Re: [PATCH RFC v4 2/7] drm: Introduce pixel_source DRM plane property

[Dmitry Baryshkov <dmitry.baryshkov@linaro.org>]

On 10/07/2023 22:51, Jessica Zhang wrote:
> 
> 
> On 6/30/2023 1:27 AM, Pekka Paalanen wrote:
>> On Fri, 30 Jun 2023 03:42:28 +0300
>> Dmitry Baryshkov <dmitry.baryshkov@linaro.org> wrote:
>>
>>> On 30/06/2023 03:25, Jessica Zhang wrote:
>>>> Add support for pixel_source property to drm_plane and related
>>>> documentation.
>>>>
>>>> This enum property will allow user to specify a pixel source for the
>>>> plane. Possible pixel sources will be defined in the
>>>> drm_plane_pixel_source enum.
>>>>
>>>> The current possible pixel sources are DRM_PLANE_PIXEL_SOURCE_FB and
>>>> DRM_PLANE_PIXEL_SOURCE_COLOR. The default value is *_SOURCE_FB.
>>>
>>> I think, this should come before the solid fill property addition. First
>>> you tell that there is a possibility to define other pixel sources, then
>>> additional sources are defined.
>>
>> Hi,
>>
>> that would be logical indeed.
> 
> Hi Dmitry and Pekka,
> 
> Sorry for the delay in response, was out of office last week.
> 
> Acked.
> 
>>
>>>>
>>>> Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
>>>> ---
>>>>    drivers/gpu/drm/drm_atomic_state_helper.c |  1 +
>>>>    drivers/gpu/drm/drm_atomic_uapi.c         |  4 ++
>>>>    drivers/gpu/drm/drm_blend.c               | 81 
>>>> +++++++++++++++++++++++++++++++
>>>>    include/drm/drm_blend.h                   |  2 +
>>>>    include/drm/drm_plane.h                   | 21 ++++++++
>>>>    5 files changed, 109 insertions(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_atomic_state_helper.c 
>>>> b/drivers/gpu/drm/drm_atomic_state_helper.c
>>>> index fe14be2bd2b2..86fb876efbe6 100644
>>>> --- a/drivers/gpu/drm/drm_atomic_state_helper.c
>>>> +++ b/drivers/gpu/drm/drm_atomic_state_helper.c
>>>> @@ -252,6 +252,7 @@ void 
>>>> __drm_atomic_helper_plane_state_reset(struct drm_plane_state 
>>>> *plane_state,
>>>>        plane_state->alpha = DRM_BLEND_ALPHA_OPAQUE;
>>>>        plane_state->pixel_blend_mode = DRM_MODE_BLEND_PREMULTI;
>>>> +    plane_state->pixel_source = DRM_PLANE_PIXEL_SOURCE_FB;
>>>>        if (plane_state->solid_fill_blob) {
>>>>            drm_property_blob_put(plane_state->solid_fill_blob);
>>>> diff --git a/drivers/gpu/drm/drm_atomic_uapi.c 
>>>> b/drivers/gpu/drm/drm_atomic_uapi.c
>>>> index a28b4ee79444..6e59c21af66b 100644
>>>> --- a/drivers/gpu/drm/drm_atomic_uapi.c
>>>> +++ b/drivers/gpu/drm/drm_atomic_uapi.c
>>>> @@ -596,6 +596,8 @@ static int drm_atomic_plane_set_property(struct 
>>>> drm_plane *plane,
>>>>            drm_property_blob_put(solid_fill);
>>>>            return ret;
>>>> +    } else if (property == plane->pixel_source_property) {
>>>> +        state->pixel_source = val;
>>>>        } else if (property == plane->alpha_property) {
>>>>            state->alpha = val;
>>>>        } else if (property == plane->blend_mode_property) {
>>>
>>> I think, it was pointed out in the discussion that drm_mode_setplane()
>>> (a pre-atomic IOCTL to turn the plane on and off) should also reset
>>> pixel_source to FB.
> 
> I don't remember drm_mode_setplane() being mentioned in the pixel_source 
> discussion... can you share where it was mentioned?

https://lore.kernel.org/dri-devel/20230627105849.004050b3@eldfell/

Let me quote it here:
"Legacy non-atomic UAPI wrappers can do whatever they want, and program
any (new) properties they want in order to implement the legacy
expectations, so that does not seem to be a problem."


> 
> I'd prefer to avoid having driver change the pixel_source directly as it 
> could cause some unexpected side effects. In general, I would like 
> userspace to assign the value of pixel_source without driver doing 
> anything "under the hood".

s/driver/drm core/

We have to remain compatible with old userspace, especially with the 
non-atomic one. If the userspace calls ioctl(DRM_IOCTL_MODE_SETPLANE), 
we have to display the specified FB, no matter what was the value of 
PIXEL_SOURCE before this ioctl.


> 
>>>
>>>> @@ -671,6 +673,8 @@ drm_atomic_plane_get_property(struct drm_plane 
>>>> *plane,
>>>>        } else if (property == plane->solid_fill_property) {
>>>>            *val = state->solid_fill_blob ?
>>>>                state->solid_fill_blob->base.id : 0;
>>>> +    } else if (property == plane->pixel_source_property) {
>>>> +        *val = state->pixel_source;
>>>>        } else if (property == plane->alpha_property) {
>>>>            *val = state->alpha;
>>>>        } else if (property == plane->blend_mode_property) {
>>>> diff --git a/drivers/gpu/drm/drm_blend.c b/drivers/gpu/drm/drm_blend.c
>>>> index 38c3c5d6453a..8c100a957ee2 100644
>>>> --- a/drivers/gpu/drm/drm_blend.c
>>>> +++ b/drivers/gpu/drm/drm_blend.c
>>>> @@ -189,6 +189,18 @@
>>>>     *    solid_fill is set up with 
>>>> drm_plane_create_solid_fill_property(). It
>>>>     *    contains pixel data that drivers can use to fill a plane.
>>>>     *
>>>> + * pixel_source:
>>>> + *    pixel_source is set up with 
>>>> drm_plane_create_pixel_source_property().
>>>> + *    It is used to toggle the source of pixel data for the plane.
>>
>> Other sources than the selected one are ignored?
> 
> Yep, the plane will only display the data from the set pixel_source.
> 
> So if pixel_source == FB and solid_fill_blob is non-NULL, 
> solid_fill_blob will be ignored and the plane will display the FB that 
> is set.

correct.

> 
> Will add a note about this in the comment docs.
> 
>>
>>>> + *
>>>> + *    Possible values:
>>
>> Wouldn't hurt to explicitly mention here that this is an enum.
> 
> Acked.
> 
>>
>>>> + *
>>>> + *    "FB":
>>>> + *        Framebuffer source
>>>> + *
>>>> + *    "COLOR":
>>>> + *        solid_fill source
>>
>> I think these two should be more explicit. Framebuffer source is the
>> usual source from the property "FB_ID". Solid fill source comes from
>> the property "solid_fill".
> 
> Acked.
> 
>>
>> Why "COLOR" and not, say, "SOLID_FILL"?
> 
> Ah, that would make more sense :)
> 
> I'll change this to "SOLID_FILL".
> 
>>
>>>> + *
>>>>     * Note that all the property extensions described here apply 
>>>> either to the
>>>>     * plane or the CRTC (e.g. for the background color, which 
>>>> currently is not
>>>>     * exposed and assumed to be black).
>>>> @@ -648,3 +660,72 @@ int drm_plane_create_solid_fill_property(struct 
>>>> drm_plane *plane)
>>>>        return 0;
>>>>    }
>>>>    EXPORT_SYMBOL(drm_plane_create_solid_fill_property);
>>>> +
>>>> +/**
>>>> + * drm_plane_create_pixel_source_property - create a new pixel 
>>>> source property
>>>> + * @plane: drm plane
>>>> + * @supported_sources: bitmask of supported pixel_sources for the 
>>>> driver (NOT
>>>> + *                     including DRM_PLANE_PIXEL_SOURCE_FB, as it 
>>>> will be supported
>>>> + *                     by default).
>>>
>>> I'd say this is too strong. I'd suggest either renaming this to
>>> extra_sources (mentioning that FB is enabled for all the planes) or
>>> allowing any source bitmask (mentioning that FB should be enabled by the
>>> caller, unless there is a good reason not to do so).
>>
>> Right. I don't see any problem with having planes of type OVERLAY that
>> support only solid_fill and no FB. Planes of type PRIMARY and CURSOR I
>> would expect to always support at least FB.
>>
>> Atomic userspace is prepared to have an OVERLAY plane fail for any
>> arbitrary reason. Legacy userspace probably should not ever see a plane
>> that does not support FB.
> 
> Got it... If we allow the possibility of FB sources not being supported, 
> then should the default pixel_source per plane be decided by the driver 
> too?
> 
> I'd forced FB support so that I could set pixel_source to FB in 
> __drm_atomic_helper_plane_state_reset(). If we allow more flexibility in 
> the default pixel_source value, I guess we can also store a 
> default_pixel_source value in the plane_state.

I'd say, the FB is a sane default. It the driver has other needs, it can 
override the value in drm_plane_funcs::reset().

> 

[skipped the rest]

-- 
With best wishes
Dmitry