lib: Emit lr and sc instructions based on -march flags

When -march=rv64im_zalrsc_zicsr is used, provide atomic operations
and locks using lr and sc instructions only.

Signed-off-by: Chao-ying Fu <cfu@mips.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20250226014727.19710-1-cfu@mips.com
Signed-off-by: Anup Patel <anup@brainfault.org>

docs/platform_requirements.md

@@ -19,6 +19,10 @@ Base Platform Requirements
 The base RISC-V platform requirements for OpenSBI are as follows:
 
 1. At least rv32ima_zicsr or rv64ima_zicsr required on all HARTs
+
+     * Users may restrict the usage of atomic instructions to lr/sc
+       via rv32im_zalrsc_zicsr or rv64im_zalrsc_zicsr if preferred
+
 2. At least one HART should have S-mode support because:
 
      * SBI calls are meant for RISC-V S-mode (Supervisor mode)

firmware/fw_base.S

@@ -59,8 +59,18 @@ _try_lottery:
 	/* Jump to relocation wait loop if we don't get relocation lottery */
 	lla	a6, _boot_lottery
 	li	a7, BOOT_LOTTERY_ACQUIRED
+#ifdef __riscv_atomic
 	amoswap.w a6, a7, (a6)
 	bnez	a6, _wait_for_boot_hart
+#elif __riscv_zalrsc
+_sc_fail:
+	lr.w	t0, (a6)
+	sc.w	t1, a7, (a6)
+	bnez	t1, _sc_fail
+	bnez	t0, _wait_for_boot_hart
+#else
+#error "need a or zalrsc"
+#endif
 
 	/* relocate the global table content */
 	li	t0, FW_TEXT_START	/* link start */

firmware/payloads/test_head.S

@@ -30,7 +30,18 @@ _start:
 	/* Pick one hart to run the main boot sequence */
 	lla	a3, _hart_lottery
 	li	a2, 1
+#ifdef __riscv_atomic
 	amoadd.w a3, a2, (a3)
+#elif __riscv_zalrsc
+_sc_fail:
+	lr.w	t0, (a3)
+	addw	t1, t0, a2
+	sc.w	t1, t1, (a3)
+	bnez	t1, _sc_fail
+	move	a3, t0
+#else
+#error "need a or zalrsc"
+#endif
 	bnez	a3, _start_hang
 
 	/* Save a0 and a1 */

lib/sbi/riscv_atomic.c

@@ -12,7 +12,7 @@
 #include <sbi/riscv_atomic.h>
 #include <sbi/riscv_barrier.h>
 
-#ifndef __riscv_atomic
+#if !defined(__riscv_atomic) && !defined(__riscv_zalrsc)
 #error "opensbi strongly relies on the A extension of RISC-V"
 #endif
 
@@ -31,6 +31,7 @@ void atomic_write(atomic_t *atom, long value)
 
 long atomic_add_return(atomic_t *atom, long value)
 {
+#ifdef __riscv_atomic
 	long ret;
 #if __SIZEOF_LONG__ == 4
 	__asm__ __volatile__("	amoadd.w.aqrl  %1, %2, %0"
@@ -43,6 +44,29 @@ long atomic_add_return(atomic_t *atom, long value)
 			     : "r"(value)
 			     : "memory");
 #endif
+#elif __riscv_zalrsc
+	long ret, temp;
+#if __SIZEOF_LONG__ == 4
+	__asm__ __volatile__("1:lr.w.aqrl	%1,%0\n"
+			     "  addw	%2,%1,%3\n"
+			     "  sc.w.aqrl	%2,%2,%0\n"
+			     "  bnez	%2,1b"
+			     : "+A"(atom->counter), "=&r"(ret), "=&r"(temp)
+			     : "r"(value)
+			     : "memory");
+#elif __SIZEOF_LONG__ == 8
+	__asm__ __volatile__("1:lr.d.aqrl	%1,%0\n"
+			     "  add	%2,%1,%3\n"
+			     "  sc.d.aqrl	%2,%2,%0\n"
+			     "  bnez	%2,1b"
+			     : "+A"(atom->counter), "=&r"(ret), "=&r"(temp)
+			     : "r"(value)
+			     : "memory");
+#endif
+#else
+#error "need a or zalrsc"
+#endif
+
 	return ret + value;
 }
 
@@ -51,6 +75,7 @@ long atomic_sub_return(atomic_t *atom, long value)
 	return atomic_add_return(atom, -value);
 }
 
+#ifdef __riscv_atomic
 #define __axchg(ptr, new, size)							\
 	({									\
 		__typeof__(ptr) __ptr = (ptr);					\
@@ -76,6 +101,39 @@ long atomic_sub_return(atomic_t *atom, long value)
 		}								\
 		__ret;								\
 	})
+#elif __riscv_zalrsc
+#define __axchg(ptr, new, size)							\
+	({									\
+		__typeof__(ptr) __ptr = (ptr);					\
+		__typeof__(new) __new = (new);					\
+		__typeof__(*(ptr)) __ret, __temp;					\
+		switch (size) {							\
+		case 4:								\
+			__asm__ __volatile__ (					\
+				"1:	lr.w.aqrl %0, %1\n"			\
+				"	sc.w.aqrl %2, %3, %1\n"			\
+				"	bnez	  %2, 1b\n"			\
+				: "=&r" (__ret), "+A" (*__ptr), "=&r" (__temp)	\
+				: "r" (__new)					\
+				: "memory");					\
+			break;							\
+		case 8:								\
+			__asm__ __volatile__ (					\
+				"1:	lr.d.aqrl %0, %1\n"			\
+				"	sc.d.aqrl %2, %3, %1\n"			\
+				"	bnez	  %2, 1b\n"			\
+				: "=&r" (__ret), "+A" (*__ptr), "=&r" (__temp)	\
+				: "r" (__new)					\
+				: "memory");					\
+			break;							\
+		default:							\
+			break;							\
+		}								\
+		__ret;								\
+	})
+#else
+#error "need a or zalrsc"
+#endif
 
 #define axchg(ptr, x)								\
 	({									\

lib/sbi/riscv_locks.c

@@ -53,7 +53,16 @@ void spin_lock(spinlock_t *lock)
 
 	__asm__ __volatile__(
 		/* Atomically increment the next ticket. */
+#ifdef __riscv_atomic
 		"	amoadd.w.aqrl	%0, %4, %3\n"
+#elif __riscv_zalrsc
+		"3:	lr.w.aqrl	%0, %3\n"
+		"	addw	%1, %0, %4\n"
+		"	sc.w.aqrl	%1, %1, %3\n"
+		"	bnez	%1, 3b\n"
+#else
+#error "need a or zalrsc"
+#endif
 
 		/* Did we get the lock? */
 		"	srli	%1, %0, %6\n"