[U-Boot] [PATCH 0/2] Make sure 85xx bss doesn't start at 0x0

[Peter Tyser <ptyser@xes-inc.com>]

On Tue, 2009-10-06 at 15:46 -0500, Kumar Gala wrote:
> On Oct 6, 2009, at 1:08 PM, Peter Tyser wrote:
> 
> > On Tue, 2009-10-06 at 19:51 +0200, Wolfgang Denk wrote:
> >> Dear Peter Tyser,
> >>
> >> In message <1254843932.24664.2083.camel@localhost.localdomain> you  
> >> wrote:
> >>>
> >>> I personally like the current implementation of putting the bss  
> >>> after
> >>> the entire U-Boot image.  It keeps U-Boot's code, malloc pool,  
> >>> stack,
> >>> bss, etc all in the same general area which is nice, and has the  
> >>> side
> >>> benefit that the bootpg won't be overwritten.
> >>
> >> OK, if you think so...
> >>
> >>> I know ORing in 0x10 is a bit ugly, but what's the real downside of
> >>> doing it?
> >>
> >> Nothing. I just hate to allocate the bss at 0x0, because this is
> >> actually incorrect - it's the result of an address overflow /
> >> truncation, and pretty much misleading to someone trying to read and
> >> understand the code. For the linked image, it does not _look_ as if
> >> the bss was located _after_ the U-Boot image, it looks detached and
> >> allocated in low RAM.
> >
> > Do you have a preference Kumar?  You're probably going to be the first
> > in line to have to deal with any resulting confusion:)
> >
> > I personally would rank the options:
> > 1. OR in an offset to the bss address and leave some good comments in
> > the linker script and commit message
> >
> > 2. Make the bss the last section like other PPC boards which would
> > result in the bootpg sometimes being overwritten
> >
> > 3. Put the bss at an arbitrary address
> 
> I don't have a preference, but maybe I missed the answer to my  
> question about where does 44x put the BSS.

The 44x boards put the bss after "the rest" of u-boot, but before the
bootpg section.  Sometimes the bss might overlap the bootpg which would
mean the bootpg would get zeroed out on bootup and the bss would "wrap
around to 0 (which is fine, just confusing).  Eg:
 
  [ 0]                   NULL            00000000 000000 000000 00      0   0  0
  [ 1] .resetvec         PROGBITS        fffffffc 03f2e4 000004 00  AX  0   0  1
  [ 2] .bootpg           PROGBITS        fffff000 03e2e8 000250 00  AX  0   0  1
  [ 3] .text             PROGBITS        fff80000 000094 0303b0 00  AX  0   0  4
  [ 4] .rodata           PROGBITS        fffb03b0 030444 00a14c 00   A  0   0  4
  [ 5] .reloc            PROGBITS        fffba500 03a594 002280 00  WA  0   0  4
  [ 6] .data             PROGBITS        fffbc780 03c814 00088c 00  WA  0   0  4
  [ 7] .data.rel.local   PROGBITS        fffbd00c 03d0a0 000a98 00  WA  0   0  4
  [ 8] .data.rel.ro.loca PROGBITS        fffbdaa4 03db38 0000b0 00  WA  0   0  4
  [ 9] .data.rel         PROGBITS        fffbdb54 03dbe8 000100 00  WA  0   0  4
  [10] .u_boot_cmd       PROGBITS        fffbdc54 03dce8 000600 00  WA  0   0  4
  [11] .bss              NOBITS          fffbe300 03e2e8 011c44 00  WA  0   0  4

> Is it possible to put it before TEXTBASE?

I looked into that originally but couldn't get it to work via the linker
script alone.  If we wanted to hardcode a bss size, we could pass "-Tbss
<TEXTBASE - HARDCODED_BSS_SIZE>" to ld to position it.  We could
allocate some relatively huge chunk of memory for it below TEXTBASE, but
I'm not sure we could make it dynamically sized.

Peter