[PATCH 0/8] drm, fbdev: rework dependencies

[Geert Uytterhoeven]

Hi Jani,

On Tue, Apr 21, 2020 at 2:58 PM Jani Nikula <jani.nikula at linux.intel.com> wrote:
> On Tue, 21 Apr 2020, Daniel Vetter <daniel at ffwll.ch> wrote:
> > On Mon, Apr 20, 2020 at 04:03:23PM +0200, Arnd Bergmann wrote:
> >> On Mon, Apr 20, 2020 at 10:14 AM Jani Nikula
> >> <jani.nikula at linux.intel.com> wrote:
> >> > On Fri, 17 Apr 2020, Jason Gunthorpe <jgg at ziepe.ca> wrote:
> >> > > On Fri, Apr 17, 2020 at 07:14:53PM +0200, Daniel Vetter wrote:
> >> > >> On Fri, Apr 17, 2020 at 05:55:45PM +0200, Arnd Bergmann wrote:
> >> > >> >
> >> > >> > If we can agree on these changes, maybe someone can merge them
> >> > >> > through the drm-misc tree.
> >> > >> >
> >> > >> > Please review
> >> > >>
> >> > >> Biggest concern I have is that usability of make menuconfig is horrible,
> >>
> >> No doubt about that, but that seems to be unrelated to the cleanup.
> >>
> >> > >> and it's very hard to find options that are hidden by depends on. You can
> >> > >> use the search interface, if you happen to know the option.
> >> > >>
> >> > >> Once you've surmounted that bar, the next one is trying to find what
> >> > >> exactly you need to enable. Which again means endless of recursive
> >> > >> screaming at Kconfig files, since make menuconfig doesn't help you at all.
> >>
> >> The changes I'm doing are mostly for fbdev, which is currently the
> >> odd one out. Most kernel subsystems today follow the documented
> >> recommendations and only use 'depends on' for things they
> >> depend on.
> >>
> >> Having fbdev be the exception causes two problems:
> >>
> >> - It does not make kconfig any easier to use overall, just less consistent
> >>   when it is the only thing that implicitly turns on dependencies and
> >>   for everything else one still has to look up what the dependencies are.
> >>
> >> - Most of the problems with circular dependencies come from mixing
> >>   the two methods, and most of the cases where they have caused
> >>   problems in the past involve fbdev in some way.
> >>
> >> I also doubt switching lots of 'depends on' to 'select' all over Kconfig
> >> would improve the situation on a global level. It would simplify the
> >> problem of turning something on without understanding the what it
> >> does, but in turn it makes it harder to turn off something else.
> >>
> >> E.g. today it is hard to turn off fbdev because that is selected by a
> >> number of (partly unrelated) options, but there was a recent discussion
> >> about getting distros to stop enabling fbdev out of security concerns.
> >
> > I've done some history digging, this is the patch that started this all:
> >
> > commit d2f59357700487a8b944f4f7777d1e97cf5ea2ed
> > Author: Ingo Molnar <mingo at elte.hu>
> > Date:   Thu Feb 5 16:03:34 2009 +0100
> >
> >     drm/i915: select framebuffer support automatically
> >
> > I.e. driver gets disabled because a new config is added which isn't
> > enabled. System doesn't boot, maintainer gets angry regression report,
> > select hack gets added.
>
> Gotta love a good commit message from a decade ago.
>
> First, it says it's a migration helper. And that the problem
> specifically is that the user has a working config *without* FB enabled
> as a starting point.
>
> Now, if the starting point for a new config *now* is less than ten years
> old, and it had i915 enabled, it'll also have FB enabled. Because
> select. The migration part has done its job, and I think we should be
> good to make some progress.

It will indeed work with "make oldconfig", as an old config with
CONFIG_DRM_I915 enabled will have CONFIG_FB set.

However, that is not true when starting with a defconfig that has
CONFIG_DRM_I915 enabled: such a defconfig will not have CONFIG_FB set,
due to the trimming process when creating a minimal defconfig.

Hence when making the change from "select FB" to "depends on FB", you
have to make sure to update the affected defconfigs, too:

$ git grep CONFIG_DRM_I915 -- "*defconfig*"
arch/x86/configs/i386_defconfig:CONFIG_DRM_I915=y
arch/x86/configs/x86_64_defconfig:CONFIG_DRM_I915=y

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds