Re: [PATCH] kbuild: document recursive dependency limitation / resolution

[josh@joshtriplett.org]

On Wed, Jul 29, 2015 at 01:09:16PM -0700, Luis R. Rodriguez wrote:
> From: "Luis R. Rodriguez" <mcgrof@suse.com>
> 
> Recursive dependency issues with kconfig are unavoidable due to
> some limitations with kconfig, since these issues are recurring
> provide a hint to the user how they can resolve these dependency
> issues and also document why such limitation exists.
> 
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Cc: James Bottomley <jbottomley@odin.com>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Paul Bolle <pebolle@tiscali.nl>
> Cc: Herbert Xu <herbert@gondor.apana.org.au>
> Cc: Takashi Iwai <tiwai@suse.de>
> Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
> Cc: Michal Marek <mmarek@suse.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: linux-kbuild@vger.kernel.org
> Cc: linux-doc@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>

As a minor nit, I would suggest saying that making Kbuild handle this
might require a full SAT solver, rather than the current phrasing that
suggests that a SAT solver is the right solution to this problem.  Let's
wait to make that conclusion until a kbuild patch shows up.

Otherwise, this seems quite sensible; thanks for documenting this bit of
tribal knowledge.

- Josh Triplett

> I've cc'd Roberto and Stefano as I think we might be able to in the
> long term use some of their work on package dependency and solvers for
> this problem [0] [1] [2]. This last part -- just consider it long term
> focused.
> 
> [0] https://upsilon.cc/~zack/research/publications/splc2010-fd-deps.pdf
> [1] https://ocaml.org/meetings/ocaml/2014/preferences-2014-09-05-slides.pdf
> [2] https://www.youtube.com/watch?v=GSOcRQvZg8w
> 
>  Documentation/kbuild/kconfig-language.txt | 22 ++++++++++++++++++++++
>  scripts/kconfig/symbol.c                  |  2 ++
>  2 files changed, 24 insertions(+)
> 
> diff --git a/Documentation/kbuild/kconfig-language.txt b/Documentation/kbuild/kconfig-language.txt
> index 350f733bf2c7..7e0510d1cef7 100644
> --- a/Documentation/kbuild/kconfig-language.txt
> +++ b/Documentation/kbuild/kconfig-language.txt
> @@ -393,3 +393,25 @@ config FOO
>  	depends on BAR && m
>  
>  limits FOO to module (=m) or disabled (=n).
> +
> +Kconfig recursive dependency limitations
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +If you've hit the Kconfig error: "recursive dependency detected" you've run
> +into a recursive dependency issue with Kconfig. Kconfig does not do recursive
> +dependency resolution, this has a few implications for Kconfig file writers. In
> +practice it means that for instance if a driver A selects a few kconfig symbols
> +another driver B which selects any of these symbols cannot negate any of the
> +symbols the driver A selected.  Because of this current limitation developers
> +who run into this type of recursive dependency issue have two diverging
> +options:
> +
> +  a) Either swap all "select FOO" to "depends on FOO" or,
> +  b) Change the offending "depends on FOO" to "select FOO"
> +
> +Kconfig's limitations can be addressed by implementing a SAT solver for it,
> +but until then, Kconfig is limitted to require developers to use one of
> +the above two mechanisms to address recursive dependency issues. For more
> +details you can refer to this thread and discussion:
> +
> +http://lkml.kernel.org/r/1432241149-8762-1-git-send-email-mcgrof@do-not-panic.com
> diff --git a/scripts/kconfig/symbol.c b/scripts/kconfig/symbol.c
> index 70c5ee189dce..4d61b7490dad 100644
> --- a/scripts/kconfig/symbol.c
> +++ b/scripts/kconfig/symbol.c
> @@ -1117,6 +1117,8 @@ static void sym_check_print_recursive(struct symbol *last_sym)
>  		if (stack->sym == last_sym)
>  			fprintf(stderr, "%s:%d:error: recursive dependency detected!\n",
>  				prop->file->name, prop->lineno);
> +			fprintf(stderr, "For a resolution refer to Documentation/kbuild/kconfig-language.txt\n");
> +			fprintf(stderr, "section \"Kconfig recursive dependency limitations\"\n");
>  		if (stack->expr) {
>  			fprintf(stderr, "%s:%d:\tsymbol %s %s value contains %s\n",
>  				prop->file->name, prop->lineno,
> -- 
> 2.3.2.209.gd67f9d5.dirty
>