Comment 3 for bug 1552693

$ cat /var/log/Xorg.0.log | grep RADEON
[ 565.640] (II) RADEON: Driver for ATI Radeon chipsets:
 ATI Mobility RADEON HD 4850, ATI Mobility RADEON HD 4850 X2,
 ATI Mobility RADEON HD 4870, ATI Mobility RADEON M98,
 ATI Mobility RADEON HD 4870, ATI Radeon 4800 Series,
 ATI RV730XT [Radeon HD 4670], ATI RADEON E4600,
 ATI Mobility Radeon HD 2400, ATI RADEON E2400, ATI RV610,
[ 565.644] (II) RADEON(0): Creating default Display subsection in Screen section
[ 565.644] (==) RADEON(0): Depth 24, (--) framebuffer bpp 32
[ 565.644] (II) RADEON(0): Pixel depth = 24 bits stored in 4 bytes (32 bpp pixmaps)
[ 565.644] (==) RADEON(0): Default visual is TrueColor
[ 565.644] (==) RADEON(0): RGB weight 888
[ 565.644] (II) RADEON(0): Using 8 bits per RGB (8 bit DAC)
[ 565.644] (--) RADEON(0): Chipset: "TAHITI" (ChipID = 0x679a)
[ 565.656] (II) RADEON(0): glamor detected, initialising EGL layer.
[ 565.656] (II) RADEON(0): KMS Color Tiling: enabled
[ 565.656] (II) RADEON(0): KMS Color Tiling 2D: enabled
[ 565.656] (II) RADEON(0): KMS Pageflipping: enabled
[ 565.656] (II) RADEON(0): SwapBuffers wait for vsync: enabled
[ 565.664] (II) RADEON(0): Output DisplayPort-0 has no monitor section
[ 565.682] (II) RADEON(0): Output DisplayPort-1 has no monitor section
[ 565.683] (II) RADEON(0): Output HDMI-0 has no monitor section
[ 565.696] (II) RADEON(0): Output DVI-0 has no monitor section
[ 565.704] (II) RADEON(0): EDID for output DisplayPort-0
[ 565.722] (II) RADEON(0): EDID for output DisplayPort-1
[ 565.722] (II) RADEON(0): Printing probed modes for output DisplayPort-1
[ 565.722] (II) RADEON(0): Modeline "1024x768"x60.0 65.00 1024 1048 1184 1344 768 771 777 806 -hsync -vsync (48.4 kHz e)
[ 565.722] (II) RADEON(0): Modeline "800x600"x60.3 40.00 800 840 968 1056 600 601 605 628 +hsync +vsync (37.9 kHz e)
[ 565.722] (II) RADEON(0): Modeline "800x600"x56.2 36.00 800 824 896 1024 600 601 603 625 +hsync +vsync (35.2 kHz e)
[ 565.722] (II) RADEON(0): Modeline "848x480"x60.0 33.75 848 864 976 1088 480 486 494 517 +hsync +vsync (31.0 kHz e)
[ 565.722] (II) RADEON(0): Modeline "640x480"x59.9 25.18 640 656 752 800 480 490 492 525 -hsync -vsync (31.5 kHz e)
[ 565.723] (II) RADEON(0): EDID for output HDMI-0
[ 565.736] (II) RADEON(0): EDID for output DVI-0
[ 565.736] (II) RADEON(0): Output DisplayPort-0 disconnected
[ 565.736] (II) RADEON(0): Output DisplayPort-1 connected
[ 565.736] (II) RADEON(0): Output HDMI-0 disconnected
[ 565.736] (II) RADEON(0): Output DVI-0 disconnected
[ 565.736] (II) RADEON(0): Using exact sizes for initial modes
[ 565.736] (II) RADEON(0): Output DisplayPort-1 using initial mode 1024x768 +0+0
[ 565.736] (II) RADEON(0): Using default gamma of (1.0, 1.0, 1.0) unless otherwise stated.
[ 565.736] (II) RADEON(0): mem size init: gart size :7fbcc000 vram size: s:c0000000 visible:beda6000
[ 565.736] (==) RADEON(0): DPI set to (96, 96)
[ 565.736] (II) RADEON(0): [DRI2] Setup complete
[ 565.736] (II) RADEON(0): [DRI2] DRI driver: radeonsi
[ 565.736] (II) RADEON(0): [DRI2] VDPAU driver: radeonsi
[ 565.736] (II) RADEON(0): Front buffer size: 3072K
[ 565.736] (II) RADEON(0): VRAM usage limit set to 2811391K
[ 565.736] (==) RADEON(0): DRI3 disabled
[ 565.736] (==) RADEON(0): Backing store enabled
[ 565.736] (II) RADEON(0): Direct rendering enabled
[ 565.759] (II) RADEON(0): Use GLAMOR acceleration.
[ 565.759] (II) RADEON(0): Acceleration enabled
[ 565.759] (==) RADEON(0): DPMS enabled
[ 565.759] (==) RADEON(0): Silken mouse enabled
[ 565.759] (II) RADEON(0): Set up textured video (glamor)
[ 565.759] (II) RADEON(0): [XvMC] Associated with GLAMOR Textured Video.
[ 565.759] (II) RADEON(0): [XvMC] Extension initialized.
[ 565.759] (II) RADEON(0): RandR 1.2 enabled, ignore the following RandR disabled message.
[ 565.763] (II) RADEON(0): Setting screen physical size to 270 x 203

From the log we can see that probe modes only find low resolutions.