linux-kernel - [PATCH 10/17] riscv: read the hart ID from mhartid on boot

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Date:   Mon, 24 Jun 2019 07:43:04 +0200
From:   Christoph Hellwig <hch@....de>
To:     Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>
Cc:     Damien Le Moal <damien.lemoal@....com>,
        linux-riscv@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Damien Le Moal <Damien.LeMoal@....com>
Subject: [PATCH 10/17] riscv: read the 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>
Signed-off-by: Christoph Hellwig <hch@....de>
---
 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 e5fa5481aa99..a4c170e41a34 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -18,6 +18,14 @@ ENTRY(_start)
 	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, mhartid
+#endif
+
 	/* Load the global pointer */
 .option push
 .option norelax
-- 
2.20.1