Re: [PATCH] Re: MatroxFB support can't be compiled into kernel

[Jeff Garzik]

Petr Vandrovec wrote:
> 
> On 26 Jan 01 at 14:29, Bernhard Rosenkraenzer wrote:
> >
> > Subject says it all - works as a module, but can't be compiled into the
> > kernel because of duplicate definitions, caused by several files including
> > matroxfb_base.h which in turn defines global_disp.
> >
> > Patch attached.
> 
> Oops. I did not tried matroxfb without multihead for so long... But...
> Should not (1) compiler optimize them out, as global_disp is used only
> by matroxfb_base.c and (2) linker merge them together? I was under
> impression that kernel uses common storage for uninitialized variables...
> I'm sure that it worked sometime in the past...
> 
> Anyway, I preffer patch bellow, as global_disp is used only by
> matroxfb_base.c, and only if CONFIG_FB_MATROX_MULTIHEAD is not set...
> 
> Linus, original complaint was against 2.4.0-ac11 - I do not know, whether
> Alan uses some new linker scripts or what's going on. In any case, can you
> apply this too? There is no reason why matrox's 'global_disp' should polute
> global namespace.

If you compile your kernels with -fno-common, this problem would show
up.  Andrea and a couple of the gcc guys, in a thread ~30 days ago,
recommended the use of -fno-common to build the kernel.  I started using
it myself, and have picked up and fixed a few problems such as the one
that your patch fixes.

I sent a patch to Alan to add -fno-common to the command line of his
2.4.0-acXX patches, but it got dropped (presumeably too experimental or
whatever).

In case anyone is curious, here is what 'info gcc' says about
-fno-common:

> `-fno-common'
>      Allocate even uninitialized global variables in the bss section of
>      the object file, rather than generating them as common blocks.
>      This has the effect that if the same variable is declared (without
>      `extern') in two different compilations, you will get an error
>      when you link them.  The only reason this might be useful is if
>      you wish to verify that the program will work on other systems
>      which always work this way.

-- 
Jeff Garzik       | "You see, in this world there's two kinds of
Building 1024     |  people, my friend: Those with loaded guns
MandrakeSoft      |  and those who dig. You dig."  --Blondie