v4.19.13 snapshot.
diff --git a/arch/arm/lib/backtrace.S b/arch/arm/lib/backtrace.S
new file mode 100644
index 0000000..7d7952e
--- /dev/null
+++ b/arch/arm/lib/backtrace.S
@@ -0,0 +1,121 @@
+/*
+ * linux/arch/arm/lib/backtrace.S
+ *
+ * Copyright (C) 1995, 1996 Russell King
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * 27/03/03 Ian Molton Clean up CONFIG_CPU
+ *
+ */
+#include <linux/kern_levels.h>
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+ .text
+
+@ fp is 0 or stack frame
+
+#define frame r4
+#define sv_fp r5
+#define sv_pc r6
+#define mask r7
+#define offset r8
+
+ENTRY(c_backtrace)
+
+#if !defined(CONFIG_FRAME_POINTER) || !defined(CONFIG_PRINTK)
+ ret lr
+ENDPROC(c_backtrace)
+#else
+ stmfd sp!, {r4 - r8, lr} @ Save an extra register so we have a location...
+ movs frame, r0 @ if frame pointer is zero
+ beq no_frame @ we have no stack frames
+
+ tst r1, #0x10 @ 26 or 32-bit mode?
+ ARM( moveq mask, #0xfc000003 )
+ THUMB( moveq mask, #0xfc000000 )
+ THUMB( orreq mask, #0x03 )
+ movne mask, #0 @ mask for 32-bit
+
+1: stmfd sp!, {pc} @ calculate offset of PC stored
+ ldr r0, [sp], #4 @ by stmfd for this CPU
+ adr r1, 1b
+ sub offset, r0, r1
+
+/*
+ * Stack frame layout:
+ * optionally saved caller registers (r4 - r10)
+ * saved fp
+ * saved sp
+ * saved lr
+ * frame => saved pc
+ * optionally saved arguments (r0 - r3)
+ * saved sp => <next word>
+ *
+ * Functions start with the following code sequence:
+ * mov ip, sp
+ * stmfd sp!, {r0 - r3} (optional)
+ * corrected pc => stmfd sp!, {..., fp, ip, lr, pc}
+ */
+for_each_frame: tst frame, mask @ Check for address exceptions
+ bne no_frame
+
+1001: ldr sv_pc, [frame, #0] @ get saved pc
+1002: ldr sv_fp, [frame, #-12] @ get saved fp
+
+ sub sv_pc, sv_pc, offset @ Correct PC for prefetching
+ bic sv_pc, sv_pc, mask @ mask PC/LR for the mode
+
+1003: ldr r2, [sv_pc, #-4] @ if stmfd sp!, {args} exists,
+ ldr r3, .Ldsi+4 @ adjust saved 'pc' back one
+ teq r3, r2, lsr #10 @ instruction
+ subne r0, sv_pc, #4 @ allow for mov
+ subeq r0, sv_pc, #8 @ allow for mov + stmia
+
+ ldr r1, [frame, #-4] @ get saved lr
+ mov r2, frame
+ bic r1, r1, mask @ mask PC/LR for the mode
+ bl dump_backtrace_entry
+
+ ldr r1, [sv_pc, #-4] @ if stmfd sp!, {args} exists,
+ ldr r3, .Ldsi+4
+ teq r3, r1, lsr #11
+ ldreq r0, [frame, #-8] @ get sp
+ subeq r0, r0, #4 @ point at the last arg
+ bleq dump_backtrace_stm @ dump saved registers
+
+1004: ldr r1, [sv_pc, #0] @ if stmfd sp!, {..., fp, ip, lr, pc}
+ ldr r3, .Ldsi @ instruction exists,
+ teq r3, r1, lsr #11
+ subeq r0, frame, #16
+ bleq dump_backtrace_stm @ dump saved registers
+
+ teq sv_fp, #0 @ zero saved fp means
+ beq no_frame @ no further frames
+
+ cmp sv_fp, frame @ next frame must be
+ mov frame, sv_fp @ above the current frame
+ bhi for_each_frame
+
+1006: adr r0, .Lbad
+ mov r1, frame
+ bl printk
+no_frame: ldmfd sp!, {r4 - r8, pc}
+ENDPROC(c_backtrace)
+
+ .pushsection __ex_table,"a"
+ .align 3
+ .long 1001b, 1006b
+ .long 1002b, 1006b
+ .long 1003b, 1006b
+ .long 1004b, 1006b
+ .popsection
+
+.Lbad: .asciz "Backtrace aborted due to bad frame pointer <%p>\n"
+ .align
+.Ldsi: .word 0xe92dd800 >> 11 @ stmfd sp!, {... fp, ip, lr, pc}
+ .word 0xe92d0000 >> 11 @ stmfd sp!, {}
+
+#endif