Dual Display Channel

Information for Chips and Technologies Users : Dual Display Channel
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## 5. Dual Display Channel

XFree86 releases later than 4.1.0 support dual-channel display on the ct69030. This support can be used to give a single display image on two screen with different refresh rates, or entirely different images on the two displays.

Dual refresh rate display can be selected with the "`DualRefresh`" option described above. However to use the dual-head support is slightly more complex. Firstly, the ct69030 chipset must be installed on a PCI bus. This is a driver limitation that might be relaxed in the future. In addition the device, screen and layout sections of the "`XF86Config`" must be correctly configured. A sample of an incomplete "`XF86Config`" is given below

>     Section "Device"
>         Identifier  "Chips and Technologies - Pipe A"
>         Driver      "chips"
>         BusID       "PCI:0:20:0"
>         Screen      0
>     EndSection

>     Section "Device"
>         Identifier  "Chips and Technologies - Pipe B"
>         Driver      "chips"
>         BusID       "PCI:0:20:0"
>         Screen      1
>     EndSection

>     Section "Screen"
>         Identifier  "Screen 0"
>         Device      "Chips and Technologies - Pipe A"
>         Monitor     "generic LCD"

>         SubSection "Display"
>             Depth           16
>             Modes           "1024x768"
>         EndSubsection
>     EndSection

>     Section "Screen"
>         Identifier  "Screen 1"
>         Device      "Chips and Technologies - Pipe B"
>         Monitor     "generic CRT"

>         SubSection "Display"
>             Depth           16
>             Modes           "1024x768"
>         EndSubsection
>     EndSection

>     Section "ServerLayout"
>         Identifier  "Main Layout"
>         Screen      "Screen 0"
>         Screen      "Screen 1" RightOf "Screen 0"
>         InputDevice "Mouse1" "CorePointer"
>         InputDevice "Keyboard1" "CoreKeyboard"
>     EndSection

The device section must include the PCI BusID. This can be found from the log file of a working single-head installation. For instance, the line

>     (--) PCI:*(0:20:0) C&T 69030 rev 97, Mem @ 0xed000000/24

appears for the case above. Additionally, the "`Screen`" option must appear in the device section. It should be noted that if a flat panel is used, this it must be allocated to "`Screen 0`".

The server can then be started with the "`+xinerama`" option as follows

>     startx -- +xinerama

For more information, read the Xinerama documentation.

It should be noted that the dual channel display options of the 69030 require the use of additional memory bandwidth, as each display channel independently accesses the video memory. For this reason, the maximum colour depth and resolution that can be supported in a dual channel mode will be reduced compared to a single display channel mode. However, as the driver does not prevent you from using a mode that will exceed the memory bandwidth of the 69030, but a warning like

>     (WW) Memory bandwidth requirements exceeded by dual-channel
>     (WW)    mode. Display might be corrupted!!!

If you see such display corruption, and you have this warning, your choices are to reduce the refresh rate, colour depth or resolution, or increase the speed of the memory clock with the the "`SetMClk`" option described above. Note that increasing the memory clock also has its own problems as described above.

Information for Chips and Technologies Users : Dual Display Channel
Previous: Modelines
Next: The Full Story on Clock Limitations