From a99632014631409483a481a6a0d77d09ded47239 Mon Sep 17 00:00:00 2001 From: Stephen Boyd Date: Thu, 3 Feb 2011 15:48:34 -0800 Subject: [PATCH] hvc_dcc: Fix bad code generation by marking assembly volatile Without marking the asm __dcc_getstatus() volatile my compiler decides it can cache the value of __ret in a register and then check the value of it continually in hvc_dcc_put_chars() (I had to replace get_wait/put_wait with 1 and fixup the branch otherwise my disassembler barfed on __dcc_(get|put)char). 00000000 : 0: ee103e11 mrc 14, 0, r3, cr0, cr1, {0} 4: e3a0c000 mov ip, #0 ; 0x0 8: e2033202 and r3, r3, #536870912 ; 0x20000000 c: ea000006 b 2c 10: e3530000 cmp r3, #0 ; 0x0 14: 1afffffd bne 10 18: e7d1000c ldrb r0, [r1, ip] 1c: ee10fe11 mrc 14, 0, pc, cr0, cr1, {0} 20: 2afffffd bcs 1c 24: ee000e15 mcr 14, 0, r0, cr0, cr5, {0} 28: e28cc001 add ip, ip, #1 ; 0x1 2c: e15c0002 cmp ip, r2 30: bafffff6 blt 10 34: e1a00002 mov r0, r2 38: e12fff1e bx lr As you can see, the value of the mrc is checked against DCC_STATUS_TX (bit 29) and then stored in r3 for later use. Marking the asm volatile produces the following: 00000000 : 0: e3a03000 mov r3, #0 ; 0x0 4: ea000007 b 28 8: ee100e11 mrc 14, 0, r0, cr0, cr1, {0} c: e3100202 tst r0, #536870912 ; 0x20000000 10: 1afffffc bne 8 14: e7d10003 ldrb r0, [r1, r3] 18: ee10fe11 mrc 14, 0, pc, cr0, cr1, {0} 1c: 2afffffd bcs 18 20: ee000e15 mcr 14, 0, r0, cr0, cr5, {0} 24: e2833001 add r3, r3, #1 ; 0x1 28: e1530002 cmp r3, r2 2c: bafffff5 blt 8 30: e1a00002 mov r0, r2 34: e12fff1e bx lr which looks better and actually works. Mark all the inline assembly in this file as volatile since we don't want the compiler to optimize away these statements or move them around in any way. Acked-by: Tony Lindgren Cc: Arnd Bergmann Acked-by: Nicolas Pitre Cc: Daniel Walker Signed-off-by: Stephen Boyd Signed-off-by: Greg Kroah-Hartman --- drivers/tty/hvc/hvc_dcc.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-) diff --git a/drivers/tty/hvc/hvc_dcc.c b/drivers/tty/hvc/hvc_dcc.c index 6470f63deb4b..155ec105e1c8 100644 --- a/drivers/tty/hvc/hvc_dcc.c +++ b/drivers/tty/hvc/hvc_dcc.c @@ -33,8 +33,7 @@ static inline u32 __dcc_getstatus(void) { u32 __ret; - - asm("mrc p14, 0, %0, c0, c1, 0 @ read comms ctrl reg" + asm volatile("mrc p14, 0, %0, c0, c1, 0 @ read comms ctrl reg" : "=r" (__ret) : : "cc"); return __ret; @@ -46,7 +45,7 @@ static inline char __dcc_getchar(void) { char __c; - asm("get_wait: mrc p14, 0, pc, c0, c1, 0 \n\ + asm volatile("get_wait: mrc p14, 0, pc, c0, c1, 0 \n\ bne get_wait \n\ mrc p14, 0, %0, c0, c5, 0 @ read comms data reg" : "=r" (__c) : : "cc"); @@ -58,7 +57,7 @@ static inline char __dcc_getchar(void) { char __c; - asm("mrc p14, 0, %0, c0, c5, 0 @ read comms data reg" + asm volatile("mrc p14, 0, %0, c0, c5, 0 @ read comms data reg" : "=r" (__c)); return __c; @@ -68,7 +67,7 @@ static inline char __dcc_getchar(void) #if defined(CONFIG_CPU_V7) static inline void __dcc_putchar(char c) { - asm("put_wait: mrc p14, 0, pc, c0, c1, 0 \n\ + asm volatile("put_wait: mrc p14, 0, pc, c0, c1, 0 \n\ bcs put_wait \n\ mcr p14, 0, %0, c0, c5, 0 " : : "r" (c) : "cc"); @@ -76,7 +75,7 @@ static inline void __dcc_putchar(char c) #else static inline void __dcc_putchar(char c) { - asm("mcr p14, 0, %0, c0, c5, 0 @ write a char" + asm volatile("mcr p14, 0, %0, c0, c5, 0 @ write a char" : /* no output register */ : "r" (c)); } -- GitLab