configs/stm32f469_disco: enable Linux DRM support

The patch [1], which is required for DRM to work properly, has been
merged into Linux version 5.14-rc3. There's a 5.14.12 now in stable, so
I took that one.

After Linux boots, /dev/fb0 will be accessible. You can control the
brightness of the display after enabling the framebuffer by running the
following commands:

~ # echo 0 0 > /sys/class/graphics/fb0/pan
~ # echo 255 >/sys/class/backlight/40016c00.dsi.0/brightness

The brightness ranges from 0 to 255, as you can see running the
command:

~ # cat /sys/class/backlight/40016c00.dsi.0/max_brightness

[1] 24b5b1978cd5 ("clk: stm32f4: fix post divisor setup for I2S/SAI PLLs")

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Acked-by: Christophe Priouzeau <christophe.priouzeau@foss.st.com>
[Arnout: add explanation to readme.txt]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

board/stmicroelectronics/stm32f469-disco/linux.fragment

@@ -1 +1,7 @@
-# CONFIG_XIP_KERNEL is not set
+# CONFIG_XIP_KERNEL is not set
+CONFIG_DRM=y
+CONFIG_DRM_STM=y
+CONFIG_DRM_STM_DSI=y
+CONFIG_DRM_PANEL_ORISETECH_OTM8009A=y
+CONFIG_FB=y
+CONFIG_BACKLIGHT_CLASS_DEVICE=y

board/stmicroelectronics/stm32f469-disco/readme.txt

@@ -29,3 +29,17 @@ ready to be dumped on a SD card. Launch the following command as root:
 
 For details about the medium image layout and its content, see the
 definition in board/stmicroelectronics/stm32f469-disco/genimage.cfg.
+
+Framebuffer
+-----------
+After Linux boots, /dev/fb0 will be accessible. You can control the
+brightness of the display after enabling the framebuffer by running the
+following commands:
+
+   # echo 0 0 > /sys/class/graphics/fb0/pan
+   # echo 255 >/sys/class/backlight/40016c00.dsi.0/brightness
+
+The brightness ranges from 0 to 255, as you can see running the
+command:
+
+   # cat /sys/class/backlight/40016c00.dsi.0/max_brightness

configs/stm32f469_disco_defconfig

@@ -1,12 +1,12 @@
 BR2_arm=y
 BR2_cortex_m4=y
-BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_5_12=y
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_5_14=y
 BR2_ROOTFS_POST_BUILD_SCRIPT="board/stmicroelectronics/common/stm32f4xx/stm32-post-build.sh board/stmicroelectronics/stm32f469-disco/post-build.sh"
 BR2_ROOTFS_POST_IMAGE_SCRIPT="support/scripts/genimage.sh"
 BR2_ROOTFS_POST_SCRIPT_ARGS="-c board/stmicroelectronics/stm32f469-disco/genimage.cfg"
 BR2_LINUX_KERNEL=y
 BR2_LINUX_KERNEL_CUSTOM_VERSION=y
-BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="5.12.11"
+BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="5.14.12"
 BR2_LINUX_KERNEL_DEFCONFIG="stm32"
 BR2_LINUX_KERNEL_CONFIG_FRAGMENT_FILES="$(LINUX_DIR)/arch/arm/configs/dram_0x00000000.config board/stmicroelectronics/stm32f469-disco/linux.fragment"
 BR2_LINUX_KERNEL_IMAGE_TARGET_CUSTOM=y