attach(dsi, device);
 		if (ret)
-			return ret;
+			goto err_release_next_bridge;
 	}
 
 	dsi->lanes = device->lanes;
@@ -1969,6 +1973,10 @@ static int samsung_dsim_host_attach(struct mipi_dsi_host *host,
 	dsi->mode_flags = device->mode_flags;
 
 	return 0;
+
+err_release_next_bridge:
+	dsi->out_bridge = NULL;
+	return ret;
 }
 
 static void samsung_dsim_unregister_te_irq(struct samsung_dsim *dsi)
@@ -1985,11 +1993,11 @@ static int samsung_dsim_host_detach(struct mipi_dsi_host *host,
 	struct samsung_dsim *dsi = host_to_dsi(host);
 	const struct samsung_dsim_plat_data *pdata = dsi->plat_data;
 
-	dsi->out_bridge = NULL;
-
 	if (pdata->host_ops && pdata->host_ops->detach)
 		pdata->host_ops->detach(dsi, device);
 
+	dsi->out_bridge = NULL;
+
 	samsung_dsim_unregister_te_irq(dsi);
 
 	drm_bridge_remove(&dsi->bridge);

-- 
2.52.0