RE: [PATCH 3/3] drm: zynqmp: Add blend mode property to graphics plane

Klymenko, Anatoliy
2025-12-04T21:16:21Z

[AMD Official Use Only - AMD Internal Distribution Only]

Hi Sean,

> -----Original Message-----
> From: Sean Anderson <sean.anderson@linux.dev>
> Sent: Monday, November 17, 2025 7:36 AM
> To: Klymenko, Anatoliy <Anatoliy.Klymenko@amd.com>; Laurent Pinchart
> <laurent.pinchart@ideasonboard.com>; Tomi Valkeinen
> <tomi.valkeinen@ideasonboard.com>; dri-devel@lists.freedesktop.org
> Cc: linux-kernel@vger.kernel.org; Mike Looijmans <mike.looijmans@topic.nl>;
> David Airlie <airlied@gmail.com>; Thomas Zimmermann
> <tzimmermann@suse.de>; Maarten Lankhorst
> <maarten.lankhorst@linux.intel.com>; Maxime Ripard <mripard@kernel.org>;
> linux-arm-kernel@lists.infradead.org; Simona Vetter <simona@ffwll.ch>; Simek,
> Michal <michal.simek@amd.com>
> Subject: Re: [PATCH 3/3] drm: zynqmp: Add blend mode property to graphics
> plane
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
>
>
> On 11/14/25 19:12, Klymenko, Anatoliy wrote:
> > [AMD Official Use Only - AMD Internal Distribution Only]
> >
> >> -----Original Message-----
> >> From: dri-devel <dri-devel-bounces@lists.freedesktop.org> On Behalf Of
> Sean
> >> Anderson
> >> Sent: Thursday, November 13, 2025 3:07 PM
> >> To: Klymenko, Anatoliy <Anatoliy.Klymenko@amd.com>; Laurent Pinchart
> >> <laurent.pinchart@ideasonboard.com>; Tomi Valkeinen
> >> <tomi.valkeinen@ideasonboard.com>; dri-devel@lists.freedesktop.org
> >> Cc: linux-kernel@vger.kernel.org; Mike Looijmans
> <mike.looijmans@topic.nl>;
> >> David Airlie <airlied@gmail.com>; Thomas Zimmermann
> >> <tzimmermann@suse.de>; Maarten Lankhorst
> >> <maarten.lankhorst@linux.intel.com>; Maxime Ripard
> <mripard@kernel.org>;
> >> linux-arm-kernel@lists.infradead.org; Simona Vetter <simona@ffwll.ch>;
> Simek,
> >> Michal <michal.simek@amd.com>
> >> Subject: Re: [PATCH 3/3] drm: zynqmp: Add blend mode property to graphics
> >> plane
> >>
> >> Caution: This message originated from an External Source. Use proper
> caution
> >> when opening attachments, clicking links, or responding.
> >>
> >>
> >> On 11/13/25 18:03, Klymenko, Anatoliy wrote:
> >> > [AMD Official Use Only - AMD Internal Distribution Only]
> >> >
> >> > Hi Sean,
> >> >
> >> > Thank you for the patch.
> >> >
> >> >> -----Original Message-----
> >> >> From: Sean Anderson <sean.anderson@linux.dev>
> >> >> Sent: Thursday, November 13, 2025 12:37 PM
> >> >> To: Laurent Pinchart <laurent.pinchart@ideasonboard.com>; Tomi
> Valkeinen
> >> >> <tomi.valkeinen@ideasonboard.com>; dri-devel@lists.freedesktop.org
> >> >> Cc: linux-kernel@vger.kernel.org; Mike Looijmans
> >> <mike.looijmans@topic.nl>;
> >> >> David Airlie <airlied@gmail.com>; Thomas Zimmermann
> >> >> <tzimmermann@suse.de>; Maarten Lankhorst
> >> >> <maarten.lankhorst@linux.intel.com>; Klymenko, Anatoliy
> >> >> <Anatoliy.Klymenko@amd.com>; Maxime Ripard <mripard@kernel.org>;
> >> linux-
> >> >> arm-kernel@lists.infradead.org; Simona Vetter <simona@ffwll.ch>;
> Simek,
> >> >> Michal <michal.simek@amd.com>; Sean Anderson
> >> >> <sean.anderson@linux.dev>
> >> >> Subject: [PATCH 3/3] drm: zynqmp: Add blend mode property to graphics
> >> plane
> >> >>
> >> >> Caution: This message originated from an External Source. Use proper
> >> caution
> >> >> when opening attachments, clicking links, or responding.
> >> >>
> >> >>
> >> >> When global alpha is enabled, per-pixel alpha is ignored. Allow
> >> >> userspace to explicitly specify whether to use per-pixel alpha by
> >> >> exposing it through the blend mode property. I'm not sure whether the
> >> >> per-pixel alpha is pre-multiplied or not [1], but apparently it *must* be
> >> >> pre-multiplied so I guess we have to advertise it.
> >> >>
> >> >> [1] All we get is "The alpha value available with the graphics stream
> >> >> will define the transparency of the graphics."
> >> >>
> >> >> Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
> >> >> ---
> >> >>
> >> >> drivers/gpu/drm/xlnx/zynqmp_kms.c | 24 ++++++++++++++++++++++--
> >> >> 1 file changed, 22 insertions(+), 2 deletions(-)
> >> >>
> >> >> diff --git a/drivers/gpu/drm/xlnx/zynqmp_kms.c
> >> >> b/drivers/gpu/drm/xlnx/zynqmp_kms.c
> >> >> index 456ada9ac003..fa1cfc16db36 100644
> >> >> --- a/drivers/gpu/drm/xlnx/zynqmp_kms.c
> >> >> +++ b/drivers/gpu/drm/xlnx/zynqmp_kms.c
> >> >> @@ -61,6 +61,13 @@ static int
> zynqmp_dpsub_plane_atomic_check(struct
> >> >> drm_plane *plane,
> >> >> if (!new_plane_state->crtc)
> >> >> return 0;
> >> >>
> >> >> + if (new_plane_state->pixel_blend_mode !=
> >> >> DRM_MODE_BLEND_PIXEL_NONE &&
> >> >> + new_plane_state->alpha >> 8 != 0xff) {
> >> >> + drm_dbg_kms(plane->dev,
> >> >> + "Plane alpha must be 1.0 when using pixel alpha\n");
> >> >> + return -EINVAL;
> >> >> + }
> >> >> +
> >> >> crtc_state = drm_atomic_get_crtc_state(state, new_plane_state-
> >crtc);
> >> >> if (IS_ERR(crtc_state))
> >> >> return PTR_ERR(crtc_state);
> >> >> @@ -117,9 +124,13 @@ static void
> >> >> zynqmp_dpsub_plane_atomic_update(struct drm_plane *plane,
> >> >>
> >> >> zynqmp_disp_layer_update(layer, new_state);
> >> >>
> >> >> - if (plane->index == ZYNQMP_DPSUB_LAYER_GFX)
> >> >> - zynqmp_disp_blend_set_global_alpha(dpsub->disp, true,
> >> >> + if (plane->index == ZYNQMP_DPSUB_LAYER_GFX) {
> >> >> + bool blend = plane->state->pixel_blend_mode ==
> >> >> + DRM_MODE_BLEND_PIXEL_NONE;
> >> >> +
> >> >> + zynqmp_disp_blend_set_global_alpha(dpsub->disp, blend,
> >> >> plane->state->alpha >> 8);
> >> >> + }
> >> >>
> >> >> /*
> >> >> * Unconditionally enable the layer, as it may have been disabled
> >> >> @@ -179,9 +190,18 @@ static int zynqmp_dpsub_create_planes(struct
> >> >> zynqmp_dpsub *dpsub)
> >> >> return ret;
> >> >>
> >> >> if (i == ZYNQMP_DPSUB_LAYER_GFX) {
> >> >> + unsigned int blend_modes =
> >> >> + BIT(DRM_MODE_BLEND_PIXEL_NONE) |
> >> >> + BIT(DRM_MODE_BLEND_PREMULTI);
> >> >
> >> > | BIT(DRM_MODE_BLEND_COVERAGE) - this is what implemented in the
> >> hardware.
> >>
> >> Do you have a datasheet (or other) reference?
> >>
> >
> > Yes https://docs.amd.com/v/u/en-US/ug1085-zynq-ultrascale-trm
>
> There is no mention of this in the TRM.
>
> | Video Blending is defined for two RGB video streams. One of these
> | streams will be graphics that have an alpha value along with RGB
> | stream. The alpha value available with the graphics stream will define
> | the transparency of the graphics. Alpha value defined for blending
> | function is always 8-bit. 1-bit alpha and 4-bit alpha are also
> | supported, but these are scaled to 8-bits before they are used for
> | alpha blending
>
> Please quote a specific section where the alpha blending formula is
> documented.
>

Yes, there is no explicit alpha blending formula in the TRM, although
absence of "premultiplied alpha" term anywhere in the doc suggests that
alpha is treated as a straight. Anyways, I cooked a quick test that proved
the straight alpha theory: https://github.com/onotole/zynqmp-dp-alpha

> >> But in any case, DRM_MODE_BLEND_PREMULTI is mandatory even if we
> >> don't support
> >> it. See drm_plane_create_blend_mode_property for details.
> >>
> >
> > No doubts here
>
> OK, so regardless of what the hardware does, we have to pretend to
> support pre-multiplied alpha.
>

Yes, we my pretend so, or we may work around backward compatibility check
in drm_plane_create_blend_mode_property by setting blend mode to
DRM_MODE_BLEND_COVERAGE immediately and later reject plane states
with DRM_MODE_BLEND_PREMULTI mode.

> --Sean

Thank you,
Anatoliy