linux-kernel - Re: [PATCH 05/29] drm/atomic_state_helper: Fix bridge state initialization

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Message-ID: <20250915-heavenly-athletic-lionfish-aa7b8b@penduick>
Date: Mon, 15 Sep 2025 13:27:00 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Thomas Zimmermann <tzimmermann@...e.de>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	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>, 
	Jyri Sarha <jyri.sarha@....fi>, Tomi Valkeinen <tomi.valkeinen@...asonboard.com>, 
	Devarsh Thakkar <devarsht@...com>, dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/29] drm/atomic_state_helper: Fix bridge state
 initialization

On Tue, Sep 02, 2025 at 03:18:17PM +0200, Thomas Zimmermann wrote:
> Hi
> 
> Am 02.09.25 um 10:32 schrieb Maxime Ripard:
> > Bridges implement their state using a drm_private_obj and an
> > hand-crafted reset implementation.
> > 
> > Since drm_private_obj doesn't have a set of reset helper like the other
> > states, __drm_atomic_helper_bridge_reset() was initializing both the
> > drm_private_state and the drm_bridge_state structures.
> > 
> > This initialization however was missing the drm_private_state.obj
> > pointer to the drm_private_obj the state was allocated for, creating a
> > NULL pointer dereference when trying to access it.
> > 
> > Fixes: 751465913f04 ("drm/bridge: Add a drm_bridge_state object")
> > Signed-off-by: Maxime Ripard <mripard@...nel.org>
> > ---
> >   drivers/gpu/drm/drm_atomic_state_helper.c | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/drm_atomic_state_helper.c b/drivers/gpu/drm/drm_atomic_state_helper.c
> > index 7142e163e618ea0d7d9d828e1bd9ff2a6ec0dfeb..b962c342b16aabf4e3bea52a914e5deb1c2080ce 100644
> > --- a/drivers/gpu/drm/drm_atomic_state_helper.c
> > +++ b/drivers/gpu/drm/drm_atomic_state_helper.c
> > @@ -707,10 +707,17 @@ void drm_atomic_helper_connector_destroy_state(struct drm_connector *connector,
> >   	__drm_atomic_helper_connector_destroy_state(state);
> >   	kfree(state);
> >   }
> >   EXPORT_SYMBOL(drm_atomic_helper_connector_destroy_state);
> > +static void __drm_atomic_helper_private_obj_reset(struct drm_private_obj *obj,
> > +						  struct drm_private_state *state)
> > +{
> > +	memset(state, 0, sizeof(*state));
> 
> This argument is guaranteed to be zero'd, I think. No need for a memset.
>
> > +	state->obj = obj;
> > +}
> > +
> >   /**
> >    * __drm_atomic_helper_private_obj_duplicate_state - copy atomic private state
> >    * @obj: CRTC object
> >    * @state: new private object state
> >    *
> > @@ -796,10 +803,11 @@ EXPORT_SYMBOL(drm_atomic_helper_bridge_destroy_state);
> >    */
> >   void __drm_atomic_helper_bridge_reset(struct drm_bridge *bridge,
> >   				      struct drm_bridge_state *state)
> >   {
> >   	memset(state, 0, sizeof(*state));
> 
> Another unnecessary memset?

I guess the two can be seen as redundant, but I'd argue the one in
__drm_atomic_helper_private_obj_reset should still be there.

What guarantees that the pointer points to a zero'd structure?

Maxime

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