[MIPS] Fix $gp usage

Now we load $gp with _GLOBAL_OFFSET_TABLE_, but this is incorrect use.
As a general principle, we should use _gp for $gp.

Thanks to linker script's help we fortunately have _gp which equals to
_GLOBAL_OFFSET_TABLE_. But once _gp gets out of alignment, we will not
be able to access to GOT entires, global variables and procedure entry
points. The right thing to do is to use _gp.

This patch also introduce a new symbol `.gpword _GLOBAL_OFFSET_TABLE_'
which holds the offset from _gp. When updating GOT entries, we use this
offset and _gp to calculate the final _GLOBAL_OFFSET_TABLE_.

This patch is originally submitted by Vlad Lungu <vlad@comsys.ro>, then
I made some change to leave over num_got_entries.

Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi@necel.com>
Cc: Vlad Lungu <vlad@comsys.ro>

diff --git a/cpu/mips/start.S b/cpu/mips/start.S
index e91e213..074d01d 100644 (file)
--- a/cpu/mips/start.S
+++ b/cpu/mips/start.S
@@ -234,11 +234,11 @@ reset:
        li      t0, CONF_CM_UNCACHED
        mtc0    t0, CP0_CONFIG
 
-       /* Initialize GOT pointer.
+       /* Initialize $gp.
        */
        bal     1f
        nop
-       .word   _GLOBAL_OFFSET_TABLE_
+       .word   _gp
        1:
        move    gp, ra
        lw      t1, 0(ra)
@@ -306,9 +306,9 @@ relocate_code:
        move    t1, a2
 
        /*
-        * Fix GOT pointer:
+        * Fix $gp:
         *
-        * New GOT-PTR = (old GOT-PTR - CFG_MONITOR_BASE) + Destination Address
+        * New $gp = (Old $gp - CFG_MONITOR_BASE) + Destination Address
         */
        move    t6, gp
        sub     gp, CFG_MONITOR_BASE
@@ -341,15 +341,22 @@ relocate_code:
        j       t0
        nop
 
+       .gpword _GLOBAL_OFFSET_TABLE_   /* _GLOBAL_OFFSET_TABLE_ - _gp  */
        .word   uboot_end_data
        .word   uboot_end
        .word   num_got_entries
 
 in_ram:
-       /* Now we want to update GOT.
+       /*
+        * Now we want to update GOT.
+        *
+        * GOT[0] is reserved. GOT[1] is also reserved for the dynamic object
+        * generated by GNU ld. Skip these reserved entries from relocation.
         */
        lw      t3, -4(t0)      /* t3 <-- num_got_entries       */
-       addi    t4, gp, 8       /* Skipping first two entries.  */
+       lw      t4, -16(t0)     /* t4 <-- (_GLOBAL_OFFSET_TABLE_ - _gp) */
+       add     t4, t4, gp      /* t4 now holds _GLOBAL_OFFSET_TABLE_   */
+       addi    t4, t4, 8       /* Skipping first two entries.  */
        li      t2, 2
 1:
        lw      t1, 0(t4)