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Subject[PATCH 1/3] hvc_dcc: Fix bad code generation by marking assembly volatile
Date
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 <hvc_dcc_put_chars>:
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 <hvc_dcc_put_chars+0x2c>
10: e3530000 cmp r3, #0 ; 0x0
14: 1afffffd bne 10 <hvc_dcc_put_chars+0x10>
18: e7d1000c ldrb r0, [r1, ip]
1c: ee10fe11 mrc 14, 0, pc, cr0, cr1, {0}
20: 2afffffd bcs 1c <hvc_dcc_put_chars+0x1c>
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 <hvc_dcc_put_chars+0x10>
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 <hvc_dcc_put_chars>:
0: e3a03000 mov r3, #0 ; 0x0
4: ea000007 b 28 <hvc_dcc_put_chars+0x28>
8: ee100e11 mrc 14, 0, r0, cr0, cr1, {0}
c: e3100202 tst r0, #536870912 ; 0x20000000
10: 1afffffc bne 8 <hvc_dcc_put_chars+0x8>
14: e7d10003 ldrb r0, [r1, r3]
18: ee10fe11 mrc 14, 0, pc, cr0, cr1, {0}
1c: 2afffffd bcs 18 <hvc_dcc_put_chars+0x18>
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 <hvc_dcc_put_chars+0x8>
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.

Cc: Tony Lindgren <tony@atomide.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Daniel Walker <dwalker@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
---
drivers/char/hvc_dcc.c | 11 +++++------
1 files changed, 5 insertions(+), 6 deletions(-)
diff --git a/drivers/char/hvc_dcc.c b/drivers/char/hvc_dcc.c
index 6470f63..155ec10 100644
--- a/drivers/char/hvc_dcc.c
+++ b/drivers/char/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));
}
--
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.


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