linux-kernel - [PATCH 08/12] riscv: read the hart ID from mhartid on boot

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Message-Id: <20191028121043.22934-9-hch@lst.de>
Date:   Mon, 28 Oct 2019 13:10:39 +0100
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-kernel@...r.kernel.org,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atish.patra@....com>
Subject: [PATCH 08/12] 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>
Reviewed-by: Anup Patel <anup@...infault.org>
Reviewed-by: Atish Patra <atish.patra@....com>
Signed-off-by: Christoph Hellwig <hch@....de>
---
 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 0ab642811028..318192c66fd8 100644
--- a/arch/riscv/include/asm/csr.h
+++ b/arch/riscv/include/asm/csr.h
@@ -99,6 +99,7 @@
 #define CSR_MCAUSE		0x342
 #define CSR_MTVAL		0x343
 #define CSR_MIP			0x344
+#define CSR_MHARTID		0xf14
 
 #ifdef CONFIG_RISCV_M_MODE
 # define CSR_STATUS	CSR_MSTATUS
diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index 5cfd2c582945..fc9973086946 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -50,6 +50,14 @@ _start_kernel:
 	csrw CSR_IE, zero
 	csrw CSR_IP, zero
 
+#ifdef CONFIG_RISCV_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