Re: [PATCH 3/4] drm/i915: Enabling pre-multiplied alpha drm property

[Damien Lespiau <damien.lespiau@intel.com>]

On Thu, Mar 20, 2014 at 03:29:42PM +0530, Sagar Arun Kamble wrote:
> Hi Damien,
> 
> On Wed, 2014-03-19 at 15:10 +0000, Damien Lespiau wrote:
> > On Sat, Mar 08, 2014 at 01:51:18PM +0530, sagar.a.kamble@intel.com wrote:
> > > From: Sagar Kamble <sagar.a.kamble@intel.com>
> > > 
> > > This patch enables property for changin the pixel format
> > > of plane to enable/disable pre-multiplied alpha format.
> > > Client has to set BIT(DRM_BLEND_PREMULTIPLIED_ALPHA) | 0x0/0x1
> > > to disable/enable pre-multiplied alpha format.
> > > 
> > > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > > Cc: Jani Nikula <jani.nikula@linux.intel.com>
> > > Cc: David Airlie <airlied@linux.ie>
> > > Signed-off-by: Srinivas, Vidya <vidya.srinivas@intel.com>
> > > Signed-off-by: Sagar Kamble <sagar.a.kamble@intel.com>
> > 
> > Huum, the alpha being premultiplied or not seems to be a property of the
> > framebuffer to me, not of the plane. It seems to me that we should
> > define alternative premultiplied DRM_FORMATs and make the sprite planes
> > advertise support for premultiplied fbs in the format list when the
> > hardware indeed supports them.
> This is what i think of usage of this property:
> 
> Composer/user mode starts using plane with XRGB format and then it wants
> to add transparency to the plane. So it will set the format to ARGB
> format and provide buffer for that plane that will have pixels with
> pre-multiplied alpha (a*r, a*g, a*b, a).
> This can be done with primary plane(CRTC) as well, however I have 
> not added this as CRTC property since CRTCs are going to be drm_plane
> soon.
> 
> Will this kind of interface for usermode to toggle the pixel format's
> alpha be useful?

I don't think so, nop.

Besides being a convoluted apocalyptic scenario, one cannot simply
change the format of the FB without re-adding it with AddFB2().

There's a usage model for the compositor to add a plane-global alpha to
a plane (fades the client provided render target) and that's indeed a
plane property.

As far I as can tell, the premultiplied alpha format ban be sued support
scanning out OpenGL blended fbs.

-- 
Damien