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

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Message-Id: <DFJ8VHH9BVEZ.2445GF73YTGSO@bootlin.com>
Date: Thu, 08 Jan 2026 14:33:21 +0100
From: "Luca Ceresoli" <luca.ceresoli@...tlin.com>
To: "Marek Szyprowski" <m.szyprowski@...sung.com>, "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>
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>
Subject: Re: [PATCH 10/12] drm/bridge: samsung-dsim:
 samsung_dsim_host_attach: use a temporary variable for the next bridge

Hi Marek,

On Thu Jan 8, 2026 at 10:26 AM CET, Marek Szyprowski wrote:
> On 07.01.2026 14:13, Luca Ceresoli wrote:
>> 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 when returning successfully.
>>
>> Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
>> ---
>>   drivers/gpu/drm/bridge/samsung-dsim.c | 14 ++++++++------
>>   1 file changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
>> index eabc4c32f6ab..b3003aa49dc3 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;
>>   	}
>> @@ -1967,6 +1968,7 @@ static int samsung_dsim_host_attach(struct mipi_dsi_host *host,
>>   	dsi->lanes = device->lanes;
>>   	dsi->format = device->format;
>>   	dsi->mode_flags = device->mode_flags;
>> +	dsi->out_bridge = next_bridge;
>>
>
> This assignment is too late, dsi->out_bridge is used (indirectly, by
> samsung_dsim_attach() called from drm_bridge_attach()) by
>
> ret = pdata->host_ops->attach(dsi, device);

Thanks for testing, reporting, and suggesting a solution. I'm not sure why
it worked on my setup, but this is indeed a bug.

> a few lines before this assignment, so the following fix has to be added:
>
> diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c
> b/drivers/gpu/drm/bridge/samsung-dsim.c index b3003aa49dc3..f88aa8ab2879
> 100644 --- a/drivers/gpu/drm/bridge/samsung-dsim.c +++
> b/drivers/gpu/drm/bridge/samsung-dsim.c @@ -1959,6 +1959,7 @@ static int
> samsung_dsim_host_attach(struct mipi_dsi_host *host, return ret; } +
> dsi->out_bridge = next_bridge; if (pdata->host_ops &&
> pdata->host_ops->attach) { ret = pdata->host_ops->attach(dsi, device);
> if (ret) @@ -1968,7 +1969,6 @@ static int
> samsung_dsim_host_attach(struct mipi_dsi_host *host, dsi->lanes =
> device->lanes; dsi->format = device->format; dsi->mode_flags =
> device->mode_flags; - dsi->out_bridge = next_bridge; return 0; }

This needed a bit of demangling :) but it looks like a correct solution.

I took a moment to understand why this did not break my setup. The answer
is I have a fsl,imx8mp-mipi-dsim, which has no .attach set in its
samsung_dsim_host_ops.

Sorry for the inconvenience. I'm fixing this in v2.

Luca

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