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.
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.
Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland
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