Re: [PATCH 2/3] drm: zynqmp: Make the video plane primary

Sean Anderson
2026-01-05T19:46:56Z

Hi Anatoliy,

Sorry for the delay.

On 12/4/25 17:33, Klymenko, Anatoliy wrote:
> [AMD Official Use Only - AMD Internal Distribution Only]
>
> Hi Sean,
>
>> -----Original Message-----
>> From: Sean Anderson <sean.anderson@linux.dev>
>> Sent: Monday, November 17, 2025 8:10 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 2/3] drm: zynqmp: Make the video plane primary
>>
>> Caution: This message originated from an External Source. Use proper caution
>> when opening attachments, clicking links, or responding.
>>
>>
>> On 11/14/25 19:57, 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 2:52 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 2/3] drm: zynqmp: Make the video plane primary
>> >>
>> >> Caution: This message originated from an External Source. Use proper
>> caution
>> >> when opening attachments, clicking links, or responding.
>> >>
>> >>
>> >> On 11/13/25 17:45, Klymenko, Anatoliy wrote:
>> >> > [AMD Official Use Only - AMD Internal Distribution Only]
>> >> >
>> >> > Hi Sean,
>> >> >
>> >> > Thanks a lot for the patch (and tackling the alpha issue in general)
>> >> >
>> >> >> -----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 2/3] drm: zynqmp: Make the video plane primary
>> >> >>
>> >> >> Caution: This message originated from an External Source. Use proper
>> >> caution
>> >> >> when opening attachments, clicking links, or responding.
>> >> >>
>> >> >>
>> >> >> The zynqmp has two planes: "video" and "graphics". The video plane
>> >> >>
>> >> >> - Is on the bottom (zpos=0) (except when chroma keying as the master
>> >> plane)
>> >> >> - Supports "live" input (e.g. from an external source)
>> >> >> - Supports RGB, YUV, and YCbCr formats, including XRGB8888
>> >> >> - Does not support transparency, except via chroma keying (colorkey)
>> >> >> - Must cover the entire screen (translation/resizing not supported)
>> >> >>
>> >> >> The graphics plane
>> >> >>
>> >> >> - Is on the top (zpos=1)
>> >> >> - Supports "live" input (e.g. from an external source)
>> >> >> - Supports RGB and YUV444 formats, but not XRGB8888
>> >> >> - Supports transparency either via
>> >> >> - Global alpha channel, which disables per-pixel alpha when enabled
>> >> >> - Per-pixel alpha, which cannot be used with global alpha
>> >> >> - Chroma keying (colorkey)
>> >> >> - Must cover the entire screen (translation/resizing not supported)
>> >> >>
>> >> >> Currently the graphics plane is the primary plane. Make the video plane
>> >> >> the primary plane:
>> >> >>
>> >> >> - The video plane supports XRGB8888, which is the default 24-bit
>> >> >> colorspace for X. This results in improved performance when compared
>> >> >> to RGB565.
>> >> >> - The graphics plane can be used as an overlay because it has a higher
>> >> >> z-pos and supports a per-pixel alpha channel. Unfortunately, clients
>> >> >> like weston cannot currently take advantage of this because they
>> >> >> expect overlay planes to support translation/resizing.
>> >> >>
>> >> >> One downside to this approach could be that the graphics plane has
>> worse
>> >> >> support for YUV and YCBCr, so it may be more difficult to compose video
>> >> >
>> >> > Not just more difficult but practically impossible:
>> >> > 1. GFX (in Xilinx terminology) plane doesn't support pixel upscaling, so
>> >> > no support for NV12, YUY2 and other common video pixel formats.
>> >> > 2. Both planes are unscalable, this means we can only output native
>> >> > display resolution video on the top plane, or display thick black frame
>> >> > around the picture. We are losing GFX masking capabilities.
>> >>
>> >> The graphics masking capabilities are the same as they always were.
>> >>
>> >> > 3. We won't be able to render subtitles on top of the video.
>> >> > Probably the only practical video player option remains here is to render
>> >> > video to a texture and embed it into graphics composition.
>> >>
>> >> OK, but none of this is currently possible either because of the global
>> >> alpha setting. So do you have a specific program in mind that plays
>> >> video and will do it with an "underlay" plane?
>> >>
>> >
>> > webOS, Roku OS, just to name a few. But most importantly, AMD has quite
>> > a few customers using "underlay" plane because of its YUV 420 capability.
>>
>> Can you link to a specific example where the plane is selected based on
>> its primary/overlay status and not based on its format support? I'm not
>> too familiar with these distros, but e.g. [1] selects the plane based on
>> the format, and doesn't care which one is primary.
>>
>
> The plane selection by the type may not be explicit. For instance, if a
> graphics compositor uses legacy API with drmModeSetCrtc, it would allocate
> the primary plane implicitly. Or some compositor may just rely on planes
> z-order. For instance, here [2] you may see exact z-order WebOS compositor
> expects from the HW planes (both VideoPlane and FullscreenPlane are used
> for the video playback, the graphics lives on MainPlane). Anyways, the
> probability of primary plane being reserved for a graphics compositor is
> too high to ignore.
> Back to example [1], in our case the renderer would fail this check:
> if ((ovr->possible_crtcs & (1 << pipe)) && !ovr->crtc_id)
> for the primary plane pre-occupied by the graphics, and this check:
> if (!format_support(ovr, p->fourcc))
> for the overlay plane lacking pixel upsampling support.
>
> [2] https://github.com/webosose/luna-surfacemanager/blob/b2fef6f73232244f6f13102aa66d2d3f5942835e/modules/weboscompositor/webossurfaceitem.cpp#L1794

Does this matter? The z-order of the planes is fixed in hardware, no
matter what software wishes it were. Regardless, the z-order does not
change with this patch, so nothing should break. And from looking around
in that repo, I cannot find any references to the primary plane, nor can
I see any DRM calls.

Plus, the purpose of the z-order stuff is to support punch-through
rendering. But that can't work on ZynqMP without the following patch to
fix the alpha channel.

--Sean

>> --Sean
>>
>> [1] https://github.com/webosose/omx-
>> components/blob/27b33e17c25023f2c7ffbbfda10169fb500dacdc/src/omxdrm/vid
>> eorenderer.cpp#L469
>>
>> >> >> streams into the window of a media player. However, no existing software
>> >> >> could rely on this because there is no way to enable the per-pixel alpha
>> >> >> channel when the graphics plane is enabled. This makes it impossible to
>> >> >
>> >> > This situation is a driver bug not a fatum.
>> >>
>> >> And yet the driver bug provides ample evidence that no one is doing this,
>> >> so we don't need to worry about it.
>> >>
>> >> --Sean
>> >>
>> >> > Per pixel alpha blending works
>> >> > just fine if we disable global alpha. We just need to avoid enabling it when
>> >> > we have alpha capable GFX plane format.
>> >> >
>> >> >> "carve out" an area in the graphics plane where the video plane shows
>> >> >> through. This limitation is addressed in the next patch, but it means we
>> >> >> do not need to worry about compatibility in this area.
>> >> >>
>> >> >> An alternate approach could be to pretend that the graphics plane
>> >> >> supports XRGB8888 by using the supported ARGB8888 mode instead
>> and
>> >> >> enabling the global alpha channel. However, this would rule out ever
>> >> >> using the per-pixel alpha channel.
>> >> >>
>> >> >> Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
>> >> >> ---
>> >> >>
>> >> >> drivers/gpu/drm/xlnx/zynqmp_kms.c | 6 +++---
>> >> >> 1 file changed, 3 insertions(+), 3 deletions(-)
>> >> >>
>> >> >> diff --git a/drivers/gpu/drm/xlnx/zynqmp_kms.c
>> >> >> b/drivers/gpu/drm/xlnx/zynqmp_kms.c
>> >> >> index c80a2d4034f3..456ada9ac003 100644
>> >> >> --- a/drivers/gpu/drm/xlnx/zynqmp_kms.c
>> >> >> +++ b/drivers/gpu/drm/xlnx/zynqmp_kms.c
>> >> >> @@ -161,8 +161,8 @@ static int zynqmp_dpsub_create_planes(struct
>> >> >> zynqmp_dpsub *dpsub)
>> >> >> if (!formats)
>> >> >> return -ENOMEM;
>> >> >>
>> >> >> - /* Graphics layer is primary, and video layer is overlay. */
>> >> >> - type = i == ZYNQMP_DPSUB_LAYER_VID
>> >> >> + /* Graphics layer is overlay, and video layer is primary. */
>> >> >> + type = i == ZYNQMP_DPSUB_LAYER_GFX
>> >> >> ? DRM_PLANE_TYPE_OVERLAY :
>> >> >> DRM_PLANE_TYPE_PRIMARY;
>> >> >> ret = drm_universal_plane_init(&dpsub->drm->dev, plane, 0,
>> >> >> &zynqmp_dpsub_plane_funcs,
>> >> >> @@ -322,7 +322,7 @@ static const struct drm_crtc_funcs
>> >> >> zynqmp_dpsub_crtc_funcs = {
>> >> >>
>> >> >> static int zynqmp_dpsub_create_crtc(struct zynqmp_dpsub *dpsub)
>> >> >> {
>> >> >> - struct drm_plane *plane = &dpsub->drm-
>> >> >> >planes[ZYNQMP_DPSUB_LAYER_GFX];
>> >> >> + struct drm_plane *plane = &dpsub->drm-
>> >> >> >planes[ZYNQMP_DPSUB_LAYER_VID];
>> >> >> struct drm_crtc *crtc = &dpsub->drm->crtc;
>> >> >> int ret;
>> >> >>
>> >> >> --
>> >> >> 2.35.1.1320.gc452695387.dirty
>> >> >
>> >> > Thank you,
>> >> > Anatoliy
>> >
>> > Thank you,
>> > Anatoliy
>
> Thank you,
> Anatoliy