fb (5) - Linux Manuals

fb: frame buffer modes file


fb.modes - frame buffer modes file


/etc/fb.modes contains an unlimited number of video mode descriptions. The general format of a video mode is:


geometry <xres> <yres> <vxres> <vyres> <depth>
timings <pixclock> <left> <right> <upper> <lower> <hslen> <vslen>
options <value>


geometry options:
visible horizontal resolution (in pixels)
visible vertical resolution (in pixels)
virtual horizontal resolution (in pixels)
virtual vertical resolution (in pixels)
display depth (in bits per pixel)

timing options:

length of one pixel (in picoseconds)
left margin (in pixels)
right margin (in pixels)
upper margin (in pixel lines)
lower margin (in pixel lines)
horizontal sync length (in pixels)
vertical sync length (in pixel lines)

other options:

the first value of this options is the default
hsync {low|high}
the horizontal sync polarity
vsync {low|high}
the vertical sync polarity
csync {low|high}
the composite sync polarity
extsync {false|true}
enable or disable external resync. If enabled the sync timings are not generated by the frame buffer device and must be provided externally instead. Note that this option may not be supported by every frame buffer device
laced {false|true}
enable or disable interlace. If enabled the display will be split in two frames, each frame contains only even and odd lines respectively. These two frames will be displayed alternating, this way twice the lines can be displayed and the vertical frequency for monitor stays the same, but the visible vertical frequency gets halved
double {false|true}
enable or disable doublescan. If enabled every line will be displayed twice and this way the horizontal frequency can easily be doubled, so that the same resolution can be displayed on different monitors, even if the horizontal frequency specification differs. Note that this option may not be supported by every frame buffer device


Generally a frame buffer display is organized as follows:

        ^           |
        | 5         |
        v           |
        ^           |
        |           |
        |           |
| 1 # | 2 # 3 | 4 |
        |           |
        | 6         |
        |           |
        v           |
        ^           |
        | 7         |
        v           |
        ^           |
        | 8         |
        v           |

1 left margin
2 xres
3 right margin
4 horizontal sync len
5 upper margin
6 yres
7 lower margin
8 vertical sync len

The area bordered with `#' is the visible display area. Horizontal and vertical frequencies can now easily be calculated, for this the sum of horizontal or vertical values are important

htotal left xres right hslen
vtotal upper yres lower vslen

The length of one line can now be calculated with pixclock

line pixclock htotal

and we have the horizontal frequency

hfreq 1E12 line 1E12 (pixclock htotal)

To get the vertical frequency vtotal must eventually adjusted. If the display is laced, vtotal must be halved or if the display is a doublescan one, vtotal must be doubled. Now we can calculate the length of one frame

if (lace)   vtotal /= 2
if (double) vtotal *= 2

frame vtotal line

and we get also the vertical frequency

vfreq 1E12 frame hfreq vtotal


fbset(8), fbdev(4)