arch/x86: Clean up bootblock assembly

We have identical gdtptr16 and gdtptr. The reference in
gdtptr_offset calculation is not accounted for when
considering --gc-sections, so to support linking
gdt_init.S separately add dummy use of gdtptr symbol.

Realmode execution already accessed gdt that was located
outside [_start16bit,_estart16bit] region. Remove latter
symbol as the former was not really a start of region,
but entry point symbol.

With the romcc bootblock solution, entry32.inc may have
been linked into romstage before, but the !ENV_BOOTBLOCK
case seems obsolete now.

Change-Id: I0a3f6aeb217ca4e38b936b8c9ec8b0b69732cbb9
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47964
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/arch/x86/bootblock.ld

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
-gdtptr16_offset = gdtptr16 & 0xffff;
+gdtptr_offset = gdtptr & 0xffff;
 nullidt_offset = nullidt & 0xffff;
 
 /* Symbol _start16bit must be aligned to 4kB to start AP CPUs with

src/cpu/x86/16bit/entry16.inc

@@ -108,7 +108,7 @@ _start16bit:
 	movw	$nullidt_offset, %bx
 	subw	%ax, %bx
 	lidt	%cs:(%bx)
-	movw	$gdtptr16_offset, %bx
+	movw	$gdtptr_offset, %bx
 	subw	%ax, %bx
 	lgdtl	%cs:(%bx)
 
@@ -127,11 +127,8 @@ _start16bit:
 	 * The gdt is defined in entry32.inc, it has a 4 Gb code segment
 	 * at 0x08, and a 4 GB data segment at 0x10;
 	 */
-.align	4
-.globl gdtptr16
-gdtptr16:
-	.word	gdt_end - gdt -1 /* compute the table limit */
-	.long	gdt		 /* we know the offset */
+__gdtptr:
+	.long	gdtptr
 
 .align	4
 .globl nullidt
@@ -139,7 +136,3 @@ nullidt:
 	.word	0	/* limit */
 	.long	0
 	.word	0
-
-.globl _estart16bit
-_estart16bit:
-	.code32

src/cpu/x86/32bit/entry32.inc

@@ -8,29 +8,11 @@
 
 	.code32
 /*
- *	When we come here we are in protected mode. We expand
- *	the stack and copies the data segment from ROM to the
- *	memory.
- *
- *	After that, we call the chipset bootstrap routine that
- *	does what is left of the chipset initialization.
- *
+ *	When we come here we are in protected mode.
  *	NOTE aligned to 4 so that we are sure that the prefetch
  *	cache will be reloaded.
- *
- *	In the bootblock there is already a ljmp to __protected_start and
- *	the reset vector jumps to symbol _start16bit in entry16.inc from
- *	the reset vectors's symbol which is _start. Therefore, don't
- *	expose the _start symbol for bootblock.
  */
 	.align	4
-#if !ENV_BOOTBLOCK
-.globl _start
-_start:
-#endif
-
-	lgdt	%cs:gdtptr
-	ljmp	$ROM_CODE_SEG, $__protected_start
 
 __protected_start:
 	/* Save the BIST value */

src/soc/amd/common/block/cpu/noncar/memlayout_x86.ld

@@ -102,7 +102,7 @@ SECTIONS
 
 #if ENV_BOOTBLOCK
 
-gdtptr16_offset = gdtptr16 & 0xffff;
+gdtptr_offset = gdtptr & 0xffff;
 nullidt_offset = nullidt & 0xffff;
 
 SECTIONS {