linux-kernel - [PATCH] riscv: ftrace: Fix the logic issue in DYNAMIC

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Message-ID: <20250703084502.394406-1-chenmiao.ku@gmail.com>
Date: Thu,  3 Jul 2025 08:45:02 +0000
From: ChenMiao <chenmiao.ku@...il.com>
To: Linux RISCV <linux-riscv@...r.kernel.org>
Cc: chenmiao <chenmiao.ku@...il.com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Alexandre Ghiti <alex@...ti.fr>,
	linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] riscv: ftrace: Fix the logic issue in DYNAMIC_FTRACE selection

From: chenmiao <chenmiao.ku@...il.com>

When I was reading the source code of ftrace, I learned that
ftrace has two types: static and dynamic. Initially, I planned
to prioritize reading the static source code, so I disabled
the enable dynamic option in RISCV.

[*]   Kernel Function Tracer
[ ]   Kernel Function Graph Tracer
[ ]   enable/disable function tracing dynamically (NEW)

However, when I tried to compile it, the build failed.

./include/linux/ftrace.h:190:16: error: implicit declaration of
function ‘arch_ftrace_get_regs’; did you mean ‘arch_ftrace_regs’?
[-Wimplicit-function-declaration]
  190 |         return arch_ftrace_get_regs(fregs);
      |                ^~~~~~~~~~~~~~~~~~~~
      |                arch_ftrace_regs

After comparing it with the ARM64 architecture, I found that
ARM64 automatically enables DYNAMIC_FTRACE by default once
FUNCTION_TRACER is turned on, and this cannot be set to "no".
Therefore, I believe the optional DYNAMIC_FTRACE setting in
RISC-V has a logic flaw—if FUNCTION_TRACER is enabled,
DYNAMIC_FTRACE should also be enabled, and vice versa. Moreover,
it's clear that RISC-V lacks the necessary support to successfully
compile the kernel when DYNAMIC_FTRACE is disabled.

[*]   Kernel Function Tracer
[ ]   Kernel Function Graph Tracer
-*-   enable/disable function tracing dynamically

Signed-off-by: chenmiao <chenmiao.ku@...il.com>
---
 arch/riscv/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 36061f473..f7fc8b460 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -97,6 +97,7 @@ config RISCV
 	select CLONE_BACKWARDS
 	select COMMON_CLK
 	select CPU_PM if CPU_IDLE || HIBERNATION || SUSPEND
+	select DYNAMIC_FTRACE if FUNCTION_TRACER
 	select EDAC_SUPPORT
 	select FRAME_POINTER if PERF_EVENTS || (FUNCTION_TRACER && !DYNAMIC_FTRACE)
 	select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY if DYNAMIC_FTRACE
-- 
2.45.2