drm/sysfb: simpledrm: Improve panel-size validation

Validate the panel size from the device-tree node against the
limitations of struct drm_display_mode. The type only stores sizes
in 16-bit fields. Fail transparently on errors; do not warn.

v3:
- move comments to a more prominent place (Thierry)
v2:
- only use initialized values in debugging output (Sashiko)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Fixes: 2a6d731a8f16 ("drm/simpledrm: Allow physical width and height configuration via panel node")
Cc: Rayyan Ansari <rayyan@ansari.sh>
Cc: <stable@vger.kernel.org> # v6.4+
Link: https://patch.msgid.link/20260625094509.157581-3-tzimmermann@suse.de

1 files changed, 46 insertions, 3 deletions

diff --git a/drivers/gpu/drm/sysfb/simpledrm.c b/drivers/gpu/drm/sysfb/simpledrm.c
index 15dcafa9d524..6bbe779ec870 100644
--- a/drivers/gpu/drm/sysfb/simpledrm.c
+++ b/drivers/gpu/drm/sysfb/simpledrm.c
@@ -193,6 +193,39 @@ simplefb_get_memory_of(struct drm_device *dev, struct device_node *of_node)
 	return res;
 }
 
+static int __simplefb_get_panel_size_mm_of(struct drm_device *dev,
+					   struct device_node *of_panel_node,
+					   const char *name)
+{
+	int ret;
+	u32 value;
+
+	ret = of_property_read_u32(of_panel_node, name, &value);
+	if (ret) {
+		drm_dbg(dev, "simplefb: cannot parse panel %s: error %d\n",
+			name, ret);
+		return ret;
+	} else if (value > U16_MAX) {
+		drm_dbg(dev, "simplefb: panel %s of %u exceeds maximum value\n",
+			name, value);
+		return -EINVAL;
+	}
+
+	return value;
+}
+
+static int simplefb_get_panel_width_mm_of(struct drm_device *dev,
+					  struct device_node *of_panel_node)
+{
+	return __simplefb_get_panel_size_mm_of(dev, of_panel_node, "width-mm");
+}
+
+static int simplefb_get_panel_height_mm_of(struct drm_device *dev,
+					   struct device_node *of_panel_node)
+{
+	return __simplefb_get_panel_size_mm_of(dev, of_panel_node, "height-mm");
+}
+
 /*
  * Simple Framebuffer device
  */
@@ -594,7 +627,7 @@ static struct simpledrm_device *simpledrm_device_create(struct drm_driver *drv,
 	struct drm_sysfb_device *sysfb;
 	struct drm_device *dev;
 	int width, height, stride;
-	int width_mm = 0, height_mm = 0;
+	u16 width_mm = 0, height_mm = 0;
 	struct device_node *panel_node;
 	const struct drm_format_info *format;
 	struct resource *res, *mem = NULL;
@@ -658,8 +691,18 @@ static struct simpledrm_device *simpledrm_device_create(struct drm_driver *drv,
 			return ERR_CAST(mem);
 		panel_node = of_parse_phandle(of_node, "panel", 0);
 		if (panel_node) {
-			simplefb_read_u32_of(dev, panel_node, "width-mm", &width_mm);
-			simplefb_read_u32_of(dev, panel_node, "height-mm", &height_mm);
+			/*
+			 * Ignore errors from parsing the physical panel
+			 * size. Using the pre-initialized sizes of 0 will
+			 * make drm_sysfb_mode() calculate a default physical
+			 * size based on a resolution of 96 dpi.
+			 */
+			ret = simplefb_get_panel_width_mm_of(dev, panel_node);
+			if (ret > 0)
+				width_mm = ret;
+			ret = simplefb_get_panel_height_mm_of(dev, panel_node);
+			if (ret > 0)
+				height_mm = ret;
 			of_node_put(panel_node);
 		}
 	} else {