linux-kernel - Re: [PATCH 2/9] drm/vc4: hdmi: Constify container

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Message-ID: <87wn5tgnpz.fsf@intel.com>
Date:   Wed, 11 Jan 2023 16:17:12 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        Maxime Ripard <maxime@...no.tech>,
        Emma Anholt <emma@...olt.net>,
        Maxime Ripard <mripard@...nel.org>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Dave Stevenson <dave.stevenson@...pberrypi.com>
Subject: Re: [PATCH 2/9] drm/vc4: hdmi: Constify container_of wrappers

On Wed, 11 Jan 2023, Thomas Zimmermann <tzimmermann@...e.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@...no.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