[U-Boot] [PATCH] relocation: Do not relocate NULL pointers.

[Peter Tyser]

On Mon, 2009-09-28 at 09:34 +0200, Joakim Tjernlund wrote:
> Peter Tyser <ptyser at xes-inc.com> wrote on 28/09/2009 06:31:28:
> >
> > On Sun, 2009-09-27 at 15:15 +0200, Joakim Tjernlund wrote:
> > > Wolfgang Denk <wd at denx.de> wrote on 23/09/2009 20:23:14:
> > > >
> > > > Dear Peter Tyser,
> > > >
> > > > In message <1253710639.3968.19.camel at ptyser-laptop> you wrote:
> > > > >
> > > > > My "fix" to the linker script was to change:
> > > > > __bss_start = .;
> > > > > into:
> > > > > __bss_start = . | 4;
> > > > >
> > > > > ie, a big hack, but it did work:)  I'll take a peek at a more proper
> > > > > link script workaround.
> > > >
> > > > 32 bit alignment of the BSS segment might not be sufficient. Be
> > > > careful!
> > >
> > > Any progress on this ?
> >
> > I've been swamped the last few days, but think I have a workaround.  I
> > hope to test it tomorrow or tues.  I'll send an email when I have a more
> > definitive answer.
> >
> > > > > Nice!  It'd be great to have the magical 20 lines of assembly put into
> > > > > some semi-understandable c.
> > > >
> > > > :-)
> > >
> > > I have worked some more on this but all boards need to be converted to use
> > > the new C-variants.
> >
> > Great!
> >
> > > Anyhow, I have also been thinking/working on making U-boot
> > > fully PIC and reached a important conclusion. The GOT holds absolute
> > > ptr values and there is not much one can do about it sans modifying gcc.
> > > So before u-boot is relocated to RAM one must manually add any offset to
> > > all global/static data and string literals. The majority of strings
> > > are passed directly to printf and friends so the offset can be added inside
> > > printf. The remaining few data accesses needs to be dealt with directly, example:
> > > -       for (init_fnc_ptr = init_sequence; *init_fnc_ptr; ++init_fnc_ptr) {
> > > +       for (init_fnc_ptr = got_off(init_sequence); *init_fnc_ptr; ++init_fnc_ptr) {
> > >
> > > Only code called before relocation to RAM needs this, mostly the _f() functions.
> > > Would this be an acceptable change?
> >
> > Could you describe the advantages of generating a fully PIC U-Boot
> > image?  I understand you could execute the image from different places
> > in flash, but on the boards I've worked with this isn't a huge concern.
> > For example, its possible to have a preliminary flash mapping that
> > U-Boot executes from, then after relocation to RAM that flash mapping
> > can be modified.  So where U-Boot initially executes from isn't all that
> > important for me.  Is there some killer feature that a fully PIC U-Boot
> > provides to make adding the got_off() workarounds you mention
> > worthwhile?
> 
> For me, it is mainly to be to have two u-boot partitions and
> be able to select one to boot from. This makes it safer to
> update u-boot in the field.

X-ES requires this same functionality of dual booting.  We generally
have a jumper on each card that swaps 2 chip select 0 and 1 so that
either can be booted from (assuming CS 0 is always used to load U-Boot).
The same U-Boot image can be programmed to both flashes.  After U-Boot
relocates to RAM, we remap the flashes (in
board/xes/xpedite5370/xpedite5370.c: flash_cs_fixup()) to provide a
common flash layout regardless of which flash we booted from.  Would it
be possible to do something similar in your hardware setup so that the
same U-Boot image could be loaded from either flash partition without
the got_off() fixups you mention above?

Best,
Peter