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Subject[PATCH 07/15] riscv: read the hart ID from mhartid on boot
Date
From: Damien Le Moal <Damien.LeMoal@wdc.com>

When in M-Mode, we can use the mhartid CSR to get the ID of the running
HART. Doing so, direct M-Mode boot without firmware is possible.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
---
arch/riscv/include/asm/csr.h | 1 +
arch/riscv/kernel/head.S | 8 ++++++++
2 files changed, 9 insertions(+)

diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
index 53d7ce74b447..64f8fe84b88f 100644
--- a/arch/riscv/include/asm/csr.h
+++ b/arch/riscv/include/asm/csr.h
@@ -81,6 +81,7 @@
#define SIE_SEIE (_AC(0x1, UL) << IRQ_S_EXT)

/* symbolic CSR names: */
+#define CSR_MHARTID 0xf14
#define CSR_MSTATUS 0x300
#define CSR_MIE 0x304
#define CSR_MTVEC 0x305
diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index bb96bb7b95d2..275c2ab1e990 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -50,6 +50,14 @@ _start_kernel:
csrw CSR_XIE, zero
csrw CSR_XIP, zero

+#ifdef CONFIG_M_MODE
+ /*
+ * The hartid in a0 is expected later on, and we have no firmware
+ * to hand it to us.
+ */
+ csrr a0, CSR_MHARTID
+#endif
+
/* Load the global pointer */
.option push
.option norelax
--
2.20.1
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