linux-kernel - [PATCH 11/17] riscv: read hart ID from mhartid on boot

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Message-Id: <20190610221621.10938-12-hch@lst.de>
Date:   Tue, 11 Jun 2019 00:16:15 +0200
From:   Christoph Hellwig <hch@....de>
To:     Palmer Dabbelt <palmer@...ive.com>
Cc:     Damien Le Moal <damien.lemoal@....com>,
        linux-riscv@...ts.infradead.org, uclinux-dev@...inux.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH 11/17] riscv: read hart ID from mhartid on boot

From: Damien Le Moal <damien.lemoal@....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@....com>
---
 arch/riscv/kernel/head.S | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index cb5691d82b0b..e05379fd8b64 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -25,6 +25,14 @@ ENTRY(_start)
 	/* Reset all registers except ra, a0,a1 */
 	call reset_regs
 
+#ifdef CONFIG_M_MODE
+	/*
+	 * The hartid in a0 is expected later on, and we have no firmware
+	 * to hand it to us.
+	 */
+	csrr a0, mhartid
+#endif
+
 	/* Load the global pointer */
 .option push
 .option norelax
-- 
2.20.1