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Reading from the DCC grabs a character from the buffer and
clears the status bit. Since this is a context-changing
operation, instructions following the character read that rely on
the status bit being accurate need to be synchronized with an
ISB.
In this case, the status bit check needs to execute after the
character read otherwise we run the risk of reading the character
and checking the status bit before the read can clear the status
bit in the first place. When this happens, the user will see the
same character they typed twice, instead of once.
Add an ISB after the read and the write, so that the status check
is synchronized with the read/write operations.
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
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The inline assembly differences for v6 vs. v7 in the hvc_dcc
driver are purely optimizations. On a v7 processor, an mrc with
the pc sets the condition codes to the 28-31 bits of the register
being read. It just so happens that the TX/RX full bits the DCC
driver is testing for are high enough in the register to be put
into the condition codes. On a v6 processor, this "feature" isn't
implemented and thus we have to do the usual read, mask, test
operations to check for TX/RX full.
Since we already test the RX/TX full bits before calling
__dcc_getchar() and __dcc_putchar() we don't actually need to do
anything special for v7 over v6. The only difference is in
hvc_dcc_get_chars(). We would test RX full, poll RX full, and
then read a character from the buffer, whereas now we will test
RX full, read a character from the buffer, and then test RX full
again for the second iteration of the loop. It doesn't seem
possible for the buffer to go from full to empty between testing
the RX full and reading a character. Therefore, replace the v7
versions with the v6 versions and everything works the same.
Acked-by: Tony Lindgren <tony@atomide.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Daniel Walker <dwalker@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
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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.
Acked-by: Tony Lindgren <tony@atomide.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Daniel Walker <dwalker@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
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