[U-Boot] [PATCH v6 1/3] riscv: Add kconfig option to run U-Boot in S-mode

[Anup Patel <anup@brainfault.org>]

This patch adds kconfig option RISCV_SMODE to run U-Boot in
S-mode. When this opition is enabled we use s<xyz> CSRs instead
of m<xyz> CSRs.

It is important to note that there is no equivalent S-mode CSR
for misa and mhartid CSRs so we expect M-mode runtime firmware
(BBL or equivalent) to emulate misa and mhartid CSR read.

In-future, we will have more patches to avoid accessing misa and
mhartid CSRs from S-mode.

Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
---
 arch/riscv/Kconfig                |  5 +++++
 arch/riscv/cpu/start.S            | 23 +++++++++++++++--------
 arch/riscv/include/asm/encoding.h |  6 ++++++
 arch/riscv/lib/interrupts.c       | 31 ++++++++++++++++++++++---------
 4 files changed, 48 insertions(+), 17 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 3e0af55e71..732a357a99 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -55,6 +55,11 @@ config RISCV_ISA_C
 config RISCV_ISA_A
 	def_bool y
 
+config RISCV_SMODE
+	bool "Run in S-Mode"
+	help
+	  Enable this option to build U-Boot for RISC-V S-Mode
+
 config 32BIT
 	bool
 
diff --git a/arch/riscv/cpu/start.S b/arch/riscv/cpu/start.S
index 15e1b8199a..3f055bdb7e 100644
--- a/arch/riscv/cpu/start.S
+++ b/arch/riscv/cpu/start.S
@@ -41,10 +41,10 @@ _start:
 	li	t0, CONFIG_SYS_SDRAM_BASE
 	SREG	a2, 0(t0)
 	la	t0, trap_entry
-	csrw	mtvec, t0
+	csrw	MODE_PREFIX(tvec), t0
 
 	/* mask all interrupts */
-	csrw	mie, zero
+	csrw	MODE_PREFIX(ie), zero
 
 	/* Enable cache */
 	jal	icache_enable
@@ -166,7 +166,7 @@ fix_rela_dyn:
 */
 	la	t0, trap_entry
 	add	t0, t0, t6
-	csrw	mtvec, t0
+	csrw	MODE_PREFIX(tvec), t0
 
 clear_bss:
 	la	t0, __bss_start		/* t0 <- rel __bss_start in FLASH */
@@ -238,17 +238,24 @@ trap_entry:
 	SREG	x29, 29*REGBYTES(sp)
 	SREG	x30, 30*REGBYTES(sp)
 	SREG	x31, 31*REGBYTES(sp)
-	csrr	a0, mcause
-	csrr	a1, mepc
+	csrr	a0, MODE_PREFIX(cause)
+	csrr	a1, MODE_PREFIX(epc)
 	mv	a2, sp
 	jal	handle_trap
-	csrw	mepc, a0
+	csrw	MODE_PREFIX(epc), a0
 
+#ifdef CONFIG_RISCV_SMODE
+/*
+ * Remain in S-mode after sret
+ */
+	li	t0, SSTATUS_SPP
+#else
 /*
  * Remain in M-mode after mret
  */
 	li	t0, MSTATUS_MPP
-	csrs	mstatus, t0
+#endif
+	csrs	MODE_PREFIX(status), t0
 	LREG	x1, 1*REGBYTES(sp)
 	LREG	x2, 2*REGBYTES(sp)
 	LREG	x3, 3*REGBYTES(sp)
@@ -281,4 +288,4 @@ trap_entry:
 	LREG	x30, 30*REGBYTES(sp)
 	LREG	x31, 31*REGBYTES(sp)
 	addi	sp, sp, 32*REGBYTES
-	mret
+	MODE_PREFIX(ret)
diff --git a/arch/riscv/include/asm/encoding.h b/arch/riscv/include/asm/encoding.h
index 9ea50ce640..97cf906aa6 100644
--- a/arch/riscv/include/asm/encoding.h
+++ b/arch/riscv/include/asm/encoding.h
@@ -7,6 +7,12 @@
 #ifndef RISCV_CSR_ENCODING_H
 #define RISCV_CSR_ENCODING_H
 
+#ifdef CONFIG_RISCV_SMODE
+#define MODE_PREFIX(__suffix)	s##__suffix
+#else
+#define MODE_PREFIX(__suffix)	m##__suffix
+#endif
+
 #define MSTATUS_UIE	0x00000001
 #define MSTATUS_SIE	0x00000002
 #define MSTATUS_HIE	0x00000004
diff --git a/arch/riscv/lib/interrupts.c b/arch/riscv/lib/interrupts.c
index 903a1c4cd5..3aff006977 100644
--- a/arch/riscv/lib/interrupts.c
+++ b/arch/riscv/lib/interrupts.c
@@ -34,17 +34,30 @@ int disable_interrupts(void)
 	return 0;
 }
 
-ulong handle_trap(ulong mcause, ulong epc, struct pt_regs *regs)
+ulong handle_trap(ulong cause, ulong epc, struct pt_regs *regs)
 {
-	ulong is_int;
+	ulong is_irq, irq;
 
-	is_int = (mcause & MCAUSE_INT);
-	if ((is_int) && ((mcause & MCAUSE_CAUSE)  == IRQ_M_EXT))
-		external_interrupt(0);	/* handle_m_ext_interrupt */
-	else if ((is_int) && ((mcause & MCAUSE_CAUSE)  == IRQ_M_TIMER))
-		timer_interrupt(0);	/* handle_m_timer_interrupt */
-	else
-		_exit_trap(mcause, epc, regs);
+	is_irq = (cause & MCAUSE_INT);
+	irq = (cause & ~MCAUSE_INT);
+
+	if (is_irq) {
+		switch (irq) {
+		case IRQ_M_EXT:
+		case IRQ_S_EXT:
+			external_interrupt(0);	/* handle external interrupt */
+			break;
+		case IRQ_M_TIMER:
+		case IRQ_S_TIMER:
+			timer_interrupt(0);	/* handle timer interrupt */
+			break;
+		default:
+			_exit_trap(cause, epc, regs);
+			break;
+		};
+	} else {
+		_exit_trap(cause, epc, regs);
+	}
 
 	return epc;
 }
-- 
2.17.1