linux-kernel - Re: [PATCH v2 3/9] drm/bridge: add drm_for_each_bridge_in_chain

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Message-ID: <20250915-bulky-visionary-hamster-3a6c10@penduick>
Date: Mon, 15 Sep 2025 14:21:04 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Luca Ceresoli <luca.ceresoli@...tlin.com>
Cc: 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>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Miguel Ojeda <ojeda@...nel.org>, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <nick.desaulniers+lkml@...il.com>, 
	Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
	Tomi Valkeinen <tomi.valkeinen@...asonboard.com>, Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>, 
	Chaoyi Chen <chaoyi.chen@...k-chips.com>, Hui Pu <Hui.Pu@...ealthcare.com>, 
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>, dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	llvm@...ts.linux.dev
Subject: Re: [PATCH v2 3/9] drm/bridge: add
 drm_for_each_bridge_in_chain_scoped()

On Tue, Aug 19, 2025 at 06:01:37PM +0200, Luca Ceresoli wrote:
> Hi Maxime,
> 
> On Tue, 19 Aug 2025 15:47:06 +0200
> Maxime Ripard <mripard@...nel.org> wrote:
> 
> > > +/**
> > > + * drm_for_each_bridge_in_chain_scoped - iterate over all bridges attached
> > > + *                                       to an encoder
> > > + * @encoder: the encoder to iterate bridges on
> > > + * @bridge: a bridge pointer updated to point to the current bridge at each
> > > + *	    iteration
> > > + *
> > > + * Iterate over all bridges present in the bridge chain attached to @encoder.
> > > + *
> > > + * Automatically gets/puts the bridge reference while iterating, and puts
> > > + * the reference even if returning or breaking in the middle of the loop.
> > > + */
> > > +#define drm_for_each_bridge_in_chain_scoped(encoder, bridge)		\
> > > +	for (struct drm_bridge *bridge __free(drm_bridge_put) =		\
> > > +	     drm_bridge_chain_get_first_bridge(encoder);		\  
> > 
> > So my understanding is that the initial value of bridge would be cleaned
> > up with drm_bridge_put...
> > 
> > > +	     bridge;							\
> > > +	     bridge = drm_bridge_get_next_bridge_and_put(bridge))  
> > 
> > ... but also when iterating?
> > 
> > So if we have more than 0 values, we put two references?
> 
> No, this is not the case. The __free action is executed only when
> exiting the entire for loop, not a single iteration.
>
> This is consistent with the fact that the loop variable is persistent
> across iterations.
>
> I tested this macro in both cases:
> 
>  * looping over the entire chain the final value of @bridge will be
>    NULL and the cleanup action won't call drm_bridge_put()
>  * breaking before the last element, @bridge is non-NULL and the
>    cleanup action does call drm_bridge_put()
> 
> See examples such as for_each_child_of_node_scoped() and other OF
> iterators which work in the same way (which is no coincidence, I used
> them as starting point for writing this patch).

Ack,

Reviewed-by: Maxime Ripard <mripard@...nel.org>

Maxime

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