[RFC] handling of alpha mode (pre-multiplied/straight) in ARGB modes

[Daniel Vetter]

On Mon, Jan 11, 2016 at 03:18:44PM +0100, Marek Szyprowski wrote:
> Dear All,
> 
> I would like to add support for configuring alpha modes: straight and
> pre-multiplied for blending to Exynos DRM driver. For those who see those
> terms for the first time:
> 1. straight alpha mode means means that all A and RGB values are from full
> 0.255 range,
> 2. pre-multiplied alpha mode means that A is from 0..255 range and RGB
> values are scaled to 0..ALPHA range (where ALPHA is the A value for the
> given pixel).
> The pre-multiplied mode is quite common in texture processing.
> 
> I did a little research and found that there is no common approach for
> defining straight or pre-multiplied alpha modes for ARGB framebuffers.
> 
> From reading the sources and the register names I found that following
> drivers use pre-multiplied modes for ARGB framebuffers:
> radeon (at least for ARGB cursors),
> amdgpu (cursor),
> intel (all ARGB planes),
> rock chip.
> 
> On the other hand, there are drm drivers which support ARGB modes and use
> straight alpha modes:
> omap,
> msm.
> 
> For the rest of drivers, which might deal with per-pixel alpha, I was not
> able to determine from the code if the pixel RGB values are interpreted as
> per-multiplied or straight:
> atmel_hlcdc,
> sti,
> mdp5,
> shmobile,
> rcar,
> vc4,
> imx.

Imo in case of doubt/mixed definitions the semantics of the big desktop
drivers should win. True generic userspace is most likely developed on
those, everything else should just adjust. And in most cases we can get
away with that on arm drivers since they tend to ship userspace and kernel
in lockstep. At least where it really matters.

> Exynos DRM driver now mixes pre-multiplied and straight modes, depending on
> the CRTC sub-driver.
> 
> While checking the code I found a following comment in
> drm/i915/intel_display.c in skl_plane_ctl_format() function:
> /*
>  * XXX: For ARBG/ABGR formats we default to expecting scanout buffers
>  * to be already pre-multiplied. We need to add a knob (or a different
>  * DRM_FORMAT) for user-space to configure that.
>  */
> 
> The question is how to cleanup this ambiguities and let generic userspace to
> properly use ARGB/ARGB modes with proper blending mode. I came up with the
> following 3 solutions:
> 
> 1. Define new fourcc values for pre-multiplied (or/and straight) alpha
> modes,
> 2. Introduce a new framebuffer flag for pre-multiplied or straight alpha (or
> both),
> 3. Introduce a new plane property for controlling alpha blending mode.
> 
> The first solution has serious compatibility problem, because we can either
> map existing fourcc to pre-multiplied (to follow radeon/amdgpu, intel and
> rockchip behavior) or straight mode. Both ways it will break some existing
> applications. To avoid compatibility problem we would need to add 2 more
> sets of fourcc and leave existing ARGB modes as 'driver dependent'.
> 
> The second solution is probably a bit less intrusive, but it suffers for the
> similar compatibility problem. To make this feature compatible with existing
> code, probably 2 flags will be needed: DRM_MODE_FB_ALPHA_FORCE_PREMULTIPLIED
> and DRM_MODE_FB_ALPHA_FORCE_STRAIGHT. This way when userspace provides no
> flag, the driver can use its default mode.
> 
> Third solution (additional plane property) has been already proposed:
> https://lkml.org/lkml/2015/6/19/330, however I found no follow-up nor
> comments. Separate property lets at least drivers to notify userspace about
> driver's default alpha mode and lets generic application to properly set the
> requested mode. The only problem is that the alpha mode is not directly
> configured on the framebuffer object, where in my opinion it should be
> stored.
> 
> Please let me know which approach You like best and which should I take for
> introducing generic way of configuring alpha mode.

One idea that was tossed around a few times is to go full generic and
implement something like glBlendFunc for planes, except that we'd also
pre-multiplied/straight versions where it makes sense. For drivers that
can't support pre-multiplied alpha they could simply leave out these
values from the blend-func property (like we already allow with e.g.
rotation, when a driver can't do 90/270°).

Wrt backwards compat a property would work well too: If it's not there
then userspace should assume old behaviour (which should be pre-multiplied
really, but might be somewhat broken on some drivers/kernel).

Damien had some RFC about all this way back, but can't find it right now.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch