[PATCH 2/2] drm: xlnx: zynqmp_kms: set preferred_depth to 16 bpp

Mikko Rapeli
2025-12-05T12:38:34Z

Xorg fails to start with defaults on AMD KV260, /var/log/Xorg.0.log:

[ 23.491] (II) Loading /usr/lib/xorg/modules/drivers/fbdev_drv.so
[ 23.491] (II) Module fbdev: vendor="X.Org Foundation"
[ 23.491] compiled for 1.21.1.18, module version = 0.5.1
[ 23.491] Module class: X.Org Video Driver
[ 23.491] ABI class: X.Org Video Driver, version 25.2
[ 23.491] (II) modesetting: Driver for Modesetting Kernel Drivers:
kms
[ 23.491] (II) FBDEV: driver for framebuffer: fbdev
[ 23.510] (II) modeset(0): using drv /dev/dri/card1
[ 23.511] (WW) Falling back to old probe method for fbdev
[ 23.511] (II) Loading sub module "fbdevhw"
[ 23.511] (II) LoadModule: "fbdevhw"
[ 23.511] (II) Loading /usr/lib/xorg/modules/libfbdevhw.so
[ 23.511] (II) Module fbdevhw: vendor="X.Org Foundation"
[ 23.511] compiled for 1.21.1.18, module version = 0.0.2
[ 23.511] ABI class: X.Org Video Driver, version 25.2
[ 23.512] (II) modeset(0): Creating default Display subsection in
Screen section
"Default Screen Section" for depth/fbbpp 24/32
[ 23.512] (==) modeset(0): Depth 24, (==) framebuffer bpp 32
[ 23.512] (==) modeset(0): RGB weight 888
[ 23.512] (==) modeset(0): Default visual is TrueColor
...
[ 23.911] (II) Loading sub module "fb"
[ 23.911] (II) LoadModule: "fb"
[ 23.911] (II) Module "fb" already built-in
[ 23.911] (II) UnloadModule: "fbdev"
[ 23.911] (II) Unloading fbdev
[ 23.912] (II) UnloadSubModule: "fbdevhw"
[ 23.912] (II) Unloading fbdevhw
[ 24.238] (==) modeset(0): Backing store enabled
[ 24.238] (==) modeset(0): Silken mouse enabled
[ 24.249] (II) modeset(0): Initializing kms color map for depth 24, 8
bpc.
[ 24.250] (==) modeset(0): DPMS enabled
[ 24.250] (II) modeset(0): [DRI2] Setup complete
[ 24.250] (II) modeset(0): [DRI2] DRI driver: kms_swrast
[ 24.250] (II) modeset(0): [DRI2] VDPAU driver: kms_swrast
...
[ 24.770] (II) modeset(0): Disabling kernel dirty updates, not
required.
[ 24.770] (EE) modeset(0): failed to set mode: Invalid argument

xorg tries to use 24 and 32 bpp which pass on the fb API but which
don't actually work on AMD KV260 when Xorg starts. As a workaround
Xorg config can set color depth to 16 using /etc/X11/xorg.conf snippet:

Section "Screen"
Identifier "Default Screen"
Monitor "Configured Monitor"
Device "Configured Video Device"
DefaultDepth 16
EndSection

But this is cumbersome on images meant for multiple different arm64
devices and boards. So instead set 16 bpp as preferred depth
in zynqmp_kms fb driver which is used by Xorg in the logic to find
out a working depth.

Now Xorg startup and bpp query using fb API works and HDMI display
shows graphics. /var/log/Xorg.0.log shows:

[ 23.219] (II) LoadModule: "fbdev"
[ 23.219] (II) Loading /usr/lib/xorg/modules/drivers/fbdev_drv.so
[ 23.219] (II) Module fbdev: vendor="X.Org Foundation"
[ 23.219] compiled for 1.21.1.18, module version = 0.5.1
[ 23.219] Module class: X.Org Video Driver
[ 23.219] ABI class: X.Org Video Driver, version 25.2
[ 23.219] (II) modesetting: Driver for Modesetting Kernel Drivers:
kms
[ 23.219] (II) FBDEV: driver for framebuffer: fbdev
[ 23.238] (II) modeset(0): using drv /dev/dri/card1
[ 23.238] (WW) Falling back to old probe method for fbdev
[ 23.238] (II) Loading sub module "fbdevhw"
[ 23.238] (II) LoadModule: "fbdevhw"
[ 23.239] (II) Loading /usr/lib/xorg/modules/libfbdevhw.so
[ 23.239] (II) Module fbdevhw: vendor="X.Org Foundation"
[ 23.239] compiled for 1.21.1.18, module version = 0.0.2
[ 23.239] ABI class: X.Org Video Driver, version 25.2
[ 23.240] (II) modeset(0): Creating default Display subsection in Screen section
"Default Screen Section" for depth/fbbpp 16/16
[ 23.240] (==) modeset(0): Depth 16, (==) framebuffer bpp 16
[ 23.240] (==) modeset(0): RGB weight 565
[ 23.240] (==) modeset(0): Default visual is TrueColor
...
[ 24.015] (==) modeset(0): Backing store enabled
[ 24.015] (==) modeset(0): Silken mouse enabled
[ 24.027] (II) modeset(0): Initializing kms color map for depth 16, 6 bpc.
[ 24.028] (==) modeset(0): DPMS enabled
[ 24.028] (II) modeset(0): [DRI2] Setup complete
[ 24.028] (II) modeset(0): [DRI2] DRI driver: kms_swrast
[ 24.028] (II) modeset(0): [DRI2] VDPAU driver: kms_swrast

Cc: Bill Mills <bill.mills@linaro.org>
Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Cc: Anatoliy Klymenko <anatoliy.klymenko@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
---
drivers/gpu/drm/xlnx/zynqmp_kms.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/drivers/gpu/drm/xlnx/zynqmp_kms.c b/drivers/gpu/drm/xlnx/zynqmp_kms.c
index ccc35cacd10cb..a42192c827af0 100644
--- a/drivers/gpu/drm/xlnx/zynqmp_kms.c
+++ b/drivers/gpu/drm/xlnx/zynqmp_kms.c
@@ -506,6 +506,7 @@ int zynqmp_dpsub_drm_init(struct zynqmp_dpsub *dpsub)
drm->mode_config.min_height = 0;
drm->mode_config.max_width = ZYNQMP_DISP_MAX_WIDTH;
drm->mode_config.max_height = ZYNQMP_DISP_MAX_HEIGHT;
+ drm->mode_config.preferred_depth = 16;

ret = drm_vblank_init(drm, 1);
if (ret)
--
2.34.1