Created attachment 44743 [details] [review]
xorg.conf

Using xf86-video-ati 6.14.1 + xinerama on

In my triplehead configuration one monitor stayed blank. According to log file no monitors were detected for the second head - instead a default frame buffer of 1024x768 was created and could be seen on whole desktop screenshots.

Managed to track the problem to drmmode_display.c and code block:

                        if (info->IsPrimary && (num != 0))
                                goto out_free_encoders;
                        else if (info->IsSecondary && (num != 1))
                                goto out_free_encoders;


Radeon HD4870 has three connectors: DVI-0, DIN, DVI-1 numbered 0,1,2

Primary screen is supposed to be connected to DVI-0 and secondary to DVI-1, but due to the code block above DVI-1 (i.e. num=2) got skipped and the secondary monitor was not detected.

Adding && (num !=  2) to the else if helped solve my problem for now. Not sure about what this code is actually supposed to do so I'll just leave the actual fixing part to someone else.