[U-Boot] [PATCH v2 7/9] riscv: do not rely on hart ID passed by previous boot stage

[Lukas Auer]

RISC-V U-Boot expects the hart ID to be passed to it via register a0 by
the previous boot stage. Machine mode firmware such as BBL and OpenSBI
do this when starting their payload (U-Boot) in supervisor mode. If
U-Boot is running in machine mode, this task must be handled by the boot
ROM. Explicitly populate register a0 with the hart ID from the mhartid
CSR to avoid possible problems on RISC-V processors with a boot ROM that
does not handle this task.

Suggested-by: Rick Chen <rick at andestech.com>
Signed-off-by: Lukas Auer <lukas.auer at aisec.fraunhofer.de>
---

Changes in v2:
- New patch to populate register a0 with the hart ID from the mhartid
CSR in machine-mode

 arch/riscv/cpu/start.S | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/riscv/cpu/start.S b/arch/riscv/cpu/start.S
index 79b753847c..d4daa6e0bf 100644
--- a/arch/riscv/cpu/start.S
+++ b/arch/riscv/cpu/start.S
@@ -36,6 +36,10 @@
 .section .text
 .globl _start
 _start:
+#ifdef CONFIG_RISCV_MMODE
+	csrr	a0, mhartid
+#endif
+
 	/* save hart id and dtb pointer */
 	mv	s0, a0
 	mv	s1, a1
-- 
2.20.1