linux-kernel - Re: [PATCH v2 34/34] drm/bridge: panel: convert to devm_drm_bridge

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Message-ID: <20250428172516.79058e22@booty>
Date: Mon, 28 Apr 2025 17:25:16 +0200
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Maxime Ripard <mripard@...nel.org>
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Subject: Re: [PATCH v2 34/34] drm/bridge: panel: convert to
 devm_drm_bridge_alloc() API

Hi Maxime,

On Mon, 28 Apr 2025 13:39:23 +0200
Maxime Ripard <mripard@...nel.org> wrote:

> On Thu, Apr 24, 2025 at 10:05:49PM +0200, Luca Ceresoli wrote:
> > This is the new API for allocating DRM bridges.
> > 
> > The devm lifetime management of this driver is peculiar. The underlying
> > device for the panel_bridge is the panel, and the devm lifetime is tied the
> > panel device (panel->dev). However the panel_bridge allocation is not
> > performed by the panel driver, but rather by a separate entity (typically
> > the previous bridge in the encoder chain).
> > 
> > Thus when that separate entoty is destroyed, the panel_bridge is not
> > removed automatically by devm, so it is rather done explicitly by calling
> > drm_panel_bridge_remove(). This is the function that does devm_kfree() the
> > panel_bridge in current code, so update it as well to put the bridge
> > reference instead.
> > 
> > Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>  
> 
> This looks fine, but we need a TODO entry to clean this up later on, and
> a comment on devm_drm_put_bridge that this is inherently unsafe and
> must not be used.

Ah, I see, OK.

Quick draft:

 /**
  * devm_drm_put_bridge - Release a bridge reference obtained via devm
  * @dev: device that got the bridge via devm
  * @bridge: pointer to a struct drm_bridge obtained via devm
  *
  * Same as drm_bridge_put() for bridge pointers obtained via devm functions
  * such as devm_drm_bridge_alloc().
+ *
+ * This function is a temporary workaround and MUST NOT be used. Manual
+ * handling of bridge lifetime is inherently unsafe.
  */

and:

-	devm_kfree(panel_bridge->panel->dev, bridge);
+       /* TODO remove this after reworking panel_bridge lifetime */
+	devm_drm_put_bridge(panel_bridge->panel->dev, bridge);
 }

Does it look good enough?

Luca

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