Re: kbuild: fixing the select problem

[Michal Marek]

On 5.5.2010 23:49, James Bottomley wrote:
> [Sam: I know you don't maintain kbuild anymore, but since you have the
> most experience, if you could find time to comment, I'd be grateful]
> 
> The select problem is that the kbuild select directive will turn a
> symbol on without reference to its dependencies.  This, in turn, means
> that either selected symbols must select their dependencies, or that
> people using select have to be aware of the selected symbol's dependency
> and build those dependencies into their symbol (leading to duplication
> and the possibility of getting the dependencies out of sync).  We use
> select for the scsi transport classes, so we run into this problem in
> SCSI quite a lot.
> 
> I think the correct fix is to make a symbol that selects another symbol
> automatically inherit all of the selected symbol's dependencies.
> 
> There seems to be a fairly easy way to do this in kbuild.  Right at the
> moment, select is handled as additional symbol values as the last point
> in the symbol tree evaluation process.  Instead, what I propose doing is
> for every select symbol, we add an extra unconditional default for the
> selected symbol of the selecting symbol's current value (this breaks a
> possible dependency cycle) and add to the dependencies of the selecting
> symbol, the symbol it's currently selecting.

Nice trick :-).


> There's one wrinkle to all of this in that the current parser for
> default values stops when it finds the first valid (i.e. whose if clause
> evaluates to true) default.  To make the above scheme work, I need to
> modify the default parser so it takes the highest tristate of all the
> valid defaults (and bumps m to y for bool).

We should check if some Kconfig file doesn't rely on this "first hit"
behavior and fix it to explicitly list the condition for a given
default. Another option would be to add
default SYM1 || SYM2
to a symbol selected by SYM1 and SYM2.


> Does this look acceptable to people?  I think it should give the desired
> result and has the added benefit that we can then strip the extra select
> overlay out of the kbuild system (making the parser slightly simpler).
> 
> If this looks like a good idea to people, I think I can code up a quick
> patch.

Other than the above, right now I don't see any issues with such approach.

On a related note, I see Vegard's GSoC project to use a sat solver for
kconfig got accepted [1]. Vegard, how is the project progressing?

[1]
http://socghop.appspot.com/gsoc/student_project/show/google/gsoc2010/psu_home/t127230762803

Michal
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