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

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Message-ID: <20250819180137.28ca89c0@booty>
Date: Tue, 19 Aug 2025 18:01:37 +0200
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Maxime Ripard <mripard@...nel.org>
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()

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).

Best regards,
Luca

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