x86: remove legacy BOOT_A20_ENABLE option

BOOT_A20_ENABLE relies on the kernel being on a system that has a full PC BIOS and requires that the A20 line be enabled to boot correctly. None of the supported platforms satisfy either requirment. Change-Id: I05805a050f5531de0348b60a4f0cb974e464b279 Signed-off-by: Dirk Brandewie <[email protected]>
mlaz · Feb 6, 2016 · 3012166 · 3012166
1 parent ca97c38
commit 3012166
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diff --git a/arch/x86/core/crt0.S b/arch/x86/core/crt0.S
@@ -107,74 +107,6 @@ __start:
 
 	cli
 
-
-/*
- * Platforms that need to enable the A20 line to boot properly wil enable this
- * option.
- */
-#ifdef CONFIG_BOOT_A20_ENABLE
-
-	/*
-	 * Set the stack pointer to just before the start of the .text
-	 * section.  This needs to be performed before any BIOS invocations.
-	 *
-	 * Note that attempting to set the stack pointer to beyond the end of
-	 * the static ELF image (indicated via the _end symbol) may result
-	 * in pointer wrap-around for images with large BSS sections
-	 * (since the processor is still in "real" mode and thus the stack
-	 * pointer is only a 16 bits).
-	 */
-
-	movl	$__start, %esp
-
-	/*
-	 * Before switching to 32-bit protected mode, Gate-A20 must be
-	 * enabled (legacy PC hardware issue).
-	 *
-	 * The classical method of enabling address line A20 via the 8042
-	 * keyboard controller will not be performed.  Instead, it will be
-	 * assumed that the board has a relatively modern BIOS which supports
-	 * the following int 15h function:
-	 *
-	 *      - int 15h: ax=2400 -> disable A20
-	 *      - int 15h: ax=2401 -> enable A20
-	 *           If successful: CF clear, AH = 00h
-	 *           On error: CF set, AH = status
-	 *           Status: 01h keyboard controller is in secure mode
-	 *                   86h function not supported
-	 *      - int 15h: ax=2402 -> query status A20
-	 *           status (0: disabled, 1: enabled) is returned in AL
-	 *      - int 15h: ax=2403 -> query A20 support (kbd or port 92)
-	 *           status (bit 0: kbd, bit 1: port 92) is returned in BX
-	 *
-	 *      Return values:
-	 *      If successful: carry flag (CF) clear, AH = 00h
-	 *      On error: carry flag (CF) set, AH = status
-	 *      Status: 01h keyboard controller is in secure mode
-	 *              86h function not supported
-	 *
-	 * If the int15h method fails, then the system control port A
-	 * (I/O port 0x92) method will be used.
-	 */
-
-	movw	$0x2401, %ax		/* ax=2401 -> enable A20 */
-	int	$0x15
-	jnc	A20Enabled		/* CF = 1 -> try I/O port 0x92 method */
-
-	/* try enabling Gate-A20 via system control port A (I/O port 0x92) */
-
-	movb	$0x02, %al
-	outb	%al, $0x92
-
-	xorl	%ecx, %ecx
-A20EnableWait:
-	inb	$0x92, %al
-	andb	$0x02, %al
-	loopz	A20EnableWait
-
-A20Enabled:
-#endif /* CONFIG_BOOT_A20_ENABLE */
-
 	/* load Interrupt Descriptor and Global Descriptor tables (IDT & GDT) */
 
 	lgdt	%cs:_GdtRom		/* load 32-bit operand size GDT */

diff --git a/misc/Kconfig b/misc/Kconfig
@@ -234,15 +234,6 @@ config PROT_MODE_SWITCH
 	This option causes the kernel to transition from real mode (16-bit)
 	to protected mode (32-bit) during its initial booting sequence.
 
-config BOOT_A20_ENABLE
-	bool "Boot A20 enable"
-	depends on PROT_MODE_SWITCH
-	default n
-	help
-	This option causes the A20 line to be enabled during the transition
-	from real mode (16-bit) to protected mode (32-bit) during its initial
-	booting sequence.
-
 endmenu
 
 menu "System Monitoring Options"