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

Tue Apr 21 13:10:21 UTC 2020

On Tue, Apr 21, 2020 at 3:05 PM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
>
> 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

To clarify what I was aiming for with my mail: I'm not worried about
fbdev here, I'm just worried that this will come back, and we'll grow
select somewhere else until it's become a big & totally horrible mess.
I think a lot of the backlight selects have also grown because of
this, so this isn't just a one-off I think.

If Arnd is happy to play "Kconfig select" whack-a-mole ever once in a
while (and deal with the intermediate compile horrors while everyone
upgrades) I'm ok with this landing. Just not terribly happy if the
underlying issue isn't fixed.
-Daniel

> 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

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch