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

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Message-ID: <bfacbd41-48c6-4123-9fdd-3d82eb682673@suse.de>
Date: Wed, 1 Oct 2025 08:45:09 +0200
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Maxime Ripard <mripard@...nel.org>
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

Hi

Am 23.09.25 um 11:33 schrieb Maxime Ripard:
> On Mon, Sep 15, 2025 at 03:12:11PM +0200, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 15.09.25 um 13:27 schrieb Maxime Ripard:
>>> 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?
>> We only call this helper after allocation AFAICT. And the DRM APIs already
>> assume that allocation clears to zero.
> Really? Do we have that documented anywhere?

I don't think it's documented anywhere, but that's what Sima told me 
when I tried to write similar code. The rule was to alloc zero'd memory 
and than have init functions only set what is necessary, but keep others 
to zero. I think it can be seen in certain places, where drivers set a 
variety of flags or fields before calling an init helper. Creating a GEM 
object from shmem is an example. [1]

[1] 
https://elixir.bootlin.com/linux/v6.17/source/drivers/gpu/drm/drm_gem_shmem_helper.c#L52

Best regards
Thomas

>
> And even then, there's nothing that requires that helper to be called
> straight-away after allocation.
>
> More importantly, do we really care about skipping them? Like, all the
> other reset helpers are doing it, it's cheap, safe, why should we remove
> it?
>
> Maxime

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)