[libflat] Remove now-obsolete flatten_real_mode call

Flat real mode will have been set up as a side-effect of the
protected-mode call invoked during install_block() for .text16.early;
there is no need to do so explicitly.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/libprefix.S

@@ -649,7 +649,10 @@ install_prealloc:
 	popl	%esi
 
 #ifndef KEEP_IT_REAL
-	/* Access high memory */
+	/* Access high memory by enabling the A20 gate.  (We will
+	 * already have 4GB segment limits as a result of calling
+	 * install_block.)
+	 */
 	pushw	%cs
 	pushw	$1f
 	pushw	%ax

src/arch/i386/transitions/libflat.S

@@ -21,124 +21,6 @@ FILE_LICENCE ( GPL2_OR_LATER )
 
 	.arch i386
 
-#define CR0_PE 1
-
-/****************************************************************************
- * flatten_real_mode
- *
- * Set up 4GB segment limits
- *
- * Parameters:
- *   none
- * Returns:
- *   none
- * Corrupts:
- *   none
- ****************************************************************************
- */
-	/* GDT for protected-mode calls */
-	.section ".text16.early.data", "aw", @progbits
-	.align 16
-flatten_gdt:
-flatten_gdt_limit:	.word flatten_gdt_length - 1
-flatten_gdt_base:	.long 0
-			.word 0 /* padding */
-flatten_cs:	/* 16-bit protected-mode flat code segment */
-	.equ    FLAT_CS, flatten_cs - flatten_gdt
-	.word   0xffff, 0
-	.byte   0, 0x9b, 0x8f, 0
-flatten_ss:	/* 16-bit protected-mode flat stack segment */
-	.equ    FLAT_SS, flatten_ss - flatten_gdt
-	.word   0xffff, 0
-	.byte   0, 0x93, 0x8f, 0
-flatten_gdt_end:
-	.equ	flatten_gdt_length, . - flatten_gdt
-	.size	flatten_gdt, . - flatten_gdt
-
-	.section ".text16.early.data", "aw", @progbits
-	.align 16
-flatten_saved_gdt:
-	.long	0, 0
-	.size	flatten_saved_gdt, . - flatten_saved_gdt
-
-	.section ".text16.early", "awx", @progbits
-	.code16
-flatten_real_mode:
-	/* Preserve registers and flags */
-	pushfl
-	pushl	%eax
-	pushw	%si
-	pushw	%gs
-	pushw	%fs
-	pushw	%es
-	pushw	%ds
-	pushw	%ss
-
-	/* Set %ds for access to .text16.early.data variables */
-	pushw	%cs
-	popw	%ds
-
-	/* Preserve original GDT */
-	sgdt flatten_saved_gdt
-
-	/* Set up GDT bases */
-	xorl	%eax, %eax
-	movw	%cs, %ax
-	shll	$4, %eax
-	addl	$flatten_gdt, %eax
-	movl	%eax, flatten_gdt_base
-	movw	%cs, %ax
-	movw	$flatten_cs, %si
-	call	set_seg_base
-	movw	%ss, %ax
-	movw	$flatten_ss, %si
-	call	set_seg_base
-
-	/* Switch temporarily to protected mode and set segment registers */
-	pushw	%cs
-	pushw	$2f
-	cli
-	data32 lgdt flatten_gdt
-	movl	%cr0, %eax
-	orb	$CR0_PE, %al
-	movl	%eax, %cr0
-	ljmp	$FLAT_CS, $1f
-1:	movw	$FLAT_SS, %ax
-	movw	%ax, %ss
-	movw	%ax, %ds
-	movw	%ax, %es
-	movw	%ax, %fs
-	movw	%ax, %gs
-	movl	%cr0, %eax
-	andb	$0!CR0_PE, %al
-	movl	%eax, %cr0
-	lret
-2:	/* lret will ljmp to here */
-
-	/* Restore GDT, registers and flags */
-	data32 lgdt flatten_saved_gdt
-	popw	%ss
-	popw	%ds
-	popw	%es
-	popw	%fs
-	popw	%gs
-	popw	%si
-	popl	%eax
-	popfl
-	ret
-	.size flatten_real_mode, . - flatten_real_mode
-
-	.section ".text16.early", "awx", @progbits
-	.code16
-set_seg_base:
-	rolw	$4, %ax
-	movw	%ax, 2(%si)
-	andw	$0xfff0, 2(%si)
-	movb	%al, 4(%si)
-	andb	$0x0f, 4(%si)
-	ret
-	.size set_seg_base, . - set_seg_base
-
 /****************************************************************************
  * test_a20_short, test_a20_long
  *
@@ -403,7 +285,7 @@ enable_a20_method:
 /****************************************************************************
  * access_highmem (real mode far call)
  *
- * Open up access to high memory in flat real mode with A20 enabled
+ * Open up access to high memory with A20 enabled
  *
  * Parameters:
  *   none
@@ -419,7 +301,5 @@ enable_a20_method:
 access_highmem:
 	/* Enable A20 line */
 	call	enable_a20
-	/* Set up 4GB limits */
-	call	flatten_real_mode
 	lret
 	.size	access_highmem, . - access_highmem