Re: [RFC PATCH 2/2] Kbuild: avoid partial linking of drivers/built-in.o

[Nicolas Pitre]

On Mon, 30 Mar 2015, Ard Biesheuvel wrote:

> On 30 March 2015 at 16:13, Michal Marek <mmarek@xxxxxxx> wrote:
> > On 2015-03-30 15:31, Ard Biesheuvel wrote:
> >> On 30 March 2015 at 15:26, Russell King - ARM Linux
> >> <linux@xxxxxxxxxxxxxxxx> wrote:
> >>> On Mon, Mar 30, 2015 at 02:38:35PM +0200, Michal Marek wrote:
> >>>> Is this a limitation of a particular ARM ABI or a limitation of a state
> >>>> of the art ARM linker or something else?
> >>>
> >>> It's a limitation of the ARM ISA.
> >>>
> >>> Normal PC-relative branches, which are emitted by the C compiler, can
> >>> branch +/- 32MB for ARM, or +/- 16MB of Thumb.  Beyond that, the address
> >>> offset is not representable in the instruction.
> >
> > Thank you both for the explanation!
> >
> >
> >>> The question is: how far do we go with allyesconfig... do we want it
> >>> to work, or is reaching the final link sufficient?
> >
> > It certainly is more useful as a test tool if the baseline is a
> > successful compile and link. Because you can have genuine link errors
> > due to missing symbols.
> >
> 
> Agreed
> 
> >
> >>> If we do tweak
> >>> stuff to allow the link to work, are we going to try running it?
> >
> > Good question. I myself always treated all{yes,mod}config as a build
> > test only and never dared to run it. Allyesconfig produces a giant
> > kernel image and allmodconfig builds binfmt_script as a module. And if
> > people used all*config for boot tests, they would probably be sending
> > patches to tweak the Kconfigs for that purpose. And this is not the case
> > as far as I can tell.
> >
> 
> Russell should confirm this, but I think running such a large kernel
> is non-trivial on ARM, since the decompressor should make room for the
> decompressed image by moving itself upward in memory, and it may
> overwrite the device tree binary in the process.

Loading the device tree at a different address should be easy.  You just 
need to load it at the top of RAM. You may also start by attempting to 
boot a plain Image (uncompressed) kernel binary.

> >> That is an excellent question, hence the RFC in the subject line.
> >>
> >> Note that the other patch, the one against kallsyms, addresses the
> >> issue where the distance between the beginning of .text and the end of
> >> .init.text  exceeds this limit, which is not as unlikely as the issue
> >> that this patch addresses, where just drivers/built-in.o in isolation
> >> already exceeds this limit.
> >>
> >> So I am quite happy to drop this, especially as we can add
> >> -ffunction-sections as well.
> >
> > What you could do is to add a Kconfig option to arch/arm/Kconfig adding
> > -ffunction-sections to the compiler flags. Then allyesconfig would
> > select it and work around the problem in a somewhat elegant way.
> >
> 
> Excellent idea! Arnd hasn't chimed in yet, but he is the one doing
> lots and lots of randconfig builds and other test builds, so I will
> wait for him to confirm that this is a useful thing to have.

I'm using -ffunction-sections as well for the kernel size reduction work 
I'm currently doing.  The linker script has to be adapted so .text.* is 
specified along .text otherwise those functions end up appended at the 
end of the binary.


Nicolas
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