[Bug 25607] drmCheckModesettingSupported fails if X loaded radeon modeset=1 just before

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=25607

--- Comment #7 from Alex Deucher <agd5f at yahoo.com> 2012-07-30 13:14:07 UTC ---
(In reply to comment #6)
> Userspace is not a problem here, many distributions have GL implementation
> switch tools already.
> 

Userspace is the problem.

> 
> The problem is that xf86-video-ati is built with KMS and radeon.ko loads with
> modeset=1 but still, X fails to load due to the initialization (bug?) issue
> reported by the OP. Restarting X fixes the problem. I do think there is
> something to fix here and it would be nice to at least fix this chicken/egg
> issue.

You need to have the kernel module loaded before starting X.  The same is true
of other KMS drivers (intel, nouveau, etc.).  If the drm is not loaded, the ddx
assumes UMS and tries to load the drm.  If it loads in KMS mode, you don't want
two drivers handling the same hw, so the ddx bails to prevent that.  If you try
and load the drm from X and assume that the kernel is in KMS mode, you break
old systems with old kernels that don't support KMS.  The other problem with
having the ddx load the drm for KMS mode is that with KMS, the drm handles all
of the chip init.  With UMS when you load the drm in UMS mode, the kernel
module  doesn't do anything at load time; it just waits until userspace inits
the hw and actually decides to use the drm.  With KMS, we have no way of
knowing when the KMS drm has finished loading or initializing the hw so we may
end up trying to use the KMS drm before it's properly loaded.  It's not a
configuration we want to support.

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