guide: add Kernel mode setting for display resolution

Signed-off-by: Dongjin Kim <tobetter@gmail.com>
Change-Id: I1d08f196cb391701b56820ed960610fa2cce5db3

guides/kmsdrm.md

@@ -0,0 +1,42 @@
+# Kernel mode setting
+
+Kernel mode setting (KMS) is a method to set the display resolution in the
+Linux kernel during boot up, this method also effectly work when:
+* you want to set the display mode with specific resolution
+* the display device is old
+* EDID data is corrupted or it contains incorrect one
+
+
+## Prerequisites
+### EDID binary
+
+You need to download and install the EDID binary file to your Linux. Please open
+the link to find out the supported EDID binaries.
+https://github.com/akatrevorjay/edid-generator
+
+Please run these commands to download the display resolution for 1920x1080.
+```bash
+$ sudo mkdir -p /usr/lib/firmware/edid/
+$ sudo wget https://github.com/akatrevorjay/edid-generator/blob/master/1920x1080.bin -P /usr/lib/firmware/edid
+```
+## Apply new resolution
+In order to apply the new display resolution with this method, the boot script
+must be updated with a kernel parameter that specify the display resoltuion so that
+the Linux kernel loads the EDID binary on booting.
+
+Add this line to **/usr/share/flash-kernel/ubootenv.d/upstream/90-misc** to select the display resolution as 1920x1080.
+> setenv bootargs "${bootargs} drm_kms_helper.edid_firmware=HDMI-A-1:edid/1920x1080.bin"
+
+So, the file **/usr/share/flash-kernel/ubootenv.d/upstream/90-misc** must be like this.
+```bash
+$ cat /usr/share/flash-kernel/ubootenv.d/upstream/90-misc 
+setenv bootargs "${bootargs} cma=800M"
+setenv bootargs "${bootargs} clk_ignore_unused"
+setenv bootargs "${bootargs} drm_kms_helper.edid_firmware=HDMI-A-1:edid/1920x1080.bin"
+```
+
+Then update the boot script to apply the change and reboot.
+```bash
+$ sudo update-bootscript
+$ sudo reboot
+```