Information for Chips and Technologies Users : Troubleshooting
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6. Troubleshooting

The cursor appears as a white box, after switching modes

There is a known bug in the H/W cursor, that sometimes causes the cursor to be redrawn as a white box, when the mode is changed. This can be fixed by moving the cursor to a different region, switching to the console and back again, or if it is too annoying the H/W cursor can be disabled by removing the "hw_cursor" option.

The cursor hot-spot isn't at the same point as the cursor

With modes on the 6555x machines that are stretched to fill the flat panel, the H/W cursor is not correspondingly stretched. This is a small and long-standing bug in the current server. You can avoid this by either using the "no_stretch" option or removing the hw_cursor" option.

The lower part of the screen is corrupted

Many DSTN screens use the top of video ram to implement a frame accelerator. This reduces the amount of video ram available to the modes. The server doesn't prevent the user from specifying a mode that will use this memory, it prints a warning on the console. The effect of this problem will be that the lower part of the screen will reside in the same memory as the frame accelerator and will therefore be corrupt. Try reducing the amount of memory consumed by the mode.

There is a video signal, but the screen doesn't sync.

You are using a mode that your screen cannot handle. If it is a non-standard mode, maybe you need to tweak the timings a bit. If it is a standard mode and frequency that your screen should be able to handle, try to find different timings for a similar mode and frequency combination. For LCD modes, it is possible that your LCD panel requires different panel timings at the text console than with a graphics mode. In this case you will need the "use_modeline" and perhaps also the "fix_panel_size" options to reprogram the LCD panel timings to sensible values.

`Wavy' screen.

Horizontal waving or jittering of the whole screen, continuously (independent from drawing operations). You are probably using a dot clock that is too high (or too low); it is also possible that there is interference with a close MCLK. Try a lower dot clock. For CRT's you can also try to tweak the mode timings; try increasing the second horizontal value somewhat.

Crash or hang after start-up (probably with a black screen).

Try the "noaccel" or "no_bitblt" options. Check that the BIOS settings are OK; in particular, disable caching of 0xa0000-0xaffff. Disabling hidden DRAM refresh may also help.

Hang as the first text is appearing on the screen on SVR4 machines.

This problem has been reported under UnixWare 1.x, but not tracked down. It doesn't occur under UnixWare 2.x and only occurs on the HiQV series of chips. It might affect some other SVR4 operating systems as well. The workaround is to turn off the use of CPU to screen acceleration with the "xaa_no_color_exp" option.

Crash, hang, or trash on the screen after a graphics operation.

This may be related to a bug in one of the accelerated functions, or a problem with the BitBLT engine. Try the "noaccel" or "no_bitblt" options. Also check the BIOS settings. It is also possible that with a high dot clock and depth on a large screen there is very little bandwidth left for using the BitBLT engine. Try reducing the clock.

Chipset is not detected.

Try forcing the chipset to a type that is most similar to what you have.

The screen is blank when starting X

One possible cause of this problem is if the kernel has been compiled with the "APM_DISPLAY_BLANK" option. It appears that this option doesn't work as specified and can cause the Xserver to blank when starting X. In all cases the kernel should be compiled without this option. If the problem remains a CRT/LCD or switch to and from the virtual console will often fix it.

Textmode is not properly restored

This has been reported on some configurations. Many laptops use the programmable clock of the 6554x chips at the console. It is not always possible to find out the setting that is used for this clock if BIOS has written the MClk after the VClk. Hence the server assumes a 25.175MHz clock at the console. This is correct for most modes, but can cause some problems. Usually this is fixed by switching between the LCD and CRT. Alternatively the user can use the "TextClockFreq" option described above to select a different clock for the text console. Another possible cause of this problem is if the kernel is compiled with the "APM_DISPLAY_BLANK" option. As mentioned before, this option should be disabled.

I can't display 640x480 on my 800x600 LCD

The problem here is that the flat panel needs timings that are related to the panel size, and not the mode size. There is no facility in the current Xservers to specify these values, and so the server attempts to read the panel size from the chip. If the user has used the "use_modeline" or "fix_panel_size" options the panel timings are derived from the mode, which can be different than the panel size. Try deleting theses options from XF86Config or using an LCD/CRT switch.

I can't get a 320x240 mode to occupy the whole 640x480 LCD

There is a bug in the 6554x's H/W cursor for modes that are doubled vertically. The lower half of the screen is not accessible. The servers solution to this problem is not to do doubling vertically. Which results in the 320x240 mode only expanded to 640x360. If this is a problem, a work around is to remove the "hw_cursor" option. The server will then allow the mode to occupy the whole 640x480 LCD.

After a suspend/resume my screen is messed up

During a suspend/resume, the BIOS controls what is read and written back to the registers. If the screen is using a mode that BIOS doesn't know about, then there is no guarantee that it will be resumed correctly. For this reason a mode that is as close to VESA like as possible should be selected. It is also possible that the VGA palette can be affected by a suspend/resume. Using an 8bpp, the colour will then be displayed incorrectly. This shouldn't affect higher depths, and is fixable with a switch to the virtual console and back.

The right hand edge of the mode isn't visible on the LCD

This is usually due to a problem with the "lcd-center" option. If this option is removed form XF86Config, then the problem might go away. Alternatively the manufacturer could have incorrectly programmed the panel size in the EGA console mode. The "fix_panel_size" can be used to force the modeline values into the panel size registers. Two machines that are known to have this problem are the "HP OmniBook 5000" and the "NEC Versa 4080".

My TFT screen has a reddish tint in 24bpp mode

The server assumes that the TFT bus width is 24bits. If this is not true then the screen will appear to have a reddish tint. This can be fixed by using the "use_18bit_bus" option. Note that the reverse is also true. If the "use_18bit_bus" is used and the TFT bus width is 24bpp, then the screen will appear reddish. Note that this option only has an effect on TFT screens.

I can't start X-windows with 16, 24 or 32bpp

Firstly, is your machine capable of 16/24/32bpp with the mode specified. Many LCD displays are incapable of using a 24bpp mode. Also you need at least a 65540 to use 16/24bpp and at least a 65550 for 32bpp. The amount of memory used by the mode will be doubled/tripled/quadrupled. The correct options to start the server with these modes are

	  startx -- -bpp 16               5-6-5 RGB ('64K color', XGA)
	  startx -- -bpp 16 -weight 555   5-5-5 RGB ('Hicolor')
	  startx -- -bpp 24               8-8-8 RGB truecolor
or with the HiQV series of chips (6555x, 68554 or 69000) you might try
	  startx -- -bpp 32               8-8-8 RGB truecolor

A general problem with the server that can manifested in many way such as drawing errors, wavy screens, etc is related to the programmable clock. Many potential programmable clock register setting are unstable. However luckily there are many different clock register setting that can give the same or very similar clocks. The clock code can be fooled into giving a different and perhaps more stable clock by simply changing the clock value slightly. For example 65.00MHz might be unstable while 65.10MHz is not. So for unexplained problems not addressed above, please try to alter the clock you are using slightly, say in steps of 0.05MHz and see if the problem goes away. Alternatively, using the "use_vclk1" option with chips later than the 65550 might also help.

For other screen drawing related problems, try the "noaccel" or "no_bitblt" options. A useful trick for all laptop computers is to switch between LCD/CRT (usually with something like Fn-F5), if the screen is having problems.

If you are having driver-related problems that are not addressed by this document, or if you have found bugs in accelerated functions, you can try contacting the XFree86 team (the current driver maintainer can be reached at or Egbert.Eich@Physik.TH-Darmstadt.DE), or post in the Usenet newsgroup "".

Information for Chips and Technologies Users : Troubleshooting
Previous: The Full Story on Clock Limitations
Next: Disclaimer