I'm trying to use fbcp to mirror /dev/fb0 (the built in framebuffer) to /dev/fb1 however i just cannot get a picture with the native resolution of /dev/fb1 (320x240). I tried different settings in /boot/config.txt (i'm only stating the difference from default):
framebuffer_width=320, framebuffer_height=240 disable_overscan=1
Kernel command line: ... bcm2708_fb.fbwidth=320 bcm2708_fb.fbheight=240 fbcp: Primary display is 720 x 480
and a second try:
disable_overscan=1 hdmi_ignore_edid=0xa5000080 hdmi_group=1 hdmi_mode=13 # 240p
Kernel command line: ... bcm2708_fb.fbwidth=720 bcm2708_fb.fbheight=240 fbcp: Primary display is 720 x 240
All this seems very inconsistent to me.
Edit: this might actually be a firmware bug. I have submitted a bug-report here: https://github.com/raspberrypi/firmware/issues/409