linux-kernel - Re: [PATCH RFC v5 02/10] drm: Introduce solid fill DRM plane property

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Date:   Fri, 18 Aug 2023 14:03:14 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Pekka Paalanen <ppaalanen@...il.com>
Cc:     Sebastian Wick <sebastian.wick@...hat.com>,
        Jessica Zhang <quic_jesszhan@...cinc.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        quic_abhinavk@...cinc.com, contact@...rsion.fr,
        laurent.pinchart@...asonboard.com, ville.syrjala@...ux.intel.com,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, freedreno@...ts.freedesktop.org,
        wayland-devel@...ts.freedesktop.org
Subject: Re: [PATCH RFC v5 02/10] drm: Introduce solid fill DRM plane property

On 18/08/2023 13:51, Pekka Paalanen wrote:
> On Fri, 4 Aug 2023 16:59:00 +0300
> Dmitry Baryshkov <dmitry.baryshkov@...aro.org> wrote:
> 
>> On Fri, 4 Aug 2023 at 16:44, Sebastian Wick <sebastian.wick@...hat.com> wrote:
>>>
>>> On Fri, Aug 4, 2023 at 3:27 PM Dmitry Baryshkov
>>> <dmitry.baryshkov@...aro.org> wrote:
>>>>
>>>> On Fri, 28 Jul 2023 at 20:03, Jessica Zhang <quic_jesszhan@...cinc.com> wrote:
>>>>>
>>>>> Document and add support for solid_fill property to drm_plane. In
>>>>> addition, add support for setting and getting the values for solid_fill.
>>>>>
>>>>> To enable solid fill planes, userspace must assign a property blob to
>>>>> the "solid_fill" plane property containing the following information:
>>>>>
>>>>> struct drm_mode_solid_fill {
>>>>>          u32 version;
>>>>>          u32 r, g, b;
>>>>> };
>>>>>
>>>>> Signed-off-by: Jessica Zhang <quic_jesszhan@...cinc.com>
>>>>> ---
>>>>>   drivers/gpu/drm/drm_atomic_state_helper.c |  9 +++++
>>>>>   drivers/gpu/drm/drm_atomic_uapi.c         | 55 +++++++++++++++++++++++++++++++
>>>>>   drivers/gpu/drm/drm_blend.c               | 30 +++++++++++++++++
>>>>>   include/drm/drm_blend.h                   |  1 +
>>>>>   include/drm/drm_plane.h                   | 35 ++++++++++++++++++++
>>>>>   include/uapi/drm/drm_mode.h               | 24 ++++++++++++++
>>>>>   6 files changed, 154 insertions(+)
>>>>>   
>>>>
>>>> [skipped most of the patch]
>>>>   
>>>>> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
>>>>> index 43691058d28f..53c8efa5ad7f 100644
>>>>> --- a/include/uapi/drm/drm_mode.h
>>>>> +++ b/include/uapi/drm/drm_mode.h
>>>>> @@ -259,6 +259,30 @@ struct drm_mode_modeinfo {
>>>>>          char name[DRM_DISPLAY_MODE_LEN];
>>>>>   };
>>>>>
>>>>> +/**
>>>>> + * struct drm_mode_solid_fill - User info for solid fill planes
>>>>> + *
>>>>> + * This is the userspace API solid fill information structure.
>>>>> + *
>>>>> + * Userspace can enable solid fill planes by assigning the plane "solid_fill"
>>>>> + * property to a blob containing a single drm_mode_solid_fill struct populated with an RGB323232
>>>>> + * color and setting the pixel source to "SOLID_FILL".
>>>>> + *
>>>>> + * For information on the plane property, see drm_plane_create_solid_fill_property()
>>>>> + *
>>>>> + * @version: Version of the blob. Currently, there is only support for version == 1
>>>>> + * @r: Red color value of single pixel
>>>>> + * @g: Green color value of single pixel
>>>>> + * @b: Blue color value of single pixel
>>>>> + */
>>>>> +struct drm_mode_solid_fill {
>>>>> +       __u32 version;
>>>>> +       __u32 r;
>>>>> +       __u32 g;
>>>>> +       __u32 b;
>>>>
>>>> Another thought about the drm_mode_solid_fill uABI. I still think we
>>>> should add alpha here. The reason is the following:
>>>>
>>>> It is true that we have  drm_plane_state::alpha and the plane's
>>>> "alpha" property. However it is documented as "the plane-wide opacity
>>>> [...] It can be combined with pixel alpha. The pixel values in the
>>>> framebuffers are expected to not be pre-multiplied by the global alpha
>>>> associated to the plane.".
>>>>
>>>> I can imagine a use case, when a user might want to enable plane-wide
>>>> opacity, set "pixel blend mode" to "Coverage" and then switch between
>>>> partially opaque framebuffer and partially opaque solid-fill without
>>>> touching the plane's alpha value.
>>>
>>> The only reason I see against this is that there might be some
>>> hardware which supports only RGB but not alpha on planes and they
>>> could then not use this property.
>>
>> Fair enough.
>>
>>> Maybe another COLOR_FILL enum value
>>> with alpha might be better? Maybe just doing the alpha via the alpha
>>> property is good enough.
>>
>> One of our customers has a use case for setting the opaque solid fill,
>> while keeping the plane's alpha intact.
> 
> Could you explain more about why they must keep plane alpha intact
> instead of reprogramming everything with atomic? Is there some
> combination that just cannot reach the same end result via userspace
> manipulation of the solid fill values with plane alpha?
> 
> Or is it a matter of userspace architecture where you have independent
> components responsible for different KMS property values?
The latter one. The goal is to be able to switch between pixel sources 
without touching any additional properties (including plane's alpha value).

-- 
With best wishes
Dmitry