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Message-Id: <DGCG90VYLUEA.3IPJ682VDMHP6@bootlin.com>
Date: Wed, 11 Feb 2026 22:27:29 +0100
From: "Luca Ceresoli" <luca.ceresoli@...tlin.com>
To: "Liu Ying" <victor.liu@....com>, "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>, "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>, "Shawn Guo"
<shawnguo@...nel.org>, "Sascha Hauer" <s.hauer@...gutronix.de>,
"Pengutronix Kernel Team" <kernel@...gutronix.de>, "Fabio Estevam"
<festevam@...il.com>
Cc: "Hui Pu" <Hui.Pu@...ealthcare.com>, "Ian Ray"
<ian.ray@...ealthcare.com>, "Thomas Petazzoni"
<thomas.petazzoni@...tlin.com>, <dri-devel@...ts.freedesktop.org>,
<imx@...ts.linux.dev>, <linux-arm-kernel@...ts.infradead.org>,
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] drm/bridge: imx8qxp-pixel-link: get/put the next
bridge
Hello Liu,
On Thu Feb 5, 2026 at 10:26 AM CET, Liu Ying wrote:
> On Thu, Feb 05, 2026 at 09:52:04AM +0100, Luca Ceresoli wrote:
>> Hello Liu,
>
> Hello Luca,
>
>>
>> On Wed Feb 4, 2026 at 7:27 AM CET, Liu Ying wrote:
>>> Hi Luca,
>>>
>>> On Tue, Feb 03, 2026 at 11:35:25AM +0100, Luca Ceresoli wrote:
>>>> This driver obtains a bridge pointer from of_drm_find_bridge() in the probe
>>>> function and stores it until driver removal. of_drm_find_bridge() is
>>>> deprecated. Move to of_drm_find_and_get_bridge() for the bridge to be
>>>> refcounted and use bridge->next_bridge to put the reference on
>>>> deallocation.
>>>>
>>>> To keep the code as simple and reliable as possible, get a reference for
>>>> each pointer that stores a drm_bridge address when it is stored and release
>>>> it when the pointer is overwritten or goes out of scope. Also remove the
>>>> intermediate selected_bridge variable to reduce the refcounted variables in
>>>> the function. The involved pointers are:
>>>>
>>>> * next_bridge loop-local variable:
>>>> - get reference by of_drm_find_and_get_bridge()
>>>> - put reference at the end of the loop iteration (__free)
>>>>
>>>> * pl->bridge.next_bridge, tied to struct imx8qxp_pixel_link lifetime:
>>>> - get reference when assigned (by copy from next_bridge)
>>>> - put reference before reassignment if reassignment happens
>>>> - put reference when the struct imx8qxp_pixel_link embedding the
>>>> struct drm_bridge is destroyed (struct drm_bridge::next_bridge)
>>>>
>>>> Additionally, split the somewhat complex if() for readability.
>>>>
>>>> Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
>>>>
>>>> ---
>>>>
>>>> Changes in v5:
>>>> - rewrite commit message after Liu's review to clarify the per-pointer
>>>> get/put idea
>>>> - split the if()s involved in selcting the bridge
>>>> - remove intermediate selected_bridge pointer
>>>> - removed Maxime's R-by, patch changed
>>>> ---
>>>> drivers/gpu/drm/bridge/imx/imx8qxp-pixel-link.c | 17 ++++++++++-------
>>>> 1 file changed, 10 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/bridge/imx/imx8qxp-pixel-link.c b/drivers/gpu/drm/bridge/imx/imx8qxp-pixel-link.c
>>>> index 91e4f4d55469..e29e099b893a 100644
>>>> --- a/drivers/gpu/drm/bridge/imx/imx8qxp-pixel-link.c
>>>> +++ b/drivers/gpu/drm/bridge/imx/imx8qxp-pixel-link.c
>>>> @@ -23,7 +23,6 @@
>>>>
>>>> struct imx8qxp_pixel_link {
>>>> struct drm_bridge bridge;
>>>> - struct drm_bridge *next_bridge;
>>>> struct device *dev;
>>>> struct imx_sc_ipc *ipc_handle;
>>>> u8 stream_id;
>>>> @@ -140,7 +139,7 @@ static int imx8qxp_pixel_link_bridge_attach(struct drm_bridge *bridge,
>>>> }
>>>>
>>>> return drm_bridge_attach(encoder,
>>>> - pl->next_bridge, bridge,
>>>> + pl->bridge.next_bridge, bridge,
>>>> DRM_BRIDGE_ATTACH_NO_CONNECTOR);
>>>> }
>>>>
>>>> @@ -260,7 +259,6 @@ static int imx8qxp_pixel_link_find_next_bridge(struct imx8qxp_pixel_link *pl)
>>>> {
>>>> struct device_node *np = pl->dev->of_node;
>>>> struct device_node *port;
>>>> - struct drm_bridge *selected_bridge = NULL;
>>>> u32 port_id;
>>>> bool found_port = false;
>>>> int reg;
>>>> @@ -297,7 +295,8 @@ static int imx8qxp_pixel_link_find_next_bridge(struct imx8qxp_pixel_link *pl)
>>>> continue;
>>>> }
>>>>
>>>> - struct drm_bridge *next_bridge = of_drm_find_bridge(remote);
>>>> + struct drm_bridge *next_bridge __free(drm_bridge_put) =
>>>> + of_drm_find_and_get_bridge(remote);
>>>> if (!next_bridge)
>>>> return -EPROBE_DEFER;
>>>>
>>>> @@ -305,12 +304,16 @@ static int imx8qxp_pixel_link_find_next_bridge(struct imx8qxp_pixel_link *pl)
>>>> * Select the next bridge with companion PXL2DPI if
>>>> * present, otherwise default to the first bridge
>>>> */
>>>> - if (!selected_bridge || of_property_present(remote, "fsl,companion-pxl2dpi"))
>>>> - selected_bridge = next_bridge;
>>>> + if (!pl->bridge.next_bridge)
>>>> + pl->bridge.next_bridge = drm_bridge_get(next_bridge);
>>>> +
>>>> + if (of_property_present(remote, "fsl,companion-pxl2dpi")) {
>>>> + drm_bridge_put(pl->bridge.next_bridge);
>>>> + pl->bridge.next_bridge = drm_bridge_get(next_bridge);
>>>> + }
>>>
>>> Can you drop the intermediate next_bridge variable to simplify the code?
>>>
>>> -8<-
>>> if (!pl->bridge.next_bridge) {
>>> pl->bridge.next_bridge = of_drm_find_and_get_bridge(remote);
>>> if (!pl->bridge.next_bridge)
>>> return -EPROBE_DEFER;
>>> }
>>>
>>> if (of_property_present(remote, "fsl,companion-pxl2dpi")) {
>>> drm_bridge_put(pl->bridge.next_bridge);
>>> pl->bridge.next_bridge = of_drm_find_and_get_bridge(remote);
>>> if (!pl->bridge.next_bridge)
>>> return -EPROBE_DEFER;
>>> }
>>> -8<-
>>
>> Potentially calling of_drm_find_and_get_bridge() twice on the same node,
>> with a put in the middle, looks poorly readable to me, even though it still
>> looks correct code.
>>
>> However I think we can do even better with an 'else if':
>>
>> if (!pl->bridge.next_bridge) {
>> pl->bridge.next_bridge = of_drm_find_and_get_bridge(remote);
>> if (!pl->bridge.next_bridge)
>> return -EPROBE_DEFER;
>> } else if (of_property_present(remote, "fsl,companion-pxl2dpi")) { <===
>> drm_bridge_put(pl->bridge.next_bridge);
>> pl->bridge.next_bridge = of_drm_find_and_get_bridge(remote);
>> if (!pl->bridge.next_bridge)
>> return -EPROBE_DEFER;
>> }
>>
>> Looks OK?
>
> Both are fine to me. TBH, I feel my version with two 'if's is a bit easier
> to read. But, I'd say up to you.
I think this is really a minor detail and there is no obvious "best"
version, so I'll send v6 as I had it ready and build-tested already,
i.e. with the 'else if' version.
Thanks for the discussion!
Luca
--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
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