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

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Date:   Mon, 5 Sep 2022 13:06:05 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        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

Hello Thomas,

On 9/5/22 12:57, Thomas Zimmermann wrote:
> 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.
>

Yes, but the check has no effect so I found it even less intuitive. Maybe
add a comment then if you wan to keep the current code?
 
> 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
>

Sure. I can add a preparatory change in v2 that adds that helper and then
use it in the follow-up patch.

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat