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

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Date:	Thu, 05 Feb 2009 11:23:36 -0800
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Randy Dunlap <randy.dunlap@...cle.com>,
	Eric Anholt <eric@...olt.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Norbert Preining <preining@...ic.at>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>,
	samr <sam@...nborg.org>
Subject: Re: 2.6.29-rc3-git6: Reported regressions from 2.6.28

Ingo Molnar wrote:
> * Randy Dunlap <randy.dunlap@...cle.com> wrote:
> 
>> Ingo Molnar wrote:
>>> * Randy Dunlap <randy.dunlap@...cle.com> wrote:
>>>
>>>> Eric Anholt wrote:
>>>>> On Wed, 2009-02-04 at 19:56 +0100, Ingo Molnar wrote:
>>>>>> * Linus Torvalds <torvalds@...ux-foundation.org> 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.
>>>> I tried what you had described in that email (from 2 weeks ago), got the
>>>> same results that you did, but kbuild does seem very confused (to me).
>>>>
>>>> reference email from 2+ weeks ago:
>>>>   http://marc.info/?l=linux-kernel&m=123197341316461&w=2
>>>>
>>>> Adding Sam to cc.
>>> Check the patch i posted in this thread earlier today, it solves this 
>>> problem.
>> I saw it.  I'd rather kconfig be fixed instead, if possible.
> 
> kconfig was not broken at all in this case. It detected a circular 
> dependency and did its work well.

Maybe.  I haven't seen an explanation for the problems that Eric
reported 2+ weeks ago.

> ( kconfig is broken in some areas - for example its misfeature of not 
>   propagating selects along dependency chains is annoying. It should at 
>   minimum warn when it sees such partial selects. But this is not one of 
>   those breakages. )


-- 
~Randy
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