[ubuntu-x] Debugging KMS (and thoughts thereof)

Geir Ove Myhr gomyhr at gmail.com
Wed Mar 17 07:49:57 UTC 2010

On Wed, Mar 17, 2010 at 2:44 AM, Bryce Harrington <bryce at canonical.com> wrote:
> Resolution Selection Fallback Issues
> ------------------------------------
> Sometimes the issue is that some problem is causing the modesetting
> logic to fail to find a suitable resolution and it goes into various
> fallbacks.  These sometimes make unsatisfactory choices in the hope of
> picking a safe default.  Usually we can spot this going on by looking at
> the Xorg.0.log, where it prints out information about each resolution
> it's considering and what timings it sees, and why it didn't pick it.
> This same thing must be going on in the kernel, but I don't see anything
> in dmesg or syslog about it, and I don't see much in the way of printk's
> in the edid drm code.  You may find it useful to add some verbosity in
> here so that there's more details in log files about why it is not
> picking resolutions.

I still see resolution picking in Xorg.0.log, even with KMS, at least
on intel. In order to make the kernel more verbose about what it's
doing, the kernel option drm.debug=0x04 can be used. It will enable
the macro DRM_DEBUG_KMS which should tell what is going on both for
resolution picking and output detection. The drm debug levels are
defined in include/drm/drmP.h [1]. I think I have also seen upstream
asking for Option ModeDebug in xorg.conf again, which used to be more
verbose about the modesetting in UMS, so possibly they have made this
work with KMS as well now, but I haven't tested this (again, this is
on intel graphics).

[1]: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=include/drm/drmP.h;h=4a3c4e441027be39da3d37b8a817ff9f918661dd;hb=HEAD

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