software/microwatt: Fix copying data to RAM and clearing BSS

This also makes us use the "small" memory model to avoid having to
use more complex constructs and adds the TOC to the linker script

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

litex/soc/cores/cpu/microwatt/core.py

@@ -41,6 +41,7 @@ class Microwatt(CPU):
         flags += "-mlittle-endian "
         flags += "-mstrict-align "
         flags += "-fno-stack-protector "
+        flags += "-mcmodel=small "
         flags += "-D__microwatt__ "
         return flags
 

litex/soc/cores/cpu/microwatt/crt0.S

@@ -41,12 +41,44 @@
 _start:
 	FIXUP_ENDIAN
 
+	/* Grab our own address */
+	bcl	20,31,$+4
+0:	mflr	%r12
+
+	/* Get our TOC */
+	addis	%r2,%r12,(.TOC.-0b)@h
+	addi	%r2,%r2,(.TOC.-0b)@l
+
+	/* Copy data from ROM to SRAM */
+	ld	%r3,_fdata@got(%r2)
+	ld	%r4,_edata@got(%r2)
+	ld	%r5,_fdata_rom@got(%r2)
+	subf	%r6,%r3,%r4
+	srdi.	%r6,%r6,3
+	beq	1f
+	li	%r10,0
+	mtctr	%r6
+0:	ldx	%r0,%r10,%r5
+	stdx	%r0,%r10,%r3
+	addi	%r10,%r10,8
+	bdnz	0b
+1:
+	/* Clear BSS */
+	ld	%r3,_fbss@got(%r2)
+	ld	%r4,_ebss@got(%r2)
+	subf	%r6,%r3,%r4
+	srdi.	%r6,%r6,3
+	beq	1f
+	li	%r0,0
+	subi	%r3,%r3,8
+	mtctr	%r6
+0:	stdu	%r0,8(%r3)
+	bdnz	0b
+1:
 	/* setup stack */
-	LOAD_IMM64(%r1, _fstack - 0x100)
+	ld	%r1,_fstack@got(%r2)
-	LOAD_IMM64(%r12, main)
+	bl	main
-	mtctr	%r12,
+	b	.
-	bctrl
-	b .
 
 #define EXCEPTION(nr)		\
 	.=  nr;      \

litex/soc/software/bios/linker.ld

@@ -25,6 +25,7 @@ SECTIONS
 		*(.rodata .rodata.* .gnu.linkonce.r.*)
 		*(.rodata1)
 		*(.got .got.*)
+		*(.toc .toc.*)
 
 		/* Make sure the file is aligned on disk as well
 		   as in memory; CRC calculation requires that. */