Hi Valery,
Already googled the Linux Framebuffer HOWTO and several articles. It seems that vesafb will not work for me (I have Matrox G400 card, on which I get a message "Cannot set vga mode, press Enter..." when booting Linux. Also, it supports VESA 2.0, but GRUB sees only modes not higher than 800x600). I also found some drivers in ./kernel/drivers/video/matrox, will check them.
what is your incentive to use the fbdev backend of libSDL? Most of the time, we are using the Linux version of Genode with X11, which is very convenient during development. (for deployment, we would not recommend the Linux version) If your graphics card is supported by X11, there is no need to fiddle with fbdev drivers of the Linux kernel.
However, from time to time, we have used libSDL directly with fbdev. So once you get the Linux driver for your graphics card running, Genode should work as expected.
Cheers Norman