ARM: Allow SMP kernels to boot on UP systems

UP systems do not implement all the instructions that SMP systems have,
so in order to boot a SMP kernel on a UP system, we need to rewrite
parts of the kernel.

Do this using an 'alternatives' scheme, where the kernel code and data
is modified prior to initialization to replace the SMP instructions,
thereby rendering the problematical code ineffectual.  We use the linker
to generate a list of 32-bit word locations and their replacement values,
and run through these replacements when we detect a UP system.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/entry-armv.S

@@ -46,7 +46,8 @@
 	 * this macro assumes that irqstat (r6) and base (r5) are
 	 * preserved from get_irqnr_and_base above
 	 */
-	test_for_ipi r0, r6, r5, lr
+	ALT_SMP(test_for_ipi r0, r6, r5, lr)
+	ALT_UP_B(9997f)
 	movne	r0, sp
 	adrne	lr, BSYM(1b)
 	bne	do_IPI
@@ -57,6 +58,7 @@
 	adrne	lr, BSYM(1b)
 	bne	do_local_timer
 #endif
+9997:
 #endif
 
 	.endm
@@ -965,11 +967,8 @@ kuser_cmpxchg_fixup:
 	beq	1b
 	rsbs	r0, r3, #0
 	/* beware -- each __kuser slot must be 8 instructions max */
-#ifdef CONFIG_SMP
-	b	__kuser_memory_barrier
-#else
-	usr_ret	lr
-#endif
+	ALT_SMP(b	__kuser_memory_barrier)
+	ALT_UP(usr_ret	lr)
 
 #endif
 

arch/arm/kernel/head.S

@@ -86,6 +86,9 @@ ENTRY(stext)
 	movs	r8, r5				@ invalid machine (r5=0)?
 	beq	__error_a			@ yes, error 'a'
 	bl	__vet_atags
+#ifdef CONFIG_SMP_ON_UP
+	bl	__fixup_smp
+#endif
 	bl	__create_page_tables
 
 	/*
@@ -333,4 +336,51 @@ __create_page_tables:
 ENDPROC(__create_page_tables)
 	.ltorg
 
+#ifdef CONFIG_SMP_ON_UP
+__fixup_smp:
+	mov	r7, #0x00070000
+	orr	r6, r7, #0xff000000	@ mask 0xff070000
+	orr	r7, r7, #0x41000000	@ val 0x41070000
+	and	r0, r9, r6
+	teq	r0, r7			@ ARM CPU and ARMv6/v7?
+	bne	__fixup_smp_on_up	@ no, assume UP
+
+	orr	r6, r6, #0x0000ff00
+	orr	r6, r6, #0x000000f0	@ mask 0xff07fff0
+	orr	r7, r7, #0x0000b000
+	orr	r7, r7, #0x00000020	@ val 0x4107b020
+	and	r0, r9, r6
+	teq	r0, r7			@ ARM 11MPCore?
+	moveq	pc, lr			@ yes, assume SMP
+
+	mrc	p15, 0, r0, c0, c0, 5	@ read MPIDR
+	tst	r0, #1 << 31
+	movne	pc, lr			@ bit 31 => SMP
+
+__fixup_smp_on_up:
+	adr	r0, 1f
+	ldmia	r0, {r3, r6, r7}
+	sub	r3, r0, r3
+	add	r6, r6, r3
+	add	r7, r7, r3
+2:	cmp	r6, r7
+	ldmia	r6!, {r0, r4}
+	strlo	r4, [r0, r3]
+	blo	2b
+	mov	pc, lr
+ENDPROC(__fixup_smp)
+
+1:	.word	.
+	.word	__smpalt_begin
+	.word	__smpalt_end
+
+	.pushsection .data
+	.globl	smp_on_up
+smp_on_up:
+	ALT_SMP(.long	1)
+	ALT_UP(.long	0)
+	.popsection
+
+#endif
+
 #include "head-common.S"

arch/arm/kernel/setup.c

@@ -36,6 +36,7 @@
 #include <asm/procinfo.h>
 #include <asm/sections.h>
 #include <asm/setup.h>
+#include <asm/smp_plat.h>
 #include <asm/mach-types.h>
 #include <asm/cacheflush.h>
 #include <asm/cachetype.h>
@@ -825,7 +826,8 @@ void __init setup_arch(char **cmdline_p)
 	request_standard_resources(&meminfo, mdesc);
 
 #ifdef CONFIG_SMP
-	smp_init_cpus();
+	if (is_smp())
+		smp_init_cpus();
 #endif
 	reserve_crashkernel();
 

arch/arm/kernel/vmlinux.lds.S

@@ -40,6 +40,11 @@ SECTIONS
 		__tagtable_begin = .;
 			*(.taglist.init)
 		__tagtable_end = .;
+#ifdef CONFIG_SMP_ON_UP
+		__smpalt_begin = .;
+			*(.alt.smp.init)
+		__smpalt_end = .;
+#endif
 
 		INIT_SETUP(16)
 
@@ -237,6 +242,12 @@ SECTIONS
 
 	/* Default discards */
 	DISCARDS
+
+#ifndef CONFIG_SMP_ON_UP
+	/DISCARD/ : {
+		*(.alt.smp.init)
+	}
+#endif
 }
 
 /*