Blackfin arch: defines and provides entry points for certain user space functions at fixed addresses

This patch defines (and provides) entry points for certain user space functions
at fixed addresses.  The Blackfin has no usable atomic instructions, but we can
ensure that these code sequences appear atomic from a user space point of view
by detecting when we're in the process of executing them during the interrupt
handler return path.  This allows much more efficient pthread lock
implementations than the bfin_spinlock syscall we're currently using.

Also provided is a small sys_rt_sigreturn stub which can be used by the signal
handler setup code.  The signal.c part will be committed separately.

Signed-off-by: Bernd Schmidt <bernd.schmidt@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>

5 files changed, 225 insertions, 1 deletions

diff --git a/arch/blackfin/kernel/Makefile b/arch/blackfin/kernel/Makefile
index 93d21406cad..b7b6de82401 100644
--- a/arch/blackfin/kernel/Makefile
+++ b/arch/blackfin/kernel/Makefile
@@ -6,7 +6,8 @@ extra-y := init_task.o vmlinux.lds
 
 obj-y := \
 	entry.o process.o bfin_ksyms.o ptrace.o setup.o signal.o \
-	sys_bfin.o time.o traps.o irqchip.o dma-mapping.o flat.o
+	sys_bfin.o time.o traps.o irqchip.o dma-mapping.o flat.o \
+	fixed_code.o
 
 obj-$(CONFIG_BF53x)		     += bfin_gpio.o
 obj-$(CONFIG_BF561)		     += bfin_gpio.o
diff --git a/arch/blackfin/kernel/fixed_code.S b/arch/blackfin/kernel/fixed_code.S
new file mode 100644
index 00000000000..99ea296c82c
--- /dev/null
+++ b/arch/blackfin/kernel/fixed_code.S
@@ -0,0 +1,132 @@
+/*
+ * This file contains sequences of code that will be copied to a
+ * fixed location, defined in <asm/atomic_seq.h>.  The interrupt
+ * handlers ensure that these sequences appear to be atomic when
+ * executed from userspace.
+ * These are aligned to 16 bytes, so that we have some space to replace
+ * these sequences with something else (e.g. kernel traps if we ever do
+ * BF561 SMP).
+ */
+#include <linux/linkage.h>
+#include <asm/entry.h>
+#include <asm/unistd.h>
+
+.text
+ENTRY(_fixed_code_start)
+
+.align 16
+ENTRY(_sigreturn_stub)
+	P0 = __NR_rt_sigreturn;
+	EXCPT 0;
+	/* Speculative execution paranoia.  */
+0:	JUMP.S 0b;
+ENDPROC (_sigreturn_stub)
+
+.align 16
+	/*
+	 * Atomic swap, 8 bit.
+	 * Inputs:	P0: memory address to use
+	 *		R1: value to store
+	 * Output:	R0: old contents of the memory address, zero extended.
+	 */
+ENTRY(_atomic_xchg32)
+	R0 = [P0];
+	[P0] = R1;
+	rts;
+ENDPROC (_atomic_xchg32)
+
+.align 16
+	/*
+	 * Compare and swap, 32 bit.
+	 * Inputs:	P0: memory address to use
+	 *		R1: compare value
+	 *		R2: new value to store
+	 * The new value is stored if the contents of the memory
+	 * address is equal to the compare value.
+	 * Output:	R0: old contents of the memory address.
+	 */
+ENTRY(_atomic_cas32)
+	R0 = [P0];
+	CC = R0 == R1;
+	IF !CC JUMP 1f;
+	[P0] = R2;
+1:
+	rts;
+ENDPROC (_atomic_cas32)
+
+.align 16
+	/*
+	 * Atomic add, 32 bit.
+	 * Inputs:	P0: memory address to use
+	 *		R0: value to add
+	 * Outputs:	R0: new contents of the memory address.
+	 *		R1: previous contents of the memory address.
+	 */
+ENTRY(_atomic_add32)
+	R1 = [P0];
+	R0 = R1 + R0;
+	[P0] = R0;
+	rts;
+ENDPROC (_atomic_add32)
+
+.align 16
+	/*
+	 * Atomic sub, 32 bit.
+	 * Inputs:	P0: memory address to use
+	 *		R0: value to subtract
+	 * Outputs:	R0: new contents of the memory address.
+	 *		R1: previous contents of the memory address.
+	 */
+ENTRY(_atomic_sub32)
+	R1 = [P0];
+	R0 = R1 - R0;
+	[P0] = R0;
+	rts;
+ENDPROC (_atomic_sub32)
+
+.align 16
+	/*
+	 * Atomic ior, 32 bit.
+	 * Inputs:	P0: memory address to use
+	 *		R0: value to ior
+	 * Outputs:	R0: new contents of the memory address.
+	 *		R1: previous contents of the memory address.
+	 */
+ENTRY(_atomic_ior32)
+	R1 = [P0];
+	R0 = R1 | R0;
+	[P0] = R0;
+	rts;
+ENDPROC (_atomic_ior32)
+
+.align 16
+	/*
+	 * Atomic ior, 32 bit.
+	 * Inputs:	P0: memory address to use
+	 *		R0: value to ior
+	 * Outputs:	R0: new contents of the memory address.
+	 *		R1: previous contents of the memory address.
+	 */
+ENTRY(_atomic_and32)
+	R1 = [P0];
+	R0 = R1 & R0;
+	[P0] = R0;
+	rts;
+ENDPROC (_atomic_ior32)
+
+.align 16
+	/*
+	 * Atomic ior, 32 bit.
+	 * Inputs:	P0: memory address to use
+	 *		R0: value to ior
+	 * Outputs:	R0: new contents of the memory address.
+	 *		R1: previous contents of the memory address.
+	 */
+ENTRY(_atomic_xor32)
+	R1 = [P0];
+	R0 = R1 ^ R0;
+	[P0] = R0;
+	rts;
+ENDPROC (_atomic_ior32)
+
+ENTRY(_fixed_code_end)
diff --git a/arch/blackfin/kernel/process.c b/arch/blackfin/kernel/process.c
index 3eff7439d8d..6b7a94ab96c 100644
--- a/arch/blackfin/kernel/process.c
+++ b/arch/blackfin/kernel/process.c
@@ -35,6 +35,7 @@
 
 #include <asm/blackfin.h>
 #include <asm/uaccess.h>
+#include <asm/fixed_code.h>
 
 #define	LED_ON	0
 #define	LED_OFF	1
@@ -350,6 +351,70 @@ unsigned long get_wchan(struct task_struct *p)
 	return 0;
 }
 
+void finish_atomic_sections (struct pt_regs *regs)
+{
+	if (regs->pc < ATOMIC_SEQS_START || regs->pc >= ATOMIC_SEQS_END)
+		return;
+
+	switch (regs->pc) {
+	case ATOMIC_XCHG32 + 2:
+		put_user(regs->r1, (int *)regs->p0);
+		regs->pc += 2;
+		break;
+
+	case ATOMIC_CAS32 + 2:
+	case ATOMIC_CAS32 + 4:
+		if (regs->r0 == regs->r1)
+			put_user(regs->r2, (int *)regs->p0);
+		regs->pc = ATOMIC_CAS32 + 8;
+		break;
+	case ATOMIC_CAS32 + 6:
+		put_user(regs->r2, (int *)regs->p0);
+		regs->pc += 2;
+		break;
+
+	case ATOMIC_ADD32 + 2:
+		regs->r0 = regs->r1 + regs->r0;
+		/* fall through */
+	case ATOMIC_ADD32 + 4:
+		put_user(regs->r0, (int *)regs->p0);
+		regs->pc = ATOMIC_ADD32 + 6;
+		break;
+
+	case ATOMIC_SUB32 + 2:
+		regs->r0 = regs->r1 - regs->r0;
+		/* fall through */
+	case ATOMIC_SUB32 + 4:
+		put_user(regs->r0, (int *)regs->p0);
+		regs->pc = ATOMIC_SUB32 + 6;
+		break;
+
+	case ATOMIC_IOR32 + 2:
+		regs->r0 = regs->r1 | regs->r0;
+		/* fall through */
+	case ATOMIC_IOR32 + 4:
+		put_user(regs->r0, (int *)regs->p0);
+		regs->pc = ATOMIC_IOR32 + 6;
+		break;
+
+	case ATOMIC_AND32 + 2:
+		regs->r0 = regs->r1 & regs->r0;
+		/* fall through */
+	case ATOMIC_AND32 + 4:
+		put_user(regs->r0, (int *)regs->p0);
+		regs->pc = ATOMIC_AND32 + 6;
+		break;
+
+	case ATOMIC_XOR32 + 2:
+		regs->r0 = regs->r1 ^ regs->r0;
+		/* fall through */
+	case ATOMIC_XOR32 + 4:
+		put_user(regs->r0, (int *)regs->p0);
+		regs->pc = ATOMIC_XOR32 + 6;
+		break;
+	}
+}
+
 #if defined(CONFIG_ACCESS_CHECK)
 int _access_ok(unsigned long addr, unsigned long size)
 {
diff --git a/arch/blackfin/kernel/setup.c b/arch/blackfin/kernel/setup.c
index 76bf2cea61d..534227f4da3 100644
--- a/arch/blackfin/kernel/setup.c
+++ b/arch/blackfin/kernel/setup.c
@@ -42,6 +42,7 @@
 #include <asm/cacheflush.h>
 #include <asm/blackfin.h>
 #include <asm/cplbinit.h>
+#include <asm/fixed_code.h>
 
 u16 _bfin_swrst;
 
@@ -404,6 +405,27 @@ void __init setup_arch(char **cmdline_p)
 
 	printk(KERN_INFO "Hardware Trace Enabled\n");
 	bfin_write_TBUFCTL(0x03);
+
+	/* Copy atomic sequences to their fixed location, and sanity check that
+	   these locations are the ones that we advertise to userspace.  */
+	memcpy((void *)FIXED_CODE_START, &fixed_code_start,
+	       FIXED_CODE_END - FIXED_CODE_START);
+	BUG_ON((char *)&sigreturn_stub - (char *)&fixed_code_start
+	       != SIGRETURN_STUB - FIXED_CODE_START);
+	BUG_ON((char *)&atomic_xchg32 - (char *)&fixed_code_start
+	       != ATOMIC_XCHG32 - FIXED_CODE_START);
+	BUG_ON((char *)&atomic_cas32 - (char *)&fixed_code_start
+	       != ATOMIC_CAS32 - FIXED_CODE_START);
+	BUG_ON((char *)&atomic_add32 - (char *)&fixed_code_start
+	       != ATOMIC_ADD32 - FIXED_CODE_START);
+	BUG_ON((char *)&atomic_sub32 - (char *)&fixed_code_start
+	       != ATOMIC_SUB32 - FIXED_CODE_START);
+	BUG_ON((char *)&atomic_ior32 - (char *)&fixed_code_start
+	       != ATOMIC_IOR32 - FIXED_CODE_START);
+	BUG_ON((char *)&atomic_and32 - (char *)&fixed_code_start
+	       != ATOMIC_AND32 - FIXED_CODE_START);
+	BUG_ON((char *)&atomic_xor32 - (char *)&fixed_code_start
+	       != ATOMIC_XOR32 - FIXED_CODE_START);
 }
 
 static int __init topology_init(void)
diff --git a/arch/blackfin/mach-common/entry.S b/arch/blackfin/mach-common/entry.S
index 40045b1386a..c4a32ea06c4 100644
--- a/arch/blackfin/mach-common/entry.S
+++ b/arch/blackfin/mach-common/entry.S
@@ -741,6 +741,10 @@ _schedule_and_signal_from_int:
 	r0 = [p0];
 	sti r0;
 
+	r0 = sp;
+	sp += -12;
+	call _finish_atomic_sections;
+	sp += 12;
 	jump.s .Lresume_userspace;
 
 _schedule_and_signal: