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 2dc3b17..f645f84 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 0d12a12..4004073 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 397ac71..926b2e7 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 b483248..9ab1926 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 ca754fd..f6b5fbf 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 */