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From:   Jani Nikula <jani.nikula@linux.intel.com>
To:     Thomas Zimmermann <tzimmermann@suse.de>,
        Maxime Ripard <maxime@cerno.tech>,
        Emma Anholt <emma@anholt.net>,
        Maxime Ripard <mripard@kernel.org>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>
Cc:     linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        Dave Stevenson <dave.stevenson@raspberrypi.com>
Subject: Re: [PATCH 2/9] drm/vc4: hdmi: Constify container_of wrappers
In-Reply-To: <38a5f052-18d5-5297-f1c7-44ca5b8285f5@suse.de>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
References: <20221207-rpi-hdmi-improvements-v1-0-6b15f774c13a@cerno.tech>
 <20221207-rpi-hdmi-improvements-v1-2-6b15f774c13a@cerno.tech>
 <38a5f052-18d5-5297-f1c7-44ca5b8285f5@suse.de>
Date:   Wed, 11 Jan 2023 16:17:12 +0200
Message-ID: <87wn5tgnpz.fsf@intel.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Wed, 11 Jan 2023, Thomas Zimmermann <tzimmermann@suse.de> wrote:
> Hi
>
> Am 07.12.22 um 17:07 schrieb Maxime Ripard:
>> None of our wrappers around container_of to access our objects from the
>> DRM object pointer actually modify the latter.
>> 
>> Let's make them const.
>> 
>> Signed-off-by: Maxime Ripard <maxime@cerno.tech>
>
> Personally, I wouldn't take this patch as it does an implicit const cast 
> on the pointer.
>
> For const correctness, it seems better to add new helpers that keep the 
> const. Those could be use in places where the caller is not allowed to 
> modify the state (i.e., atomic_update, et al).
>
> Something like this:
>
> const struct vc4_hdmi *
> const_connector_to_vc4_hdmi(const struct drm_connector *connector)
> {
> 	return container_of(connector, const struct vc4_hdmi,
> 		connector);
> }

See container_of_const().

BR,
Jani.

>
> Best regards
> Thomas
>
>> ---
>>   drivers/gpu/drm/vc4/vc4_hdmi.h | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>> 
>> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.h b/drivers/gpu/drm/vc4/vc4_hdmi.h
>> index dc3ccd8002a0..023ea64ef006 100644
>> --- a/drivers/gpu/drm/vc4/vc4_hdmi.h
>> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.h
>> @@ -224,13 +224,13 @@ struct vc4_hdmi {
>>   };
>>   
>>   static inline struct vc4_hdmi *
>> -connector_to_vc4_hdmi(struct drm_connector *connector)
>> +connector_to_vc4_hdmi(const struct drm_connector *connector)
>>   {
>>   	return container_of(connector, struct vc4_hdmi, connector);
>>   }
>>   
>>   static inline struct vc4_hdmi *
>> -encoder_to_vc4_hdmi(struct drm_encoder *encoder)
>> +encoder_to_vc4_hdmi(const struct drm_encoder *encoder)
>>   {
>>   	struct vc4_encoder *_encoder = to_vc4_encoder(encoder);
>>   	return container_of(_encoder, struct vc4_hdmi, encoder);
>> @@ -244,7 +244,7 @@ struct vc4_hdmi_connector_state {
>>   };
>>   
>>   static inline struct vc4_hdmi_connector_state *
>> -conn_state_to_vc4_hdmi_conn_state(struct drm_connector_state *conn_state)
>> +conn_state_to_vc4_hdmi_conn_state(const struct drm_connector_state *conn_state)
>>   {
>>   	return container_of(conn_state, struct vc4_hdmi_connector_state, base);
>>   }
>> 

-- 
Jani Nikula, Intel Open Source Graphics Center