On Mon, Jan 18, 2016 at 11:14:41AM +0100, Marek Szyprowski wrote:
Hello,
On 2016-01-13 15:58, Ville Syrjälä wrote:
On Wed, Jan 13, 2016 at 03:13:35PM +0100, Marek Szyprowski wrote:
On 2016-01-11 16:22, Ville Syrjälä wrote:
On Mon, Jan 11, 2016 at 04:07:50PM +0100, Daniel Vetter wrote:
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:
- 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:
- 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.
I remembe he said he wanted to work on it, but I don't remember seeing anything come out of that. I guess he got swallowed by patchwork, or I just missed the patch.
This mail might include some of my more recent thoughts on this matter http://lists.freedesktop.org/archives/intel-gfx/2014-April/042956.html
I also found this patch: http://patchwork.freedesktop.org/patch/23000/
However I still don't get a complete picture how it would work. If I got it right, the real blending function is defined by a pair of the values from the proposed enums, where one applies to the background (the blending result of deeper planes) and the second applies to the processed plane. Some combinations of the proposed enums doesn't make much sense (and especially changing the constant zero and one means simply changing the order of the planes), so I wonder if it might be easier to simply create an enum with all sane values for blending and document it really well? This way drivers will simply initialize only the supported blending modes.
That was my idea more or less too. We'd still model things based on the GL stuff, but we only advertize specific src+dst blend factor combinations. Ie. a single enum property where each value contains both the src and dst blend factor.
Okay, I've did some analysis of what Exynos hardware can do and it turned out that there only a few modes which really makes sense. Assuming that the order of planes is first determined by zpos properties, we already have well defined what is background (result of blending of all deeper planes) and what is blended plane (the terms source and destination used in previous discussion about blending are not very intuitive for me).
Now lets consider the basic blending equation: ResultRGB = A * backgroundRGB + B * planeRGB
where: planeRGB is RGB pixel values of blended plane backgroundRGB is RGB pixel values of plane's background (i.e. result of previous blending or RGB pixel values of deeper plane)
where A and B can be one of: ZERO, ONE, CONST_ALPHA (constant plane property), (1-CONST_ALPHA), PIXEL_ALPHA (alpha value from given pixel), (1-PIXEL_ALPHA), PIXEL_CONST_ALPHA (CONST_ALPHA * PIXEL_ALPHA), (1-PIXEL_CONST_ALPHA)
This gives following useful modes:
- NO_BLENDING: A=ZERO, B=ONE - no blending, global and per-pixel alpha
ignored (regardless selected fourcc mode) 2. CONST_ALPHA_BLEND: A=(1-CONST_ALPHA), B=CONST_ALPHA - blending with global plane alpha, per-pixel alpha ignored (regardless selected fourcc mode) 3. PIXEL_ALPHA_BLEND: A=(1-PIXEL_ALPHA), B=PIXEL_ALPHA - blending with per-pixel alpha, assuming plane with alpha pre-multiplied RGB pixel values (global alpha ignored, needs fourcc mode with alpha channel otherwise same as 1) 4. PIXEL_CONST_ALPHA_BLEND: A=(1-PIXEL_CONST_ALPHA), B=PIXEL_CONST_ALPHA
- blending with both global and per-pixel alpha multiplied, assuming
plane with alpha pre-multiplied RGB pixel values (needs fourcc mode with alpha channel otherwise same as 2)
If we want to solve pre-multiplied/straight alpha modes issue by introducing additional blending mode, we should consider following: 5. PIXEL_ALPHA_BLEND_STRAIGHT: A=(1-PIXEL_ALPHA), B=PIXEL_ALPHA - blending with per-pixel alpha, assuming plane with straight RGB pixel values (to be multiplied by pixel alpha value), 6. PIXEL_CONST_ALPHA_BLEND_STRAIGHT: A=(1-CONST_PIXEL_ALPHA), B=CONST_PIXEL_ALPHA - blending with both global and per-pixel alpha, assuming plane with straight RGB pixel values (to be multiplied by pixel alpha value)
Are there any other useful modes implemented by your hardware? The above defined modes are quite easy to get embedded into single enum and have well defined behavior.
IIRC some of our hardware had a bunch more options available. To avoid putting too many limitations into the API (which I'm sure we'd end up regretting at some point if history is any indication) I'd just call the enum values as: S_ONE_D_ZERO S_ONE_D_ONE_MINUS_SRC_ALPHA S_SRC_ALPHA_D_ONE_MINUS_SRC_ALPHA etc. or something like that. I suppose the actual strings shouldn't matter too much if people would just use the values and we'd define the values to have real meaning. Eg. ((dst_factor << whatever) | src_factor)
As far as pre-multiplication goes, I think a separate bool property would be good enough. And I've always found it easier to think in terms of which operations the hardware will perform, so I'd call it something like "src_premultiply", and when it's true the hardware will do an extra premultiplication step before blending, and when it's false no extra step is performed.