riscv: read the hart ID from mhartid on boot

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>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

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

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