* Nicolas Pitre <nico@xxxxxxxxxxx> [101220 13:38]:
> On Mon, 20 Dec 2010, Stephen Boyd wrote:
>
> > 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@xxxxxxxxxxx>
> > Cc: Arnd Bergmann <arnd@xxxxxxxx>
> > Cc: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>
> > Cc: Daniel Walker <dwalker@xxxxxxxxxxxxxx>
> > Signed-off-by: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
>
> Acked-by: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>
Acked-by: Tony Lindgren <tony@xxxxxxxxxxx>
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