linux-kernel - Re: [PATCH] drm/simpledrm: Drop superfluous primary plane .atomic

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Message-ID: <43e3dead-6089-c342-5050-34d7325bcb24@suse.de>
Date:   Mon, 5 Sep 2022 12:57:58 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        linux-kernel@...r.kernel.org
Cc:     Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/simpledrm: Drop superfluous primary plane
 .atomic_check return logic

Hi Javier

Am 31.08.22 um 13:12 schrieb Javier Martinez Canillas:
> The simpledrm_primary_plane_helper_atomic_check() function is more complex
> than needed. It first checks drm_atomic_helper_check_plane_state() returns
> value to decide whether to return this or zero.
> 
> But it could just return that function return value directly. It also does
> a check if new_plane_state->visible isn't set, but returns zero regardless.
> 
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
> ---
> 
>   drivers/gpu/drm/tiny/simpledrm.c | 15 ++++-----------
>   1 file changed, 4 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/gpu/drm/tiny/simpledrm.c b/drivers/gpu/drm/tiny/simpledrm.c
> index a81f91814595..0be47f40247a 100644
> --- a/drivers/gpu/drm/tiny/simpledrm.c
> +++ b/drivers/gpu/drm/tiny/simpledrm.c
> @@ -485,21 +485,14 @@ static int simpledrm_primary_plane_helper_atomic_check(struct drm_plane *plane,
>   	struct drm_plane_state *new_plane_state = drm_atomic_get_new_plane_state(new_state, plane);
>   	struct drm_crtc *new_crtc = new_plane_state->crtc;
>   	struct drm_crtc_state *new_crtc_state = NULL;
> -	int ret;
>   
>   	if (new_crtc)
>   		new_crtc_state = drm_atomic_get_new_crtc_state(new_state, new_crtc);
>   
> -	ret = drm_atomic_helper_check_plane_state(new_plane_state, new_crtc_state,
> -						  DRM_PLANE_NO_SCALING,
> -						  DRM_PLANE_NO_SCALING,
> -						  false, false);
> -	if (ret)
> -		return ret;
> -	else if (!new_plane_state->visible)
> -		return 0;
> -
> -	return 0;
> +	return drm_atomic_helper_check_plane_state(new_plane_state, new_crtc_state,
> +						   DRM_PLANE_NO_SCALING,
> +						   DRM_PLANE_NO_SCALING,
> +						   false, false);

I'm undecided on this change. I know it's correct and more to the point. 
But the call's logic is non-intuitive: the call either returns an error 
or we have to test ->visible afterwards. So I wrote it explicitly.

I saw that your change to ssd130x also uses the pattern. If we find more 
such drivers, we could implement the atomic check as a helper. I suggest 
drm_plane_helper_atomic_check_fixed() in drm_plane_helper.c

Best regards
Thomas

>   }
>   
>   static void simpledrm_primary_plane_helper_atomic_update(struct drm_plane *plane,

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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