[PATCH v3 1/1] ARM: Thumb-2: Symbol manipulation macros for function body copying

[Dave Martin]

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 macros to help with such cases.

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.

Signed-off-by: Dave Martin <dave.martin at linaro.org>
---
KernelVersion: v2.6.37

 arch/arm/include/asm/fncpy.h |  110 ++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 110 insertions(+), 0 deletions(-)
 create mode 100644 arch/arm/include/asm/fncpy.h

diff --git a/arch/arm/include/asm/fncpy.h b/arch/arm/include/asm/fncpy.h
new file mode 100644
index 0000000..6399265
--- /dev/null
+++ b/arch/arm/include/asm/fncpy.h
@@ -0,0 +1,110 @@
+/*
+ * arch/arm/include/asm/fncpy.h - helper macros for function body copying
+ *
+ * Copyright (C) 2011 Linaro Limited
+ *
+ * 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.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+/*
+ * 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.
+ *
+ *
+ * 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);
+ *
+ * ... do any required D-side/I-side synchronisation ...
+ *
+ * ... 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>
+
+#include <asm/cacheflush.h>
+
+/* Function pointer casting macros */
+
+/* Cast function pointer to integer: */
+#define __funcp_to_uint(funcp) ({				\
+		uintptr_t __result;				\
+								\
+		asm("" : "=r" (__result) : "0" (funcp));	\
+		__result;					\
+	})
+
+/* Cast integer to function pointer with type matching funcp: */
+#define __uint_to_funcp(i, funcp) ({			\
+		typeof(funcp) __result;			\
+							\
+		asm("" : "=r" (__result) : "0" (i));	\
+		__result;				\
+	})
+
+
+/* Function symbol manipulation macros */
+
+/*
+ * FSYM_REBASE: Determine the correct function pointer for funcp,
+ * after the function has been copied to dest_buf:
+ */
+#define FSYM_REBASE(funcp, dest_buf)					\
+	__uint_to_funcp((uintptr_t)(dest_buf) | FSYM_TYPE(funcp), funcp)
+
+/*
+ * FSYM_BASE: Determine the base address in memory of the function funcp
+ * FSYM_TYPE: Determine the instruction set type (ARM/Thumb) of funcp
+ * (both defined below)
+ */
+
+#ifdef CONFIG_THUMB2_KERNEL
+#define FSYM_BASE(funcp) ((void *)(__funcp_to_uint(funcp) & ~(uintptr_t)1))
+#define FSYM_TYPE(funcp) (__funcp_to_uint(funcp) & 1)
+#else /* !CONFIG_THUMB2_KERNEL */
+#define FSYM_BASE(funcp) ((void *)__funcp_to_uint(funcp))
+#define FSYM_TYPE(funcp) 0
+#endif /* !CONFIG_THUMB2_KERNEL */
+
+/* Function copy helper */
+#define fncpy(dest_buf, funcp, size) ({				\
+		memcpy(dest_buf, FSYM_BASE(funcp), size);	\
+		flush_icache_range((unsigned long)(dest_buf),	\
+			(unsigned long)(dest_buf) + (size));	\
+								\
+		FSYM_REBASE(funcp, dest_buf);			\
+	})
+
+#endif /* !__ASM_FNCPY_H */
-- 
1.7.1