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18. How do I convert XFree86 mode-lines into framebuffer device timings?

If you have XFree86 (X11) installed on your machine, and you can use it successfully, it is a simple matter to convert the mode-lines in your XF86Config to the required timings needed by the framebuffer devices.

The framebuffer device requires the following fields

  • pixclock - pixel clock in pico seconds
  • left_margin - time fron sync to picture
  • right_margin - time from picture to sync
  • upper_margin - time from sync to picture
  • lower_margin - time from picture to sync
  • hsync_len - length of horizontal sync
  • vsync_len - length of vertical sync

An XFree86 mode line has the following fields

Modeline  "1280x1024" DCF HR SH1 SH2 HFL VR SV1 SV2 VFL

It is necessary to do some simple calculations to translate the XF86 mode-lines into a set of framebuffer device timings. As an example, we shall examine how to convert a mode-line taken from my XF86Config file.

Modeline  "1280x1024" 110.00 1280 1328 1512 1712 1024 1025 1028 1054

First, calculate the required pixclock rate. XFree86 uses megahertz whilst framebuffer devices uses picoseconds (Why, I don't know). Divide one million by DCF. For example, 1,000,000 / 110.0 = 9090.9091

Now we need to calculate the horizontal timings.

  • left_margin = HFL - SH2
  • right_margin = SH1 - HR
  • hsync_len = SH2 - SH1

In our example, this would be:

  • left_margin = 1712 - 1512 = 200
  • right_margin = 1328 - 1280 = 48
  • hsync_len = 1512 - 1328 = 184

And now we need to calculate the vertical timings.

  • upper_margin = VFL - SV2
  • lower_margin = SV1 - VR
  • vsync_len = SV2 - SV1

For our example, this would be:

  • upper_margin = 1054 - 1028 = 26
  • lower_margin = 1025 - 1024 = 1
  • vsync_len = 1028 - 1025 = 3

Now we can use this information to set up the framebuffer for the desired mode. For example, for the matroxfb framebuffer, it requires:

video=matrox:xres:<>,yres:<>,depth:<>,left:<>,right:<>,hslen:<>,upper:<>,lower:<>,vslen:<>

I put in my /etc/lilo.conf the following line:

append = "video=matrox:xres:1280,yres:1024,depth:32,left:200,right:48,hslen:184,upper:26,lower:0,vslen:3"

Note that in this case the pixclock isn't used. It's only necessary if you don't like the default pixclock rates. You can supply this as a parameter as well. Setting the pixclock is documented in other parts of this HOWTO.


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