/*
* Copyright 2010 Tilera Corporation. All Rights Reserved.
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation, version 2.
*
* This program is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or
* NON INFRINGEMENT. See the GNU General Public License for
* more details.
*
* Support routines for atomic operations. Each function takes:
*
* r0: address to manipulate
* r1: pointer to atomic lock guarding this operation (for ATOMIC_LOCK_REG)
* r2: new value to write, or for cmpxchg/add_unless, value to compare against
* r3: (cmpxchg/xchg_add_unless) new value to write or add;
* (atomic64 ops) high word of value to write
* r4/r5: (cmpxchg64/add_unless64) new value to write or add
*
* The 32-bit routines return a "struct __get_user" so that the futex code
* has an opportunity to return -EFAULT to the user if needed.
* The 64-bit routines just return a "long long" with the value,
* since they are only used from kernel space and don't expect to fault.
* Support for 16-bit ops is included in the framework but we don't provide
* any (x86_64 has an atomic_inc_short(), so we might want to some day).
*
* Note that the caller is advised to issue a suitable L1 or L2
* prefetch on the address being manipulated to avoid extra stalls.
* In addition, the hot path is on two icache lines, and we start with
* a jump to the second line to make sure they are both in cache so
* that we never stall waiting on icache fill while holding the lock.
* (This doesn't work out with most 64-bit ops, since they consume
* too many bundles, so may take an extra i-cache stall.)
*
* These routines set the INTERRUPT_CRITICAL_SECTION bit, just
* like sys_cmpxchg(), so that NMIs like PERF_COUNT will not interrupt
* the code, just page faults.
*
* If the load or store faults in a way that can be directly fixed in
* the do_page_fault_ics() handler (e.g. a vmalloc reference) we fix it
* directly, return to the instruction that faulted, and retry it.
*
* If the load or store faults in a way that potentially requires us
* to release the atomic lock, then retry (e.g. a migrating PTE), we
* reset the PC in do_page_fault_ics() to the "tns" instruction so
* that on return we will reacquire the lock and restart the op. We
* are somewhat overloading the exception_table_entry notion by doing
* this, since those entries are not normally used for migrating PTEs.
*
* If the main page fault handler discovers a bad address, it will see
* the PC pointing to the "tns" instruction (due to the earlier
* exception_table_entry processing in do_page_fault_ics), and
* re-reset the PC to the fault handler, atomic_bad_address(), which
* effectively takes over from the atomic op and can either return a
* bad "struct __get_user" (for user addresses) or can just panic (for
* bad kernel addresses).
*
* Note that if the value we would store is the same as what we
* loaded, we bypass the store. Other platforms with true atomics can
* make the guarantee that a non-atomic __clear_bit(), for example,
* can safely race with an atomic test_and_set_bit(); this example is
* from bit_spinlock.h in slub_lock() / slub_unlock(). We can't do
* that on Tile since the "atomic" op is really just a
* read/modify/write, and can race with the non-atomic
* read/modify/write. However, if we can short-circuit the write when
* it is not needed, in the atomic case, we avoid the race.
*/
#include <linux/linkage.h>
#include <asm/atomic_32.h>
#include <asm/page.h>
#include <asm/processor.h>
.section .text.atomic,"ax"
ENTRY(__start_atomic_asm_code)
.macro atomic_op, name, bitwidth, body
.align 64
STD_ENTRY_SECTION(__atomic\name, .text.atomic)
{
movei r24, 1
j 4f /* branch to second cache line */
}
1: {
.ifc \bitwidth,16
lh r22, r0
.else
lw r22, r0
addi r28, r0, 4
.endif
}
.ifc \bitwidth,64
lw r23, r28
.endif
\body /* set r24, and r25 if 64-bit */
{
seq r26, r22, r24
seq r27, r23, r25
}
.ifc \bitwidth,64
bbnst r27, 2f
.endif
bbs r26, 3f /* skip write-back if it's the same value */
2: {
.ifc \bitwidth,16
sh r0, r24
.else
sw r0, r24
.endif
}
.ifc \bitwidth,64
sw r28, r25
.endif
mf
3: {
move r0, r22
.ifc \bitwidth,64
move r1, r23
.else
move r1, zero
.endif
sw ATOMIC_LOCK_REG_NAME, zero
}
mtspr INTERRUPT_CRITICAL_SECTION, zero
jrp lr
4: {
move ATOMIC_LOCK_REG_NAME, r1
mtspr INTERRUPT_CRITICAL_SECTION, r24
}
#ifndef CONFIG_SMP
j 1b /* no atomic locks */
#else
{
tns r21, ATOMIC_LOCK_REG_NAME
moveli r23, 2048 /* maximum backoff time in cycles */
}
{
bzt r21, 1b /* branch if lock acquired */
moveli r25, 32 /* starting backoff time in cycles */
}
5: mtspr INTERRUPT_CRITICAL_SECTION, zero
mfspr r26, CYCLE_LOW /* get start point for this backoff */
6: mfspr r22, CYCLE_LOW /* test to see if we've backed off enough */
sub r22, r22, r26
slt r22, r22, r25
bbst r22, 6b
{
mtspr INTERRUPT_CRITICAL_SECTION, r24
shli r25, r25, 1 /* double the backoff; retry the tns */
}
{
tns r21, ATOMIC_LOCK_REG_NAME
slt r26, r23, r25 /* is the proposed backoff too big? */
}
{
bzt r21, 1b /* branch if lock acquired */
mvnz r25, r26, r23
}
j 5b
#endif
STD_ENDPROC(__atomic\name)
.ifc \bitwidth,32
.pushsection __ex_table,"a"
.word 1b, __atomic\name
.word 2b, __atomic\name
.word __atomic\name, __atomic_bad_address
.popsection
.endif
.endm
atomic_op _cmpxchg, 32, "seq r26, r22, r2; { bbns r26, 3f; move r24, r3 }"
atomic_op _xchg, 32, "move r24, r2"
atomic_op _xchg_add, 32, "add r24, r22, r2"
atomic_op _xchg_add_unless, 32, \
"sne r26, r22, r2; { bbns r26, 3f; add r24, r22, r3 }"
atomic_op _or, 32, "or r24, r22, r2"
atomic_op _andn, 32, "nor r2, r2, zero; and r24, r22, r2"
atomic_op _xor, 32, "xor r24, r22, r2"
atomic_op 64_cmpxchg, 64, "{ seq r26, r22, r2; seq r27, r23, r3 }; \
{ bbns r26, 3f; move r24, r4 }; { bbns r27, 3f; move r25, r5 }"
atomic_op 64_xchg, 64, "{ move r24, r2; move r25, r3 }"
atomic_op 64_xchg_add, 64, "{ add r24, r22, r2; add r25, r23, r3 }; \
slt_u r26, r24, r22; add r25, r25, r26"
atomic_op 64_xchg_add_unless, 64, \
"{ sne r26, r22, r2; sne r27, r23, r3 }; \
{ bbns r26, 3f; add r24, r22, r4 }; \
{ bbns r27, 3f; add r25, r23, r5 }; \
slt_u r26, r24, r22; add r25, r25, r26"
jrp lr /* happy backtracer */
ENTRY(__end_atomic_asm_code)