linux-kernel - Re: [PATCH 06/10] ARM: ftrace: add Thumb-2 support

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Date:	Tue, 16 Mar 2010 10:23:54 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Rabin Vincent <rabin@....in>
Cc:	linux-kernel@...r.kernel.org,
	Abhishek Sagar <sagar.abhishek@...il.com>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 06/10] ARM: ftrace: add Thumb-2 support

Hi Rabin,

Rabin Vincent <rabin@....in> wrote:
> Fix the mcount routines to build and run on a kernel built with the
> Thumb-2 instruction set:
>
>  - Without the BSYM, the following assembler errors appear:
>
>    entry-common.S: Assembler messages:
>    entry-common.S:179: Error: invalid immediate for address calculation (value = 0x00000004)

I'm still confused by this. I think that's a compiler problem but need
to get some feedback from toolchain people.

> --- a/arch/arm/kernel/entry-common.S
> +++ b/arch/arm/kernel/entry-common.S
> @@ -167,7 +167,8 @@ ENTRY(__gnu_mcount_nc)
>  	stmdb	sp!, {r0-r3, lr}
>  	ldr	r0, =ftrace_trace_function
>  	ldr	r2, [r0]
> -	adr	r0, ftrace_stub
> + THUMB(	orr	r2, r2, #1		)
> +	adr	r0, BSYM(ftrace_stub)

If the ftrace_stub isn't .globl, the code compiles fine. My approach
would be something like this:

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index d085033..5f5aef6 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -122,7 +122,7 @@ ENTRY(__gnu_mcount_nc)
 	stmdb sp!, {r0-r3, lr}
 	ldr r0, =ftrace_trace_function
 	ldr r2, [r0]
-	adr r0, ftrace_stub
+	adr r0, 1f
 	cmp r0, r2
 	bne gnu_trace
 	ldmia sp!, {r0-r3, ip, lr}
@@ -132,8 +132,9 @@ gnu_trace:
 	ldr r1, [sp, #20]			@ lr of instrumented routine
 	mov r0, lr
 	sub r0, r0, #MCOUNT_INSN_SIZE
-	mov lr, pc
+	adr lr, BSYM(2f)
 	mov pc, r2
+2:
 	ldmia sp!, {r0-r3, ip, lr}
 	mov pc, ip
 
@@ -141,7 +142,7 @@ ENTRY(mcount)
 	stmdb sp!, {r0-r3, lr}
 	ldr r0, =ftrace_trace_function
 	ldr r2, [r0]
-	adr r0, ftrace_stub
+	adr r0, 1f
 	cmp r0, r2
 	bne trace
 	ldr lr, [fp, #-4]			@ restore lr
@@ -160,6 +161,7 @@ trace:
 
 	.globl ftrace_stub
 ftrace_stub:
+1:
 	mov pc, lr
 
 #endif /* CONFIG_FUNCTION_TRACER */

-- 
Catalin
--
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