Hi Dmitry,

On Tue, 31 Dec 2024 16:57:38 +0200
Dmitry Baryshkov [off-list ref] wrote:

On Tue, Dec 31, 2024 at 11:40:02AM +0100, Luca Ceresoli wrote:

quoted

Instead of using dsi->out_bridge during the bridge search process, use a
temporary variable and assign dsi->out_bridge only on successful
completion.

The main goal is to be able to drm_bridge_get() the out_bridge before
setting it in dsi->out_bridge, which is done in a later commit. Setting
dsi->out_bridge as in current code would leave a use-after-free window in
case the bridge is deallocated by some other thread between
'dsi->out_bridge = devm_drm_panel_bridge_add()' and drm_bridge_get().  

I don't think that's how refcounting should work. Any of the functions
that give you the bridge should also increase refcount, requiring manual
_put() call afterwards. We might need a separate API for that.

You're perfectly right.

quoted

This change additionally avoids leaving an ERR_PTR value in dsi->out_bridge
on failure. This is not necessarily a problem but it is not clean.

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

---

This patch was added in v5.
---
 drivers/gpu/drm/bridge/samsung-dsim.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
index f8b4fb8357659018ec0db65374ee5d05330639ae..c4d1563fd32019efde523dfc0863be044c05a826 100644
--- a/drivers/gpu/drm/bridge/samsung-dsim.c
+++ b/drivers/gpu/drm/bridge/samsung-dsim.c

@@ -1705,6 +1705,7 @@ static int samsung_dsim_host_attach(struct mipi_dsi_host *host,
 	struct device *dev = dsi->dev;
 	struct device_node *np = dev->of_node;
 	struct device_node *remote;
+	struct drm_bridge *out_bridge;
 	struct drm_panel *panel;
 	int ret;
 

@@ -1740,21 +1741,23 @@ 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);
+		out_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);
+		out_bridge = of_drm_find_bridge(remote);
+		if (!out_bridge)
+			out_bridge = ERR_PTR(-EINVAL);
 	}  

While looking at this patch, I think we should migrate the driver to
drm_of_find_panel_or_bridge().

Indeed, the code here is duplicating drm_of_find_panel_or_bridge(). I'm
going to convert it.

Then your patch might add a function
close to devm_drm_of_get_bridge() or drmm_of_get_bridge().

...which would return a bridge pointer, with refcount already
incremented. Sure, except I think it should _not_ be a drmm, as
the bridge might itself disappear while the card keeps existing.

Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com