[RFC PATCH 1/3] drm: Introduce color fill properties for drm plane

[Dmitry Baryshkov]

On 29/10/2022 01:59, Jessica Zhang wrote:
> Add support for COLOR_FILL and COLOR_FILL_FORMAT properties for
> drm_plane. In addition, add support for setting and getting the values
> of these properties.
> 
> COLOR_FILL represents the color fill of a plane while COLOR_FILL_FORMAT
> represents the format of the color fill. Userspace can set enable solid
> fill on a plane by assigning COLOR_FILL to a uint64_t value, assigning
> the COLOR_FILL_FORMAT property to a uint32_t value, and setting the
> framebuffer to NULL.
> 
> Signed-off-by: Jessica Zhang <quic_jesszhan at quicinc.com>

Planes report supported formats using the drm_mode_getplane(). You'd 
also need to tell userspace, which formats are supported for color fill. 
I don't think one supports e.g. YV12.

A bit of generic comment for the discussion (this is an RFC anyway). 
Using color_fill/color_fill_format properties sounds simple, but this 
might be not generic enough. Limiting color_fill to 32 bits would 
prevent anybody from using floating point formats (e.g. 
DRM_FORMAT_XRGB16161616F, 64-bit value). Yes, this can be solved with 
e.g. using 64-bit for the color_fill value, but then this doesn't sound 
extensible too much.

So, a question for other hardware maintainers. Do we have hardware that 
supports such 'color filled' planes? Do we want to support format 
modifiers for filling color/data? Because what I have in mind is closer 
to the blob structure, which can then be used for filling the plane:

struct color_fill_blob {
     u32 pixel_format;
     u64 modifiers4];
     u32 pixel_data_size; // fixme: is this necessary?
     u8 pixel_data[];
};

And then... This sounds a lot like a custom framebuffer.

So, maybe what should we do instead is to add new DRM_MODE_FB_COLOR_FILL 
flag to the framebuffers, which would e.g. mean that the FB gets stamped 
all over the plane. This would also save us from changing if (!fb) 
checks all over the drm core.

Another approach might be using a format modifier instead of the FB flag.

What do you think?

-- 
With best wishes
Dmitry