ARM: add fncpy.h from linux v5.10

Description from the linux commit 5756e9dd0de6d5c307773f8f734c0684b3098fdd:

    ARM: 6640/1: Thumb-2: Symbol manipulation macros for function body copying

    In low-level board support code, there is sometimes a need to
    copy a function body to another location at run-time.

    A straightforward call to memcpy doesn't work in Thumb-2,
    because bit 0 of external Thumb function symbols is set to 1,
    indicating that the function is Thumb.  Without corrective
    measures, this will cause an off-by-one copy, and the copy
    may be called using the wrong instruction set.

    This patch adds an fncpy() macro to help with such copies.

    Particular care is needed, because C doesn't guarantee any
    defined behaviour when casting a function pointer to any other
    type.  This has been observed to lead to strange optimisation
    side-effects when doing the arithmetic which is required in
    order to copy/move function bodies correctly in Thumb-2.

    Thanks to Russell King and Nicolas Pitre for their input
    on this patch.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 70 insertions, 0 deletions

diff --git a/arch/arm/include/asm/fncpy.h b/arch/arm/include/asm/fncpy.h
new file mode 100644
index 0000000000..9d87ad72e9
--- /dev/null
+++ b/arch/arm/include/asm/fncpy.h
@@ -0,0 +1,70 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * arch/arm/include/asm/fncpy.h - helper macros for function body copying
+ *
+ * Copyright (C) 2011 Linaro Limited
+ */
+
+/*
+ * These macros are intended for use when there is a need to copy a low-level
+ * function body into special memory.
+ *
+ * For example, when reconfiguring the SDRAM controller, the code doing the
+ * reconfiguration may need to run from SRAM.
+ *
+ * NOTE: that the copied function body must be entirely self-contained and
+ * position-independent in order for this to work properly.
+ *
+ * NOTE: in order for embedded literals and data to get referenced correctly,
+ * the alignment of functions must be preserved when copying.  To ensure this,
+ * the source and destination addresses for fncpy() must be aligned to a
+ * multiple of 8 bytes: you will be get a BUG() if this condition is not met.
+ * You will typically need a ".align 3" directive in the assembler where the
+ * function to be copied is defined, and ensure that your allocator for the
+ * destination buffer returns 8-byte-aligned pointers.
+ *
+ * Typical usage example:
+ *
+ * extern int f(args);
+ * extern uint32_t size_of_f;
+ * int (*copied_f)(args);
+ * void *sram_buffer;
+ *
+ * copied_f = fncpy(sram_buffer, &f, size_of_f);
+ *
+ * ... later, call the function: ...
+ *
+ * copied_f(args);
+ *
+ * The size of the function to be copied can't be determined from C:
+ * this must be determined by other means, such as adding assmbler directives
+ * in the file where f is defined.
+ */
+
+#ifndef __ASM_FNCPY_H
+#define __ASM_FNCPY_H
+
+#include <linux/types.h>
+#include <linux/string.h>
+
+/*
+ * Minimum alignment requirement for the source and destination addresses
+ * for function copying.
+ */
+#define FNCPY_ALIGN 8
+
+#define fncpy(dest_buf, funcp, size) ({					\
+	uintptr_t __funcp_address;					\
+	typeof(funcp) __result;						\
+									\
+	asm("" : "=r" (__funcp_address) : "0" (funcp));			\
+									\
+	memcpy(dest_buf, (void const *)(__funcp_address & ~1), size);	\
+									\
+	asm("" : "=r" (__result)					\
+		: "0" ((uintptr_t)(dest_buf) | (__funcp_address & 1)));	\
+									\
+	__result;							\
+})
+
+#endif /* !__ASM_FNCPY_H */