Re: 2.6.29-rc3-git6: Reported regressions from 2.6.28

[Eric Anholt]

On Wed, 2009-02-04 at 19:56 +0100, Ingo Molnar wrote:
> * Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Wed, 4 Feb 2009, Norbert Preining wrote:
> > > 
> > > The problem is that if you have a configuration under 2.6.28 without 
> > > CONFIG_FB and just call make oldconfig, or even make config and don't 
> > > know that you loose the DRM. And I was using make oldconfig (there is a 
> > > graphical config?? ;-))
> > 
> > Sure. It's inconvenient, no question about that. I asked the i915 people 
> > to look into not requiring CONFIG_FB, and I hope they will, but my point 
> > is that I don't think we can consider "small one-time inconvenience" to be 
> > a "regression".
> 
> if you mean that as a general principle, there's four very real downsides in 
> my opinion.
> 
> Firstly, we could have done better (and still can do better), via various 
> easy and non-intrusive measures:
> 
>    - We could add a runtime warning:
> 
>       for example a WARN_ONCE("please enable CONFIG_DRM_I915 and CONFIG_FB") 
>       that there's no DRM because CONFIG_FB is not selected and oldconfig 
>       loses the I915 setting silently - placed in a key DRM ioctl, would 
>       have gone a long way addressing the issue. Testers do notice kernel 
>       warnings that pop up when their X gets slow. (This approach might also 
>       have the added bonus of warning folks who enable the wrong driver for 
>       the hardware.)
> 
>    - Or we could add a more thoughtful Kconfig migration:
> 
>       Rename DRM_I915 to DRM_I915_FB [which it really is now], and keep
>       DRM_I915 as a non-interactive migration helper: if set, it 
>       auto-selects both FB and DRM_I915_FB.
> 
>       While CONFIG_FB is an interactive Kconfig option so a select can be 
>       dangerous to a correct dependency tree, it seems safe to do in this 
>       specific case because it seems to be a rather leaf entry with no 
>       dependencies.

I tried select FB.  It's the right thing to do.  It doesn't work.  I
posted to the mailing list two weeks ago about the insane dependency
chain that kbuild comes up with and fails on when we do this, and got
silence.

Believe me, I hate this inconvenience to users even more than each of
you do, because I get to deal with the reports.  But I haven't had the
time to sit down and figure out what drugs kbuild is on, or even how to
work around it (despite IRC help from a few other kernel guys).

The alternative I can see is to ifdef the code for something that will
be on by default and which stable userland will require in 6 months.
That seems wrong.

-- 
Eric Anholt
eric@xxxxxxxxxx                         eric.anholt@xxxxxxxxx

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