sh: GUSA atomic rollback support.

This implements kernel-level atomic rollback built on top of gUSA,
as an alternative non-IRQ based atomicity method. This is generally
a faster method for platforms that are lacking the LL/SC pairs that
SH-4A and later use, and is only supportable on legacy cores.

Signed-off-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

5 files changed, 34 insertions, 58 deletions

diff --git a/arch/sh/Kconfig b/arch/sh/Kconfig
index 2dc3b177193c..f645f8416f1c 100644
--- a/arch/sh/Kconfig
+++ b/arch/sh/Kconfig
@@ -692,7 +692,7 @@ source "kernel/Kconfig.preempt"
 
 config GUSA
         def_bool y
-        depends on !SMP
+        depends on !SMP && SUPERH32
         help
           This enables support for gUSA (general UserSpace Atomicity).
           This is the default implementation for both UP and non-ll/sc
@@ -704,6 +704,16 @@ config GUSA
           This should only be disabled for special cases where alternate
           atomicity implementations exist.
 
+config GUSA_RB
+        bool "Implement atomic operations by roll-back (gRB) (EXPERIMENTAL)"
+        depends on GUSA && CPU_SH3 || (CPU_SH4 && !CPU_SH4A)
+        help
+          Enabling this option will allow the kernel to implement some
+          atomic operations using a software implemention of load-locked/
+          store-conditional (LLSC). On machines which do not have hardware
+          LLSC, this should be more efficient than the other alternative of
+          disabling insterrupts around the atomic sequence.
+
 endmenu
 
 menu "Boot options"
diff --git a/arch/sh/kernel/cpu/sh3/entry.S b/arch/sh/kernel/cpu/sh3/entry.S
index 0d12a124055c..4004073f98cd 100644
--- a/arch/sh/kernel/cpu/sh3/entry.S
+++ b/arch/sh/kernel/cpu/sh3/entry.S
@@ -13,8 +13,9 @@
 #include <linux/linkage.h>
 #include <asm/asm-offsets.h>
 #include <asm/thread_info.h>
-#include <asm/cpu/mmu_context.h>
 #include <asm/unistd.h>
+#include <asm/cpu/mmu_context.h>
+#include <asm/page.h>
 
 ! NOTE:
 ! GNU as (as of 2.9.1) changes bf/s into bt/s and bra, when the address
@@ -409,6 +410,27 @@ ENTRY(handle_exception)
         ! Using k0, k1 for scratch registers (r0_bank1, r1_bank),
         ! save all registers onto stack.
         !
+
+#ifdef CONFIG_GUSA
+        ! Check for roll back gRB (User and Kernel)
+        mov     r15, k0
+        shll    k0
+        bf/s    1f
+         shll   k0
+        bf/s    1f
+         stc    spc, k1
+        stc     r0_bank, k0
+        cmp/hs  k0, k1          ! test k1 (saved PC) >= k0 (saved r0)
+        bt/s    2f
+         stc    r1_bank, k1
+
+        add     #-2, k0
+        add     r15, k0
+        ldc     k0, spc         ! PC = saved r0 + r15 - 2
+2:      mov     k1, r15         ! SP = r1
+1:
+#endif
+
         stc     ssr, k0         ! Is it from kernel space?
         shll    k0              ! Check MD bit (bit30) by shifting it into...
         shll    k0              !       ...the T bit
diff --git a/arch/sh/kernel/entry-common.S b/arch/sh/kernel/entry-common.S
index 397ac71d97f1..926b2e7b11c1 100644
--- a/arch/sh/kernel/entry-common.S
+++ b/arch/sh/kernel/entry-common.S
@@ -176,25 +176,6 @@ work_notifysig:
         jmp     @r1
          lds    r0, pr
 work_resched:
-#if defined(CONFIG_GUSA) && !defined(CONFIG_PREEMPT)
-        ! gUSA handling
-        mov.l   @(OFF_SP,r15), r0       ! get user space stack pointer
-        mov     r0, r1
-        shll    r0
-        bf/s    1f
-         shll   r0
-        bf/s    1f
-         mov    #OFF_PC, r0
-        !                                 SP >= 0xc0000000 : gUSA mark
-        mov.l   @(r0,r15), r2           ! get user space PC (program counter)
-        mov.l   @(OFF_R0,r15), r3       ! end point
-        cmp/hs  r3, r2                  ! r2 >= r3? 
-        bt      1f
-        add     r3, r1                  ! rewind point #2
-        mov.l   r1, @(r0,r15)           ! reset PC to rewind point #2
-        !
-1:
-#endif
         mov.l   1f, r1
         jsr     @r1                             ! schedule
          nop
diff --git a/arch/sh/kernel/process_32.c b/arch/sh/kernel/process_32.c
index b48324867eee..9ab1926b9d10 100644
--- a/arch/sh/kernel/process_32.c
+++ b/arch/sh/kernel/process_32.c
@@ -322,25 +322,6 @@ struct task_struct *__switch_to(struct task_struct *prev,
         unlazy_fpu(prev, task_pt_regs(prev));
 #endif
 
-#if defined(CONFIG_GUSA) && defined(CONFIG_PREEMPT)
-        {
-                struct pt_regs *regs;
-
-                preempt_disable();
-                regs = task_pt_regs(prev);
-                if (user_mode(regs) && regs->regs[15] >= 0xc0000000) {
-                        int offset = (int)regs->regs[15];
-
-                        /* Reset stack pointer: clear critical region mark */
-                        regs->regs[15] = regs->regs[1];
-                        if (regs->pc < regs->regs[0])
-                                /* Go to rewind point */
-                                regs->pc = regs->regs[0] + offset;
-                }
-                preempt_enable_no_resched();
-        }
-#endif
-
 #ifdef CONFIG_MMU
         /*
          * Restore the kernel mode register
diff --git a/arch/sh/kernel/signal_32.c b/arch/sh/kernel/signal_32.c
index ca754fd42437..f6b5fbfe75c4 100644
--- a/arch/sh/kernel/signal_32.c
+++ b/arch/sh/kernel/signal_32.c
@@ -507,24 +507,6 @@ handle_signal(unsigned long sig, struct k_sigaction *ka, siginfo_t *info,
                                                 ctrl_inw(regs->pc - 4));
                                 break;
                 }
-#ifdef CONFIG_GUSA
-        } else {
-                /* gUSA handling */
-                preempt_disable();
-
-                if (regs->regs[15] >= 0xc0000000) {
-                        int offset = (int)regs->regs[15];
-
-                        /* Reset stack pointer: clear critical region mark */
-                        regs->regs[15] = regs->regs[1];
-                        if (regs->pc < regs->regs[0])
-                                /* Go to rewind point #1 */
-                                regs->pc = regs->regs[0] + offset -
-                                        instruction_size(ctrl_inw(regs->pc-4));
-                }
-
-                preempt_enable_no_resched();
-#endif
         }
 
         /* Set up the stack frame */