meta-lxatac-bsp: linux-lxatac: add workaround for broken fbdev emulation

Starting with commit 86634fa4e6aef ("Merge v6.10-rc6 into drm-next") the
tacd fails to write to the /dev/fb0 device with a SIGBUS error.

This merge commit is the first affected commit because it brings together
these two commits for the first time:

- commit 9317ff69170d3 ("drm/tiny/panel-mipi-dbi: Use fbdev-dma")
- commit d92a7580392ad ("drm/fbdev-dma: Only set smem_start is enable per
   module option")

The first one changes panel-mipi-dbi from being a `drm_fbdev_generic`
device to being a `drm_fbdev_dma` device.
The second one prevents `drm_fbdev_dma` devices from leaking the memory
address of the framebuffer to userspace unless the
`drm_kms_helper.drm_leak_fbdev_smem=1` kernel commandline option is set.

This being an issue is likely a bug in the panel-mipi-dbi driver that
should be fixed there, but this workaround makes sure we can use 6.11
until the driver is fixed.

Signed-off-by: Leonard Göhrs <[email protected]>

meta-lxatac-bsp/recipes-core/images/lxatac-core-image-base.bbappend

@@ -1,7 +1,14 @@
 inherit bootspec
 
 BOOTSPEC_EXTRALINE = "linux-appendroot true\n"
-BOOTSPEC_OPTIONS += "rootwait"
+
+# The drm_leak_fbdev_smem option is a workaround required since Linux 6.11
+# because without it the tacd will crash when attempting to write to the
+# framebuffer. This is likely a bug in the panel-mipi-dbi driver or the tacd
+# that should be fixed there.
+# Once drm_leak_fbdev_smem is removed the kernel config option
+# CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM should be disabled as well.
+BOOTSPEC_OPTIONS += "rootwait drm_kms_helper.drm_leak_fbdev_smem=1"
 
 IMAGE_INSTALL:append = "\
     kernel-image \

meta-lxatac-bsp/recipes-kernel/linux/files/defconfig

@@ -3826,7 +3826,7 @@ CONFIG_DRM_KMS_HELPER=y
 # CONFIG_DRM_DEBUG_MODESET_LOCK is not set
 CONFIG_DRM_FBDEV_EMULATION=y
 CONFIG_DRM_FBDEV_OVERALLOC=100
-# CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM is not set
+CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM=y
 # CONFIG_DRM_LOAD_EDID_FIRMWARE is not set
 CONFIG_DRM_GEM_DMA_HELPER=y