Hi Guanqion Xie,
We modified mxc_hdmi_edid_rebuild_modelist function in drivers/video/mxc/mxc_hdmi.c as following.
fb_destroy_modelist(&hdmi->fbi->modelist);
fb_add_videomode(&vga_mode, &hdmi->fbi->modelist);
fb_add_videomode(&wxga_mode, &hdmi->fbi->modelist); <- add
fb_add_videomode(&wuxga_mode, &hdmi->fbi->modelist); <- add
After adding the above code, user will be able to change HDMI resolation to 1280x800 by the following command after Linux BSP boot.

Best Regards,
Satoshi Shimoda