[PATCH v4 4/7] alpha: provide ftrace return address support for lockdep

Lockdep uses ftrace_return_address() to report useful call sites for
lock acquisition and IRQ-state tracking diagnostics. Provide the Alpha
architecture hook using the compiler return-address builtin when frame
pointers are available.

Return zero when frame pointers are disabled, matching the existing
fallback behavior of architectures that cannot provide a reliable return
address.

This is a preparatory change for enabling lockdep support on Alpha.

Reviewed-by: Matt Turner <[email protected]>
Tested-by: Matt Turner <[email protected]>
Signed-off-by: Magnus Lindholm <[email protected]>
---
 arch/alpha/include/asm/ftrace.h | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/arch/alpha/include/asm/ftrace.h b/arch/alpha/include/asm/ftrace.h
index 40a8c178f10d..7ec44134c804 100644
--- a/arch/alpha/include/asm/ftrace.h
+++ b/arch/alpha/include/asm/ftrace.h
@@ -1 +1,29 @@
-/* empty */
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_ALPHA_FTRACE_H
+#define _ASM_ALPHA_FTRACE_H
+
+#ifdef CONFIG_FRAME_POINTER
+
+static void *alpha_ftrace_return_address0(void)
+	noinline notrace;
+static void *alpha_ftrace_return_address0(void)
+{
+	return __builtin_return_address(0);
+}
+
+#define ftrace_return_address0 alpha_ftrace_return_address0()
+
+/*
+ * __builtin_return_address() requires a constant integer argument.
+ * Keep this as a macro so the value is seen at the callsite.
+ */
+#define ftrace_return_address(n) __builtin_return_address(n)
+
+#else  /* !CONFIG_FRAME_POINTER */
+
+#define ftrace_return_address0 0UL
+#define ftrace_return_address(n) ((void)(n), 0UL)
+
+#endif /* CONFIG_FRAME_POINTER */
+
+#endif /* _ASM_ALPHA_FTRACE_H */
-- 
2.53.0