linux-kernel - Re: [PATCH 3/3] drm: zynqmp: Add blend mode property to graphics plane

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Message-ID: <7f7423e9-809d-4023-be01-db59e6dd755e@linux.dev>
Date: Mon, 17 Nov 2025 10:35:57 -0500
From: Sean Anderson <sean.anderson@...ux.dev>
To: "Klymenko, Anatoliy" <Anatoliy.Klymenko@....com>,
 Laurent Pinchart <laurent.pinchart@...asonboard.com>,
 Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
 "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 Mike Looijmans <mike.looijmans@...ic.nl>, David Airlie <airlied@...il.com>,
 Thomas Zimmermann <tzimmermann@...e.de>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>,
 "linux-arm-kernel@...ts.infradead.org"
 <linux-arm-kernel@...ts.infradead.org>, Simona Vetter <simona@...ll.ch>,
 "Simek, Michal" <michal.simek@....com>
Subject: Re: [PATCH 3/3] drm: zynqmp: Add blend mode property to graphics
 plane

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@...ts.freedesktop.org> On Behalf Of Sean
>> Anderson
>> Sent: Thursday, November 13, 2025 3:07 PM
>> To: Klymenko, Anatoliy <Anatoliy.Klymenko@....com>; Laurent Pinchart
>> <laurent.pinchart@...asonboard.com>; Tomi Valkeinen
>> <tomi.valkeinen@...asonboard.com>; dri-devel@...ts.freedesktop.org
>> Cc: linux-kernel@...r.kernel.org; Mike Looijmans <mike.looijmans@...ic.nl>;
>> David Airlie <airlied@...il.com>; Thomas Zimmermann
>> <tzimmermann@...e.de>; Maarten Lankhorst
>> <maarten.lankhorst@...ux.intel.com>; Maxime Ripard <mripard@...nel.org>;
>> linux-arm-kernel@...ts.infradead.org; Simona Vetter <simona@...ll.ch>; Simek,
>> Michal <michal.simek@....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@...ux.dev>
>> >> Sent: Thursday, November 13, 2025 12:37 PM
>> >> To: Laurent Pinchart <laurent.pinchart@...asonboard.com>; Tomi Valkeinen
>> >> <tomi.valkeinen@...asonboard.com>; dri-devel@...ts.freedesktop.org
>> >> Cc: linux-kernel@...r.kernel.org; Mike Looijmans
>> <mike.looijmans@...ic.nl>;
>> >> David Airlie <airlied@...il.com>; Thomas Zimmermann
>> >> <tzimmermann@...e.de>; Maarten Lankhorst
>> >> <maarten.lankhorst@...ux.intel.com>; Klymenko, Anatoliy
>> >> <Anatoliy.Klymenko@....com>; Maxime Ripard <mripard@...nel.org>;
>> linux-
>> >> arm-kernel@...ts.infradead.org; Simona Vetter <simona@...ll.ch>; Simek,
>> >> Michal <michal.simek@....com>; Sean Anderson
>> >> <sean.anderson@...ux.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@...ux.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.

>> 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.

--Sean