Red Hat Bugzilla – Bug 81015
radeon driver refuses to use dotclocks < 20Mhz
Last modified: 2007-11-30 17:10:31 EST
Description of problem:
When using the radeon driver, modelines with dotclocks of less than 20 Mhz are
ignored. Using a tweaked driver (from the .src.rpm) which ignores this
limitation, the card uses modelines with lower dotclocks without any hassles.
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. Put a modeline with a dotclock < 20 Mhz into XF86Config, use it
Modeline gets ignored due to too low dotclock (according to /var/log/XFree86.*.log)
Modeline gets used or (if the card is lying about its capabilities) a
configuration option should exist which overrides the limitd determined by XFree86.
Sample modeline to reproduce?
Modeline "720x576" 13.875 720 744 808 888 576 582 588 625 -hsync -vsync
interlace # H 15625 [Hz], V 50 [Hz]
That's 720x576 PAL, let me know if you want an NTSC one -- I have some NTSC
modelines, but they're not tested.
Created attachment 89971 [details]
Patch that ensures a minimum dotclock of >= 12.5 Mhz
Do not use this at home kids. It might break your VGA card, your monitor or
Seriously: This patch forces the driver to use a minimum dotlock of 12.5 Mhz
(or less). I don't know if there are any Radeons that can't handle that (mine
does) and how they would react in that case. This should preferably be a
configuration option ('Option "EnsureTVDotClocks" "yes"'? -- we might have to
ensure that the max dotclock is at minimum 17.5 Mhz -- yeah right :o)), but I
don't know how to implement such options and I'm currently to busy/lazy to
This type of thing would be a feature enhancement, and not really a bug
fix. Due to the potentially hardware-damaging nature of such a change,
I won't consider something like this for our current development. It's
IMHO something that should be requested upstream in order to be given
Setting to DEFERRED
Created attachment 95161 [details]
Patch implementing an option to force a specific minimum dotclock on Radeon cards
This patch implements the "ForceMinDotclockenstein" option in the Radeon
driver. It accepts a frequency as its argument (like in "12.5Mhz"). This patch
documents the option in the radeon man page.
Created attachment 95511 [details]
Patch implementing an option to force a specific minimum dotclock on Radeon
Revised patch as suggested by Kevin Martin:
- rename option to "ForceMinDotClock"
- fix code duplication
File this patch upstream, deferred.
Fixed in xorg-x11-6.7.0-0.5.