riscv: Add SOC early init support

Add a mechanism for early SoC initialization for platforms that need
additional hardware initialization not possible through the regular
device tree and drivers mechanism. With this, a SoC specific
initialization function can be called very early, before DTB parsing
is done by parse_dtb() in Linux RISC-V kernel setup code.

This can be very useful for early hardware initialization for No-MMU
kernels booted directly in M-mode because it is quite likely that no
other booting stage exist prior to the No-MMU kernel.

Example use of a SoC early initialization is as follows:

static void vendor_abc_early_init(const void *fdt)
{
	/*
	 * some early init code here that can use simple matches
	 * against the flat device tree file.
	 */
}
SOC_EARLY_INIT_DECLARE("vendor,abc", abc_early_init);

This early initialization function is executed only if the flat device
tree for the board has a 'compatible = "vendor,abc"' entry;

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/soc.h

@@ -0,0 +1,23 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Copyright (C) 2020 Western Digital Corporation or its affiliates.
+ */
+
+#ifndef _ASM_RISCV_SOC_H
+#define _ASM_RISCV_SOC_H
+
+#include <linux/of.h>
+#include <linux/linkage.h>
+#include <linux/types.h>
+
+#define SOC_EARLY_INIT_DECLARE(name, compat, fn)			\
+	static const struct of_device_id __soc_early_init__##name	\
+		__used __section(__soc_early_init_table)		\
+		 = { .compatible = compat, .data = fn  }
+
+void soc_early_init(void);
+
+extern unsigned long __soc_early_init_table_start;
+extern unsigned long __soc_early_init_table_end;
+
+#endif

arch/riscv/kernel/Makefile

@@ -11,6 +11,7 @@ endif
 extra-y += head.o
 extra-y += vmlinux.lds
 
+obj-y	+= soc.o
 obj-y	+= cpu.o
 obj-y	+= cpufeature.o
 obj-y	+= entry.o

arch/riscv/kernel/head.S

@@ -234,6 +234,7 @@ clear_bss_done:
 	call kasan_early_init
 #endif
 	/* Start the kernel */
+	call soc_early_init
 	call parse_dtb
 	tail start_kernel
 

arch/riscv/kernel/soc.c

@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2020 Western Digital Corporation or its affiliates.
+ */
+#include <linux/init.h>
+#include <linux/libfdt.h>
+#include <asm/pgtable.h>
+#include <asm/soc.h>
+
+/*
+ * This is called extremly early, before parse_dtb(), to allow initializing
+ * SoC hardware before memory or any device driver initialization.
+ */
+void __init soc_early_init(void)
+{
+	void (*early_fn)(const void *fdt);
+	const struct of_device_id *s;
+	const void *fdt = dtb_early_va;
+
+	for (s = (void *)&__soc_early_init_table_start;
+	     (void *)s < (void *)&__soc_early_init_table_end; s++) {
+		if (!fdt_node_check_compatible(fdt, 0, s->compatible)) {
+			early_fn = s->data;
+			early_fn(fdt);
+			return;
+		}
+	}
+}

arch/riscv/kernel/vmlinux.lds.S

@@ -28,6 +28,12 @@ SECTIONS
 	__init_begin = .;
 	INIT_TEXT_SECTION(PAGE_SIZE)
 	INIT_DATA_SECTION(16)
+	. = ALIGN(8);
+	__soc_early_init_table : {
+		__soc_early_init_table_start = .;
+		KEEP(*(__soc_early_init_table))
+		__soc_early_init_table_end = .;
+	}
 	/* we have to discard exit text and such at runtime, not link time */
 	.exit.text :
 	{