(--) PCI: (5:16:0:0) 102b:0522:103c:31fa rev 2, Mem @ 0xbf000000/16
777216, 0xfddfc000/16384, 0xfd000000/8388608, BIOS @ 0x????????/65536
(--) PCI: (7:18:0:0) 1002:7143:1787:3000 rev 0, Mem @ 0xc0000000/26
8435456, 0xfdff0000/65536, I/O @ 0x0000e000/256, BIOS @ 0x????????/131072
(--) PCI: (7:18:0:1) 1002:7163:1787:3001 rev 0, Mem @ 0xfdfe0000/65536

のように PCI device があるとして

1002:7143:1787:3000 Radeon X1300/X1550 Series
x11/modular-xorg-server-1.12.4 Modular X11 server from modular X.org
x11/xf86-video-radeonhd-1.3.0nb2 Modular X.org driver for Radeon HD series
wip/xf86-video-modesetting-0.7.0 Generic modesetting driver

の組合せの場合
sudo X -configure した時

(==) Using system config directory "/usr/pkg/share/X11/xorg.conf.d"
Number of created screens does not match number of detected devices.
  Configuration failed.
Server terminated with error (2). Closing log file.

自分でいろいろ試して用意した xorg.conf を使って sudo xdm とした場合

[  4634.990] (II) RADEONHD: version 1.3.0, built from dist of git branch master,
 commit 8cbff7bf

[  4634.990] (--) Using wscons driver
[  4634.991] (WW) Falling back to old probe method for modesetting
[  4634.991] (II) modesetting(0): using default device
[  4634.991] (WW) VGA arbiter: cannot open kernel arbiter, no multi-card support
[  4634.991] (II) modesetting(0): Creating default Display subsection in Screen 
section                                                                         
        "Screen0" for depth/fbbpp 24/24                                         
[  4634.991] (==) modesetting(0): Depth 24, (==) framebuffer bpp 24             
[  4634.991] (==) modesetting(0): RGB weight 888                                
[  4634.991] (==) modesetting(0): Default visual is TrueColor                   
[  4634.991] (II) modesetting(0): ShadowFB: preferred YES, enabled YES          
[  4634.991] (EE) modesetting(0): KMS doesn't support dumb interface
[  4634.991] (EE) modesetting(0): KMS setup failed                              
[  4634.991] (II) UnloadModule: "modesetting"                                   
[  4634.991] (EE) Screen(s) found, but none have a usable configuration.        
[  4634.991]                                                                    
Fatal server error:                                                             
[  4634.991] no screens found                                                   

某所には 「fglrx を外せ」とあるが、そのようなものは入っていない。 問題の場所は次のところ

m08@makoto 23:41:50/130612(..wip/xf86-video-modesetting)% \
cat -n work/xf86-video-modesetting-0.7.0/src/drmmode_display.c | sed -n 1229,1240p

  1229  Bool drmmode_pre_init(ScrnInfoPtr pScrn, drmmode_ptr drmmode, int cpp)
  1230  {
  1231          int i, num_dvi = 0, num_hdmi = 0;
  1232          int ret;
  1233          uint64_t value = 0;
  1234  
  1235          /* check for dumb capability */
  1236          ret = drmGetCap(drmmode->fd, DRM_CAP_DUMB_BUFFER, &value);
  1237          if (ret > 0 || value != 1) {
  1238                  xf86DrvMsg(pScrn->scrnIndex, X_ERROR, "KMS doesn't support dumb interface\n");
  1239                  return FALSE;
  1240          }

m08@makoto 23:38:38/130612(..x11/libdrm)% 
cat -n work/libdrm-2.4.45/xf86drm.c |sed -n '813,827p'
   813  int drmGetCap(int fd, uint64_t capability, uint64_t *value)
   814  {
   815          struct drm_get_cap cap = { capability, 0 };
   816          int ret;
   817  
   818          ret = drmIoctl(fd, DRM_IOCTL_GET_CAP, &cap);
   819          if (ret)
   820                  return ret;
   821  
   822          *value = cap.value;
   823          return 0;
   824  }

m08@makoto 16:38:17/130617(..x11/libdrm)% \
cat -n work/libdrm-2.4.45/include/drm/
drm.h |sed -n '781,785p'
   781  #define DRM_CAP_DUMB_BUFFER 0x1
   782  #define DRM_CAP_VBLANK_HIGH_CRTC   0x2
   783  #define DRM_CAP_DUMB_PREFERRED_DEPTH 0x3
   784  #define DRM_CAP_DUMB_PREFER_SHADOW 0x4
   785  #define DRM_CAP_PRIME 0x5

08@makoto 16:32:18/130617(..x11/libdrm)% \
cat -n work/libdrm-2.4.45/xf86drm.c |sed -n '158,170p'
   158  /**
   159   * Call ioctl, restarting if it is interupted
   160   */
   161  int
   162  drmIoctl(int fd, unsigned long request, void *arg)
   163  {
   164      int ret;
   165  
   166      do {
   167          ret = ioctl(fd, request, arg);
   168      } while (ret == -1 && (errno == EINTR || errno == EAGAIN));
   169      return ret;
   170  }

m08@makoto 16:44:12/130617(..x11/libdrm)% \
cat -n work/libdrm-2.4.45/include/drm/drm.h |sed -n '656p'
   656  #define DRM_IOCTL_GET_CAP   DRM_IOWR(0x0c, struct drm_get_cap)